- 


FIG.  323.     PLANO-CONVEX  LENS  WITH  LEAST  ABERRATION,  §809 


FIG.  322.     PLANO-CONVEX  LENS  WITH  GREATEST  ABERRATION,  §809 


FIG.  320.     CROSSING  OF  LIGHT  RAYS  WITH  GREATEST  ABERRATION,  §809 


FIG.  323A.     LIGHT  CONE  WITH  THE  RADIANT  ABOVE  THE  OPTIC  Axis,  §57 


OPTIC    PROJECTION 


PRINCIPLES,  INSTALLATION  AND  USE 


OF  THE 


MAGIC  LANTERN 
PROJECTION  MICROSCOPE 
REFLECTING  LANTERN 
MOVING  PICTURE  MACHINE 


'  FULLY  ILLUSTRATED  WITH  PLATES  AND  WITH 
OVER  400  TEXT-FIGURES 


By  SIMON  HENRY  GAGE 

Professor  of  Histology  and  Embryology,  Emeritus,  Cornell  University 
AND 

HENRY  PHELPS  GAGE,  Ph.D. 


ITHACA,  NEW  YORK 

COMSTOCK  PUBLISHING  COMPANY 

1914 


COPYRIGHT   1914 

COMSTOCK   PUBLISHING   CO., 

ITHACA,   N.   Y. 


PRESS  OF  W.  F.  HUMPHREY.  QSNEVX",  N.  Y. 


TO 

JACOB  GOULD  SCHURMAN 

IN  GRATEFUL  RECOGNITION  OF  HIS  ABLE  AND 
DEVOTED  SERVICE  TO  CORNELL  UNIVERSITY,  OF 
HIS  BREADTH  OF  SYMPATHY  FOR  ALL  ART  AND 
ALL  SCIENCE,  AND  OF  THE  ENCOURAGEMENT 
AND  SUPPORT  WHICH  HAVE  MADE  THE  PRESENT 
WORK  POSSIBLE,  THIS  VOLUME  IS  DEDICATED. 


295710 


PREFACE 

OUR  aim  in  the  preparation  of  this  work  on  Optic  Projection 
has  been  to  explain  the  underlying  principles  on  which  the 
art  depends,  and  to  give  such  simple  and  explicit  directions 
that  any  intelligent  person  can  succeed  in  all  the  fields  of  projec- 
tion; and  our  hope  is  that  the  book  will  serve  to  make  more 
general  this  graphic  art  by  means  of  which  many  persons  can  be 
appealed  to  at  the  same  time  and  in  the  most  striking  manner. 
Furthermore  we  believe  that  this  art  has  great,  undeveloped 
possibilities  for  giving  pleasure,  arousing  interest  and  kindling 
enthusiasm,  in  that  it  provides  for  the  rapid  demonstration  of 
maps,  diagrams  and  pictures  of  all  kinds,  the  structure  and  develop- 
ment of  animals  and  plants,  many  of  the  actual  phenomena  of 
physics  and  chemistry,  and  finally. scenes  from  nature  and  from  life, 
even  with  their  natural  motions  and  colors. 

The  authors  have  received  most  generous  aid  from  many  indi- 
viduals and  many  manufacturers;  and  most  loyal  service  from 
those  who  have  helped  to  put  the  book  in  its  present  actual  form. 

Manufacturers  have  not  only  answered  our  numerous  questions, 
but  have  put  at  our  disposal  valuable  apparatus  for  experiment. 
They  have  also  loaned  us  electrotypes  of  their  apparatus. 

We  feel  especially  indebted  to  the  Department  of  Physics  of 
Cornell  University  for  the  help  given  by  different  members  of  the 
staff,  and  for  the  use  of  a  research  room  and  apparatus  for  the 
numerous  photometric  and  other  determinations  required.  Pro- 
fessor George  S.  Moler  of  that  department  read  over  the  manu- 
script, and  gave  us  many  valuable  hints  derived  from  his  experience 
of  over  40  years  with  all  kinds  of  projection  apparatus. 

While  we  have  both  joined  in  the  preparation  of  the  entire 
work,  each  holds  himself  especially  responsible  for  certain  chapters 
as  follows : 

The  senior  author  for  10  chapters  (I-V,  VII-X  and  XII). 

The  junior  author  for  5  chapters  (VI,  XI,  XIII-XV). 

SIMON  HENRY  GAGE, 
October  4,  1914.  HENRY  PHELPS  GAGE. 


CONTENTS 

Introduction pp.  3-7 

CHAPTER  I 
Magic  Lantern  with  Direct  Current 

Fig.  1-38;   §  1-99, pp.  9-67 

Apparatus  and  material  for  Ch.  I,  §  i;  Works  of  reference,  §  2;  Magic 
Lanterns,  §  3-19;  Perfection  and  brilliancy  of  the  screen  image,  §  20; 
Suggestions  for  the  lecturer  or  demonstrator,  §  21-25;  Suggestions  to  the 
operator,  §  26-41 ;  Projection  of  horizontal  objects  with  a  vertical  objec- 
tive, §  42;  Projection  with  multiple  lanterns,  §  43-46;  Moving  slides  for 
single  lanterns,  §  47-49;  Stereoscopic  screen  images,  §  50;  Centering  the 
parts  of  the  magic  lantern,  §51-54;  Correct  separation  of  the  parts,  §  55- 
56;  Optical  test  for  centering,  §  57-58;  Centering  the  vertical  objective, 
§  59-60;  Troubles  with  the  magic  lantern,  §  61-93;  Breaking  of  conden- 
ser lenses,  §  94-97;  Examples  of  American  magic  lanterns,  §  99,  fig.  32- 
38;  Summary  of  Ch.  I,  §  99,. 


CHAPTER  II 
Magic  Lantern  with  Alternating  Current 

Fig-  39-40;    §  100-119 •  •  -PP-  68-77 

Apparatus  and  material  required,  §  100;  Comparison  of  direct  and  alternat- 
ing current,  §  102-103;  Installation  with  alternating  current,  §  104-113; 
Use  of  the  magic  lantern  with  alternating  current,  §  114-115;  Troubles 
with  alternating  current  lanterns,  §  Ii6-n8;  Summary  of  Ch.  II,  §  119. 


CHAPTER  III 
Magic  Lantern  for  Use  on  the  House  Electric  Lighting  System 

Fig.  41-55;  §  120-148..  ...pp.  78-99 

Apparatus  and  material,  §  120;  Magic  lantern  with  small  current  for  home 
and  laboratory,  §  122-126;  Arc  lamps  for  the  house  circuit,  §  127-131: 
Turning  the  arc  lamp  on  and  off,  §  132-135;  Magic  lantern  with  mazda 
lamp,  §136-139;  Magic  lanterns  with  Nernst  lamp,  §  140-146;  Troubles 
in  Ch.  Ill,  §  147;  Summary  of  Ch.  Ill,  §  148. 


vi  OPTIC    PROJECTION 

CHAPTER  IV 

Magic  Lantern  with  the  Lime  Light 
Fig.  56-63;  §  150-186 pp.  100-118 

Apparatus  and  material,  §  150;  Lime  light  with  oxygen  and  hydrogen,  §  152- 
158;  Management  of  the  lime  light  lantern,  §  159-173;  The  lime  light 
with  oxygen  and  illuminating  gas,  §  174-176;  The  lime  light  with  oxygen 
and  ether  vapors,  §  177-179;  Troubles  with  the  lime  light,  §  180-185; 
Summary  of  Chapter  IV,  §  186. 


CHAPTER  V 

Magic  Lantern  with  a  Petroleum  Lamp,  with  Gas,  Acetylene,  and  Alcohol 

Lamps 

Fig.  64-73;  §  190-224 pp.  H9-I37 

Apparatus  and  material,  §  190;  Oil  and  gas  lamps,  §  192-195;  Magic  lantern 
with  kerosene  lamp,  §  196-202;  Lantern  with  mantle  gas  lamps,  §  203- 
207;  Lantern  with  acetylene  lamp,  §  208-213;  Lantern  with  alcohol 
lamp  and  mantle,  §  214-219;  Troubles  in  Ch.  V,  §  220-223;  Summary 
of  Ch.  V,  §  224. 

CHAPTER  VI 
Magic  Lantern  with  Sunlight;  Heliostats 

Fig.  74-87;  §  230-265 pp.  138-165 

Apparatus  and  material  for  Ch.  VI,  §  230;  Light  from  the  sun,  and 
heliostats,  §  232-233;  Installation  and  use  of  heliostats,  §  234-238,  239- 
248;  Heliostats  for  the  southern  hemisphere,  §  249-255;  Condenser  for 
sunlight,  §  256-258;  Conduct  of  an  exhibition  with  sunlight,  §  259-262; 
Troubles  with  sunlight,  §  263-264;  Summary  of  Ch.  VI,  §  265. 


CHAPTER  VII 
Projection  of  Images  of  Opaque  Objects 

Fig.  88-1 1 1 ;  §  270-297 pp.  166-199 

Apparatus  and  material  for  Ch.  VII,  §  270;  Images  of  opaque  objects,  §  272; 
Comparison  of  the  projection  of  opaque  and  transparent  objects,  §  273- 
280;  Combined  projections,  §  281-282;  Opaque  projection  demonstra- 
tions, §  283-292 ;  Erect  images  with  opaque  projection,  §  293-295 ;  Troubles 
with  opaque  projections,  §  296;  Summary  of  Ch.  VII,  §  297. 


CONTENTS  vn 

CHAPTER  VIII 
Preparation  of  Lantern  Slides 

Fig,  112-120;  §  310-340 pp.  200-220 

Apparatus  and  material  for  Ch.  VIII,  §  310;  Sizes  of  lantern  slides,  and 
necessary  condensers,  §  312-314;  Making  lantern  slides  direct,  §  315-324; 
Photographic  lantern  slides,  marking,  mounting  and  coloring  them,  §  325- 
337;  Storing  lantern  slides,  §  338;  Troubles  in  making  lantern  slides, 
§  339;  Summary  of  Ch.  VIII,  §  340. 

CHAPTER  IX 
The  Projection  Microscope 

Fig.  121-178;  §  350-441 pp.  221-318 

Apparatus  and  references,  §  350-351;  General  on  micro-projection,  §  352- 
354;  Objectives,  amplifiers,  oculars,  visibility  of  objects,  diopter,  §  355- 
359;  Room  and  screen,  §360;  Arc  lamp  and  wiring,  fine  adjustment,  con- 
denser, water-cell  stage,  mechanical  stage,  §  361-369;  Blackening  appara- 
tus, §  370-371 ;  Hoods  for  objectives  and  shield  for  stray  light,  §  372-373; 
Centering  the  projection  microscope,  §  374-376;  Table  of  candle  power, 
§  377?  Use  of  the  projection  microscope,  §  379-390;  Magnification  of 
screen  images,  §  391-392;  Projection  with  an  ordinary  microscope,  §  393- 
396;  Projection  of  horizontal  objects  with  a  vertical  microscope,  §  397; 
Sample  objects  for  micro-projection,  §  399;  Conduct  of  an  exhibition, 
§  400;  Demonstration  with  high  powers,  §  401-411;  Alternating 
current  for  micro-projection,  §  412-416;  Micro-projection  with  the  house- 
current,  §  417-418;  Micro-projection  with  sunlight,  §  419-421;  Micro- 
projection  with  lime  light,  §  422-423;  Home-made  projection  apparatus, 
§  424-431;  Combined  micro-projection  and  lantern  slide  projection, 
§  432;  Projection  microscopes  on  the  market,  §  433-434;  Troubles  in 
micro-projection,  §  435-440;  Summary  of  Ch.  IX,  §  441. 

CHAPTER  X 
Drawing  and  Photography  with  Projection  Apparatus 

Fig.  179-220;   §  450-549 pp.  319-389 

Apparatus  and  material,  §  450;  Drawing  with  projection  apparatus,  general, 
§  452;  Room  for  drawing,  §  453-455;  Projection  apparatus  for  draw- 
ing, §  456-460;  Light  for  drawing,  §  461-463;  Drawing  with  the  magic 
lantern,  §  464-468;  Drawing  with  the  reflecting  lantern,  §  469-470; 
Drawing  with  a  photographic  camera,  §  471;  Drawing  with  the  projec- 
tion microscope,  §  472-485;  Drawing  with  the  house  current,  §  486-491; 
Microscope  to  use  with  the  house  current,  §  492-503;  Avoidance  of  heat, 


viri  OPTIC  PROJECTION 

§  504;  Stray  light,  §  505-506;  Magnification  of  drawings,  §  507-508; 
509-510;  Drawings  for  models,  §  511;  Erect  images  in  the  drawings, 
§  512-516;  517-526;  Drawings  for  publication  with  projection  appara- 
tus, §  527-530;  Drawings  and  their  lettering,  §  531;  Photography  with 
projection  apparatus,  §  532-547;  Troubles  met  in  Ch.  X,  §  548;  Sum 
mary  of  Ch.  X,  §  549. 

CHAPTER  XI 
Moving  Pictures 

Fig.  221-236;  §  550-599- ••  --PP.  39°-438 

Apparatus  and  material,  §  550;  Introduction,  §  552;  Auditorium,  screen 
and  operating  room,  §  553-557;  Current,  lamps  and  moving  picture 
machine,  §  558-574;  Installation  of  a  moving  picture  outfit,  §  575; 
Optics  of  moving  picture  projection,  §  576-578;  Magic  lantern  with  the 
moving  picture  machine,  §  579;  Management  of  the  lamp,  moving  pic- 
ture machine  mechanism,  §580-589;  flicker,  §590-592;  General  precau- 
tions, §  593;  Splicing  films,  §  594;  Winding  and  rewinding,  §  595; 
Danger  of  fire,  §  596;  Conduct  of  an  exhibition,  §  597;  Home  projectors 
and  advertising  magic  lanterns,  §  598;  Troubles  with  moving  pictures, 
§  599;  Summary  of  Ch.  XI,  §  5991. 

CHAPTER  XII 
Projection  Rooms  and  Screens 

Fig.  237-251 ;   §  600-642 pp.  439-473 

Apparatus  and  material  for  Ch.  XII,  §  600;  Suitable  room  for  projection,  and 
its  lighting,  §  602-611;  Position  of  the  projection  apparatus  in  the  room, 
§  612-620;  Screen  for  the  image,  §  621-628;  629-632;  Size  of  screens  and 
screen  images,  §  633-640;  Troubles  with  rooms  and  screens,  §  641; 
Summary  of  Ch.  XII,  §  642. 

CHAPTER  XIII 

Electric  Currents  and  their  Measurement;   Arc  Lamps,  Wiring  and  Control; 
Candle-Power  of  Arc  Lamps  for  Projection 

Fig.  252-308;  §  650-782 pp.  474-571 

Apparatus  and  material  for  Ch.  XIII,  §  650;  Electric  currents,  kinds  and 
comparison,  §  652-653;  Electric  units,  §  654-661;  Electric  measure- 
ments and  apparatus,  §  662-671;  672-674;  Power  factor,  cycle,  frequency, 
§  675-677;  Special  dynamo  for  arc  lamps,  §  678-680;  Current  rectifiers, 
§  681-683;  135  and  25  cycle  currents  for  projection,  §  684-685;  Wiring 
for  arc  lamp  from  dynamo  back  to  dynamo,  §  686;  Amperages  for 
different  purposes,  short  circuit,  ground,  insulation  of  wires,  §  687-690; 


CONTENTS  ix 

Regulations  for  wiring,  §  691-692;  693-700;  Polarity  tests  for  direct 
current,§  701-703;  Wiring  the  three- wire  automatic  lamp,  §  704;  Wiring 
for  alternating  current,  §  705-710;  Switches,  circuit  breakers  and  fuses 
§  711-722;  Resistors  or  rheostats,  §  723-735;  Reactors,  inductors, 
choke-coils,  etc.,  §  736-738;  Transformer,  §  739;  The  electric  arc,  §  740- 
743;  The  use  of  ballast  (rheostats,  etc.),  §  744 — 748;  The  arc  lamp,  light 
and  heat  from,  §  749-752;  Carbons  and  their  position,  §  753;  Table  show- 
ing size  and  wear  of  carbons,  §  753a;  Candle  power  of  arc  lamps,  §  754- 
762;  Candle  power,  and  energy  required,  §  763-768;  Distribution  of  light 
intensity  in  different  directions,  §  769-771;  Intrinsic  brilliancy  of  the 
crater,  §  773;  Visible  and  invisible  radiation,  §  774;  Radiant  efficiency 
of  arc  lamps,  §775-776;  Energy  required  for  moving  picture  projection, 
§  779-78i;  Effect  of  opacity  in  the  film,  §  782. 

CHAPTER  XIV 
Optics  of  Projection 

Fig.  309-349;   §  790-865 pp.  572-620 

Reflection  and  refraction,  §  792-801;  Lenses,  §  802-808;  Spherical  and 
chromatic  aberration  in  lenses,  §  809-810;  Image  formation  with  the 
magic  lantern,  §  811-817;  Focus  of  Condenser  and  objective,  §  818; 
Types  of  condensers,  §  819-821;  Image  formation  with  moving  pictures, 
§  822-832;  Image  formation  with  the  projection  microscope,  §  833-838; 
Light  losses,  §  839-843;  Energy  losses,  §  844-854;  Effect  of  aperture, 
§  855-856;  Brightness  of  the  screen  image,  §  857;  Microscopic  image 
and  aperture,  §  858-863;  Koehler  method  of  illumination,  §  864-865. 

CHAPTER  XV 
Uses  of  Projection  in  Physics;    Normal  and  Defective  Vision 

Fig.  350-402;  §  875-932 pp.  621-672 

Apparatus  and  material  for  Ch.  XV,  §  875 ;  Introduction,  §  877-878 ;  Experi- 
ments with  polarized  light,  §  879-884;  Projection  of  spectra,  §  885-900; 
Absorption  spectra,  §  901-902;  Emission  spectra,  §  903-905;  Ultra- 
violet light,  photography,  §  906-908;  Abbe  diffraction  theory,  §  909-911; 
Dark  ground  illumination,  striae,  §  912-915;  Normal  vision  and  eye 
defects,  §  916-932. 

Appendix.  Brief  Historical  Statement  on  the  Origin  and  Develop- 
ment of  Projection  Apparatus 673 

Projection  Apparatus  and  Accessories  in  the  Open  Market;  Manu- 
facturers    688 

Bibliography  on  Projection 693 

Index  of  names  and  subjects 705 


INTRODUCTION 

IN  THE  following  pages,  Projection  Apparatus  of  various  forms 
and  with  various  sources  of  light  have  been  considered  from 
a  three-fold  standpoint: 

(1)  The  standpoint  of  the  actual  user  of  the  apparatus. 

(2)  The  standpoint  of  the  manufacturer. 

(3)  The  standpoint  of  the  student  for  whom  an  understanding 

of  the  principles  involved  is  of  fundamental  importance. 

From  the  first  and  second  standpoints  simple  "rule  of  thumb" 
would  answer,  and  in  many  cases  has  answered  to  bring  about 
fairly  good  results.  For  example,  the  toy  magic  lanterns  so  much 
in  evidence  at  Christmas  time,  are  almost  exact  copies  of  the  first 
magic  lantern  shown  by  Walgenstein  in  1665.  The  only  striking 
difference  is  that  instead  of  a  candle  or  lamp  without  a  chimney 
such  as  he  used,  there  is  now  a  small  petroleum  lamp  with  a  glass 
chimney. 

But  for  adapting  projection  apparatus  to  new  conditions  and 
applying  it  to  new  uses  with  the  greatest  efficiency,  the  user  and 
the  manufacturer  must  comprehend  the  fundamental  optical  and 
mechanical  principles  involved.  In  a  word,  to  make  good  projec- 
tion apparatus  and  to  produce  good  projection  in  the  different 
fields,  the  manufacturer  and  the  user  must  know  the  principles, 
and  then  they  must  build  and  must  use  the  apparatus  in  accordance 
with  those  principles. 

Besides  the  optical  and  mechanical  principles  involved  in  the 
apparatus,  it  seems  to  the  authors  that  the  physiology  of  vision 
should  have  prime  consideration,  because,  after  all,  it  is  not  only 
the  possibility  of  producing  a  brilliant  screen  image  that  must  be 
thought  of,  but  also  the  possibility  that  the  observer  get  a  satis- 
factory impression  of  that  image.  With  the  magic  lantern  and 
arc  light  it  is  very  easy  to  get  screen  images  as  brilliant  as  daylight 
scenes  in  nature.  These  brilliant  images  are  best  seen  when  the 
eyes  of  the  observers  are  adapted  to  daylight  vision.  If  now,  as  is 


4  INTRODUCTION 

possible  with  modern  combined  apparatus,  the  brilliant  screen 
image  of  the  transparency  is  replaced  by  a  relatively  dim  image 
projected  by  the  opaque  lantern,  it  will  appear  exceedingly  dim 
until  the  eyes  can  be  adjusted  to  twilight  vision.  If  the  operation 
is  reversed  after  the  eyes  are  adapted  to  dim  light,  the  brilliant 
screen  image  of  the  transparency  will  dazzle  the  eyes. 

It  is  then,  not  only  the  dead  machine  that  must  be  considered, 
but  also  the  living  machine — the  eye.  It  is  for  the  eye  that  all  the 
work  is  done,  and  perfection  can  be  gained  only  by  understanding 
the  workings  of  the  two  machines,  and  adapting  the  dead  machine 
to  the  physiologic  laws  governing  the  living  machine. 

Our  aim  in  writing  this  book  then  has  been  to  show  how  good 
results  can  be  most  easily  and  certainly  obtained  in  all  the  forms  of 
projection  by  obeying  the  laws  of  physiology  as  well  as  those  of 
optics  and  mechanics. 

Naturally,  most  users  of  projection  apparatus  will  employ  that 
which  is  regularly  manufactured,  but  in  many  institutions  not  all 
of  the  desired  apparatus  can  be  afforded.  Furthermore,  every  one 
who  is  to  do  any  special  work  in  projection  must  be  capable  ot 
combining  and  adapting  apparatus  for  those  special  needs.  Hence, 
we  have  indicated  how  home-made  apparatus  can  be  got  up,  and 
how  apparatus  designed  for  one  purpose  can  be  utilized  for  other 
purposes.  We  have  done  this  for  two  reasons,  first,  because  we 
feel  sure  that  a  great  gain  in  efficiency  can  be  made  in  teaching  by 
the  use  of  the  magic  lantern,  the  projection  microscope  and  other 
forms  of  projection  apparatus,  and  secondly,  because  the  con- 
struction or  adaptation  of  projection  apparatus  gives  one  an 
intimate  and  working  knowledge  which  more  than  pays  for  all  the 
time  and  trouble. 

In  examining  the  apparatus  of  many  different  makers  we  have 
been  impressed  with  the  general  excellence  of  the  apparatus  and 
also  with  certain  general  defects. 

The  defects  seem  to  us  almost  wholly  due  to  the  fact  that  the 
manufacturers  of  apparatus  and  the  users  of  the  same  are  not 
intimately  enough  associated,  and,  therefore,  are  not  so  mutually 
helpful  as  seems  desirable. 


INTRODUCTION  5 

The  manufacturer  naturally  advertises  the  possibilities  of  his 
apparatus  as  if  he  expected  it  to  be  used  under  the  most  favorable 
conditions,  and  operated  by  men  skilled  in  the  use  of  optical  instru- 
ments, and  the  results  to  be  judged  by  persons  of  experience  who 
do  not  expect  the  impossible. 

For  example,  if  one  reads  the  statements  concerning  the  projec- 
tion of  pictures  in  books,  photographs,  postal  cards  and  actual 
objects,  the  impression  would  be  very  strong  that  the  screen  pic- 
tures so  produced  were  every  bit  as  satisfactory  as  those  of  lantern 
slides,  and  just  as  easily  produced.  In  speaking  with  many 
individuals  we  have  found  the  belief  is  very  general  that  with  the 
new  apparatus  nothing  is  simpler  than  to  get  good  screen  images 
of  objects,  pictures,  etc.,  with  all  their  natural  colors,  and  that  the 
expense  of  lantern  slides  can  be  wholly  done  away  with.  But  we 
have  yet  to  find  the  actual  user  of  such  apparatus  who  found  his 
sanguine  expectations  fully  realized. 

Modern  opaque  projection  is  marvelous  in  its  accomplishments, 
but  what  is  gained  in  the  use  of  actual  objects,  books,  etc.,  is  lost  in 
the  relative  dimness  of  the  screen  image,  in  the  expense  and  diffi- 
culty of  managing  the  apparatus,  and  in  the  large  electric  current 
needed  to  give  even  tolerable  screen  images. 

Judging  from  our  observations  the  manufacturers  have  not  fully 
realized  the  lack  of  optical  and  mechanical  knowledge  and  instinct 
in  many  users  of  projection  apparatus.  Naturally,  the  user  of  the 
apparatus  wants  results,  and  he  wants  the  apparatus  to  give  the 
results  without  trouble. 

Perhaps  the  most  striking,  as  also  it  seems  to  us  the  most  easily 
obviated  defect,  is,  that  with  many  parts  of  the  apparatus,  it  is  just 
as  possible  to  insert  them  in  the  wrong  position  as  in  the  right 
position.  For  example,  in  most  of  the  apparatus  we  have  examined 
the  condenser  is  so  mounted  that  it  can  be  put  with  either  end 
facing  the  arc  lamp.  So  with  many  other  parts,  they  can  be  put  in 
a  wrong  position  just  as  easily  as  in  a  right  position. 

In  our  opinion  there  are  five  fundamental  rules  in  the  production 
of  projection  apparatus  that  the  manufacturers  should  follow : 


6  INTRODUCTION 

1.  The  optical  parts  should  be  arranged  on  one  longitudinal 

axis  and  fixed  in  that  position,  except  that  the  projection 
objective  must  be  movable  along  the  axis  for  focusing. 

2.  The  radiant  or  source  of  light  should  be  adjustable  in  every 

direction  to  insure  proper  centering  of  the  light  along  the 
optic  axis,  and  to  insure  the  proper  relative  position  of  the 
source  of  light  and  the  condenser. 

3 .  The  object  carrier  for  lantern  slides  is  preferably  fixed  in  one 

position;  but  the  stage  of  the  microscope  and  the  object 
holder  of  most  other  kinds  of  apparatus  should  be  movable 
along  the  longitudinal  axis  so  that  the  object  can  be  put  in 
the  cone  of  light  where  it  will  be  fully  and  most  brilliantly 
lighted. 

4.  The  parts  should  be  constructed  so  that  either  (a)  it  makes 

no  difference  how  they  are  placed  or  (b)  so  that  they  can- 
not be  put  together  wrong.  (See  footnotes  to  4-5,  p.  7). 

5.  Every  part  of  the  apparatus  should  be  dull  black  to  avoid 

reflections. 

Of  course  for  experimental  apparatus  the  more  adjustable  each 
part  is  the  greater  are  its  possibilities,  but  for  apparatus  to  use  for 
definite  purposes  we  believe  that  no  unnecessary  adjustments 
should  be  possible. 

The  custom  followed  by  many  manufacturers  of  sending  an 
illustrated  pamphlet  giving  instructions  for  installing  and  using 
their  apparatus,  is  wholly  commendable.  In  addition  it  would  be 
advisable  in  some  cases  to  attach  tags  to  the  different  parts,  stating 
their  purpose  and  connections  (fig.  45). 

All  of  the  apparatus  and  all  of  the  experiments  discussed  in  this 
book  have  been  personally  tested  or  observed  by  us  to  make  sure 
that  they  will  work;  and  we  have  tried  to  give  directions  and 
methods  which  are  intelligible,  and  which  will  most  easily  produce 
the  desired  results. 

Finally,  the  authors  of  this  book  most  earnestly  advise  any  one 
who  is  to  use  projection  apparatus  to  go  to  some  place  where  the 
facilities  are  abundant,  and  where  there  is  someone  skillful  in 
using  them.  This  will  give  him  a  standard  of  what  can  be  accom- 


INTRODUCTION  7 

plished  and  what  can  reasonably  be  expected.  The  learner  will 
find  that  in  such  a  place  the  apparatus,  the  room,  the  screen  and 
the  light  are  all  adapted  to  the  purpose  to  be  served. 

Good  projection,  like  any  other  skilled   operation,  requires 
knowledge,  facilities  and  experience. 


There  is  a  very  trenchant  expression  used  in  shops  and  in  laboratories  which 
seems  to  us  to  cover  the  ground.  It  is:  "Fool  Proof." 

From  the  testimony  of  many  who  are  especially  skilled  in  machinery  and  in 
the  use  of  apparatus,  and  from  our  own  personal  experience,  the  "fool  proof" 
construction  of  apparatus  is  not  only  necessary  for  the  careless  and  unskilled, 
but  much  appreciated  by  the  most  skilled  and  careful.  When  one  is  absorbed 
in  the  principles  and  complexities  which  some  experiment  is  meant  to  elucidate, 
it  is  a  great  advantage  to  have  the  apparatus  which  is  to  be  used  so  constructed 
that  it  will  go  together  in  the  right  way  with  the  least  conscious  effort  on  the 
part  of  the  user.  The  user  ought  not  to  be  compelled  to  make  a  special  study 
of  the  apparatus  every  time  it  is  assembled.  It  is  the  business  of  the  manu- 
facturer to  put  thought  into  the  construction  of  the  apparatus,  and  it  is  the 
user's  business  to  work  out  problems  with  it. 

From  time  immemorial  it  has  been  the  habit  of  mankind  to  make  tools, 
implements  and  more  elaborate  apparatus  with  smooth  and  glistening  surfaces, 
bright  colors  often  being  added  to  heighten  the  effect.  The  microscope  and 
other  optical  apparatus  naturally  followed  the  fashion. 

While  to  many  workers  in  optics  there  early  came  the  fundamental  apprecia- 
tion that  the  clearest  images  were  possible  only  when  absolutely  no  light 
reached  the  eye  except  from  the  image  field,  still  polished  brass  and  nickel 
finish  persisted,  and  the  dazzling  reflections  when  bright  lights  were  used,  often 
overwhelmed  the  image  which  it  was  the  sole  purpose  of  the  apparatus  to  make 
visible. 

During  the  last  few  years  the  knowledge  of  the  best  conditions  for  clear 
images  has  asserted  itself  more  and  more,  and  the  mirror  surfaces  of  optical 
apparatus  have  gradually  disappeared.  At  first  the  dull  black  apparatus  was 
prepared  only  for  the  few  who  could  demand  and  pay  for  a  special  finish.  The 
advantage  of  the  dull  finish  of  optical  apparatus  is  so  apparent  when  once  seen 
and  used  that  now  it  is  becoming  very  common. 

The  great  advantage  of  such  dull  black,  non-reflecting  surfaces  for  the  out- 
side as  well  as  for  the  inside  of  optical  instruments  became  apparent  to  the 
senior  author  by  the  accident  of  a  laboratory  fire  (1900)  during  which  the 
lacquer  of  his  best  microscope  was  blackened  by  the  dense  smoke. 

The  ordinary  point  of  view  ten  to  fifteen  years  ago  that  optical  apparatus 
should  of  course  have  a  bright  brass  or  nickel  finish  is  well  illustrated  by  this 
incident:  The  senior  author  was  having,  by  special  contract,  a  microscope 
with  all  its  accessories  made  dull  black.  A  visitor,  interested  in  optical  goods, 
going  through  the  factory  noticed  this  lone,  black  microscope  among  the 
brilliant  array  and  asked:  "When  are  you  going  to  bury  that  one?" 


CHAPTER  I 

THE  MAGIC  LANTERN  WITH  DIRECT  CURRENT  ARC 
LAMP  AND  ITS  USE. 

§  1.    Apparatus  and  Material  for  Chapter  I: 

Suitable  projection  room  with  screen  (Ch.  XII) ;  Magic  lantern 
(§  3-19) ;  Arc  lamp,  automatic  or  hand-feed,  with  fine  adjustments, 
lamp-house  and  wiring  for  current  up  to  25  amperes  (fig.  3); 
Cored  carbons  adapted  to  the  current  (§  7 53 a) ;  Rheostat;  Lantern 
table  (Ch.  XII) ;  Double-pole,  knife  switch  (§  8) ;  Ammeter  (§  7); 
Incandescent  lamp  or  flashlight  (§  14-15);  Gloves  with  asbestos 
patches  (§  27);  Lantern  slides;  Opera-glasses  (§  38);  Testing 
incandescent  lamp  (§  6 1 ,  fig,  21);  Fuses ;  Extra  condenser  lenses  to 
replace  cracked  ones  (§  94);  Screw  driver  and  pliers. 

§  2.    Historical  Summary  and  Works  of  Reference : 

For  a  historical  summary  of  the  invention  and  use  of  the  Magic 
Lantern,  see  the  Appendix. 

The  reader  will  find  many  good  hints  in  the  following  works  on 
Projection.  For  the  full  titles,  see  the  Bibliography. 

R.  C.  Bayley. — Modern  Magic  Lanterns  and  their  Management. 

H.  Fourtier. — La  Pratique  des  Projections. 

Hassack  and  Rosenberg. — Die  Projektionsapparate. 

T.  C.  Hepworth.— The  Book  of  the  Lantern. 

R.  Neuhauss. — Lehrbuch  der  Projektion. 

C.  G.  Norton. — The  Lantern  and  How  to  Use  It. 

F.  P.  Wimmer. — Praxis  der  Makro — und  Mikro-Projektion. 

Lewis  Wright. — Optical  Projection. 

The  latest  information  and  many  useful  hints  may  be  found  in 
the  catalogues  of  the  manufacturers  (see  Appendix). 


MAGIC  LANTERN 

§  3.  The  Magic  Lantern  as  the  standard  for  projection  appara- 
tus.— The  magic  lantern  may  be  taken  as  the  standard  example  of 
projection  apparatus,  for  it  is  in  the  most  common  use  and  is  the 
simplest  instrument  for  image  projection. 


10 


MAGIC  LANTERN  WITH   DIRECT  CURRENT         [Cn.  I 


Condenser 


KS 


FIG.  i.    SIMPLEST  FORM  OF  MODERN  MAGIC  LANTERN  WITH  ARC 

LAMP 


It  consists  of  an  arc  lamp  with  suitable  connections  to  the  current  supply, 
a  rheostat  and  a  table  switch;  a  double  condenser,  lantern-slide  holder  and 
projection  objective. 

Arc  Lamp    This  is  a  mechanism  for  holding  and  feeding  the  carbons. 

h  c    Horizontal  (upper)  and 

v  c     Vertical  (lower)  carbons. 

5  5     Set  screws  for  holding  the  carbons  in  place. 

In  In  Insulation  between  the  carbon  holders,  and  the  rest  of  the  lamp  to 
prevent  a  short  circuit. 

/  5     Feeding  mechanism  for  moving  the  carbons. 

cl    Clamp  for  fixing  the  lamp  in  any  position  on  its  vertical  support. 

SW    Supply  wires  to  the  lamp  socket  or  wall  receptacle. 

So     Lamp  socket. 

K    Key  of  the  socket  switch. 

S — P     Separable  attachment  plug. 

L  W    Supply  wires  from  the  cap  of  the  attachment  plug  to  the  table  switch. 

K  S  Double-pole  knife  switch  on  the  table  for  turning  the  current  off  and 
on  the  arc  lamp. 

Rheostat  for  controlling  the  current.     It  is  inserted  in  one  wire. 

Condenser  In  this  simple  form  it  is  composed  of  two  plano-convex  lenses 
with  the  convexities  facing  each  other. 

i  2     The  two  elements  of  the  condenser. 

L  S    Lantern  slide  close  to  the  plane  face  of  the  2d  condenser  lens. 

Axis  Axis  The  straight  line  passing  from  the  source  of  light  along  the 
optic  axis  of  the  condenser  and  the  objective  to  the  image  screen. 

Objective  The  projection  objective  for  giving  a  clear  image  of  the  lantern 
slide  on  the  screen. 

c  The  center  of  the  objective  where  the  rays  from  the  condenser  should 
cross. 

Image  Screen  The  white  screen  upon  which  the  image  of  the  lantern  slide 
is  projected  by  the  objective. 


If  the  principles  governing  the  magic  lantern  are  mastered,  and 
one  gains  skill  in  handling  it,  the  more  difficult  forms  of  projection 
will  offer  no  great  obstacles. 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  n 

§  4.  Standard  source  of  light. — With  all  forms  of  present  day 
projection  the  direct  current  arc  light  is  taken  as  the  standard 
because,  next  to  the  sun,  it  is  the  most  perfect  light  source  available. 
In  many  places  it  is  to  be  had  during  the  entire  twenty-four  hours, 
and  is  the  safest  and  most  easily  managed  light  capable  of  furnish- 
ing sufficient  illumination  for  use  with  all  kinds  of  apparatus,  from 
the  simplest  magic  lantern  to  the  moving  picture  machine  and  the 
compound  microscope. 

MAGIC  LANTERN  WITH  DIRECT  CURRENT  ARC  LIGHT 

Except  the  projection  table,  the  room  and  screen,  (for  which  see 
§  424  and  Ch.  XII,)  the  essential  elements  of  a  magic  lantern  and 
their  arrangement  are  shown  in  fig.  i,  2,  3.  They  are  as  follows: 

§  5.  Wires  for  the  electric  current. — There  must  be  two  wires 
for  carrying  the  current  extending  from  the  main  line  to  the  electric 
lamp.  One  wire,  the  positive  (+),  conveys  the  current  to  the 
upper  carbon  of  the  lamp,  and  the  other,  the  negative  ( — ),  conveys 
the  current  from  the  lower  carbon  back  to  the  main  line  (fig.  1,2) 
(see  also  Ch.  XIII). 

§  6.  Rheostat. — This  device  must  be  placed  in  the  circuit  along 
either  the  positive  or  the  negative  wire,  it  makes  no  difference 
which.  In  figures  i  and  2  it  is  placed  in  the  negative  wire. 
It  serves  as  a  balance,  and  limits  the  amount  of  current  pas- 
sing through  the  lamp  (§  744-748). 

§  7.  Ammeter. — This  indicates  the  amount  of  current  flowing. 
It  is  not  necessary,  like  the  rheostat,  but  is  very  desirable,  for  with 
the  information  it  gives,  the  operator  can  determine  whether  any 
defects  in  the  brightness  of  the  screen  image  are  due  to  the  lack  of 
current,  or  whether  something  else  is  at  fault  (see  Troubles. 

§  61-95-) 

The  ammeter  is  placed  along  one  wire  the  same  as  the  rheostat 
(fig.  i,  2). 

In  case  no  ammeter  is  used  the  rheostat  can  be  calibrated  and 
marked  when  the  apparatus  is  installed  (see  §  729). 


12  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 

§  8.  Double-pole  switch. — It  is  important  to  have  a  double-pole 
switch  near  the  lamp.  By  its  means  the  operator  can  at  any  time 
turn  the  current  on  or  off  the  lamp.  When  the  switch  is  open  no 
current  can  reach  the  lamp  (fig.  1-3). 

§  9.  Arc  Lamp ;  automatic  type. — The  lamp  is  needed  to  hold 
the  carbons,  and  to  provide  a  mechanism  for  moving  them  toward 
each  other  as  they  burn  away  (see  §  12).  The  lamp  may  be  of  the 
automatic  type  in  which  there  is  a  magnetic  release  or  motor  for  the 
mechanism,  so  that  the  carbons  are  brought  nearer  together  when- 
ever the  arc  gets  too  long.  If  it  is  properly  designed  and  con- 
structed, the  lamp  will  burn  continuously  as  long  as  the  switch  is 
closed,  and  the  carbons  last.  There  should  also  be  a  hand-feed 
mechanism  in  these  arc  lamps,  so  that  slight  modifications  may  be 
made  by  hand  when  necessary ;  furthermore,  there  must  be  arrange- 
ments for  moving  one  or  both  carbons  separately  to  correct  any 
irregularity  in  the  wasting  away  of  the  carbons. 

§  10.  Fine  adjustments. — There  must  be  adjusting  screws  by 
'means  of  which  the  lamp  can  be  slightly  raised  or  lowered,  or  moved 
to  the  right  or  to  the  left,  to  enable  the  operator  to  keep  the  crater 
of  the  positive  carbon  exactly  in  the  axis.  This  is  to  compensate 
for  the  slight  change  in  position  of  the  crater  as  the  carbons  burn 
away  (fig.  3). 

§  11.  Arc  lamp,  hand-feed  type. — In  this  form  of  arc  lamp  the 
operator  must  work  the  mechanism  by  hand.  The  carbons  usually 
have  to  be  moved  nearer  together  every  four  or  five  minutes.  As 
with  the  automatic  type,  one  or  both  carbons  should  be  movable 
independently,  and  there  should  be  fine  adjustments  (§  9,  10). 

§  12.  Carbon  Terminals. — As  a  light  source  for  projection, 
carbon  terminals  or  electrodes  are  used  in  the  arc  lamp.  With  a 
direct  current  the  carbons  burn  away  unequally,  the  upper,  positive 
carbon,  wasting  about  twice  as  fast  as  the  lower,  negative  carbon. 
If  the  carbons  are  of  equal  size  and  quality,  the  mechanism  of  the 
lamp  must  move  the  upper  carbon  about  twice  as  fast  as  the  lower 
one.  Sometimes  a  lamp  with  equal  motion  for  the  upper  and  lower 


CH.  I] 


MAGIC  LATERN  WITH  DIRECT  CURRENT 


Condenser 


HC 


FIG.  2.     MAGIC  LANTERN  WITH  TRIPLE  CONDENSER  AND 
WATER-CELL. 

H  C,  V  C  Horizontal  or  upper  and  vertical  or  lower  carbon  of  an  arc  lamp. 
The  upper  carbon  furnishes  the  light. 

D  -f-  C  Supply  wires  for  the  electric  current.  The  positive  wire  (-}-)  goes 
to  the  upper  carbon  (H  C) ,  and  the  negative  wire  ( — )  comes  from  the  lower 
carbon  (V  C).  The  arrows  indicate  the  direction  of  the  electric  current. 

F    Fuses  where  the  supply  wires  for  the  lamp  connect  with  the  main  line. 

L  Incandescent  lamp  with  wire  guard.  It  is  connected  with  the  supply 
wires  before  the  table  switch  (S)  and  the  resister  (R),  hence  it  can  be  used 
while  the  arc  lamp  is  running  or  when  it  is  turned  off  (See  also  fig.  4). 

S    Double-pole,  knife  switch  for  turning  the  current  on  or  off  the  arc  lamp. 

R     Rheostat  for  controlling  the  current.     It  is  inserted  in  one  wire. 

A  Ammeter  to  indicate  the  amount  of  current  being  used.  It  is  inserted 
in  one  wire. 

Condenser  This  consists  of  a  meniscus  next  the  arc  light,  and  of  two  plano- 
convex lenses  with  a  water-cell  between  them.  The  lenses  must  be  arranged 
as  here  indicated. 

W  Water-cell  placed  between  the  plano-convex  lenses  of  the  condenser. 
It  absorbs  much  of  the  radiant  heat. 

L  S     Lantern  slide  close  to  the  condenser. 

-' Axis  Axis    The  straight  line  pas'sing  from  the  source  of  light  along  the 
optic  axis  of  the  condenser  and  the  objective  to  the  screen. 

Objective  Projection  objective  serving  to  give  a  clear  image  of  the  lantern 
slide  on  the  screen. 

C    Center  of  the  objective  where  the  rays  from  the  condenser  should  cross. 

Screen  Image  The  image  of  the  lantern  slide  formed  by  the  objective  on 
the  white  screen. 


carbons  is  used  and  the  upper  carbon  is  enough  larger  than  the 
lower  one,  so  that  the  two  shorten  at  the  same  rate. 

In  our  experience  it  is  more  satisfactory  to  have  both  carbons 
with  soft  cores,  but  some  advocate  and  use  a  large  soft-cored  carbon 
above  and  a  smaller  solid  carbon  below  (fig.  299). 


14  MAGIC  LANTERN  WITH  DIRECT  CURRENT        [Cn.  I 

§  13.  Lamp-house. — This  is  a  metal  box  in  which  the  arc  lamp 
is  enclosed.  It  should  be  of  good  size,  and  be  well  ventilated  by 
means  of  openings  at  the  bottom,  and  a  flue  at  the  top.  There 
should  be  one  or  more  large  doors,  so  that  the  lamp  can  be  reached 
for  changing  the  carbons  and  making  any  necessary  adjustments. 
Opposite  the  crater  at  the  end  of  the  positive  carbon  there  should 
be  a  window  about  2  to  3  cm.  (2  in.)  square  so  that  the  ends  of  the 
carbons  can  be  observed  when  the  lamp  is  burning  without  opening 
the  door.  This  window  should  be  provided  with  a  combination  of 
red  and  green,  or  red  and  blue  glass,  or  with  smoky  mica  or  with 
deeply  tinted  glass  so  that  the  eyes  will  not  be  injured  when  look- 
ing at  the  crater  (fig.  133,  147). 

§  14.  Incandescent  lamp. — If  experiments  are  to  be  made  it  is 
desirable  to  have  an  incandescent  lamp  with  wire  guard  to  use  in 
connection  with  the  lantern.  It  should  have  a  flexible  cord  of 
sufficient  length  so  that  it  can  be  carried  to  any  desired  position. 
This  lamp  must  be  connected  with  the  supply  wires  before  the 
rheostat  is  inserted;  then  it  will  burn  brightly  while  the  arc  lamp 
is  going.  By  consulting  fig.  2,  it  will  be  seen  that  the  two  wires  for 
this  lamp  are  connected  one  with  each  of  the  supply  wires.  That 
is  the  incandescent  lamp  is  not  connected  with  one  wire  like  the 
rheostat  and  the  ammeter  but  with  both  wires. 

§  15.  Electric  flash-light. — An  electric  flash-light  is  a  great 
convenience  about  a  lantern;  and  is  almost  a  necessity  when  an 
incandescent  light  (fig.  1,2)  is  absent.  It  should  lock,  so  that  it 
will  burn  continuously ;  then  carbons  may  be  changed  by  its  light 
and  other  corrections  made.  It  is  an  absolutely  safe  light  also. 

§  16.  Incandescent  lamp  to  burn  when  the  arc  lamp  is  turned 
off. — To  avoid  the  great  darkness  in  the  room  when  the  arc  lamp  is 
turned  out,  it  is  advantageous  to  have  an  incandescent  lamp  con- 
nected with  the  line,  as  indicated  in  fig.  4. 

§  17.  Condenser.— This  collects  the  light  from  the  arc  lamp 
and  directs  it  through  the  objective.  In  passing  from  the  con- 
denser to  the  objective  it  passes  though  the  lantern  slide  or  other 
object  whose  image  is  to  be  projected  (fig.  i,  24), 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  15 


FIG.  3.     ARC  LAMP  FOR  PROJECTION,  WITH  WIRING,  SWITCHES 
AND  FUSES 


Supply  Wires    The  conductors  from  the  supply  to  the  outlet  box. 

Outlet  Box.  An  iron  box  receiving  the  supply  wires  at  one  end  and  giving 
exit  to  them  from  the  other. 

Fuses  &  Switch  Two  cartridge  fuses  in  the  circuit  and  a  double-pole  knife 
switch  beyond  the  fuses.  The  fuses  are  present  to  avoid  accident  in  case  of  a 
short  circuit  and  the  switch  to  turn  the  current  on  or  off  as  desired. 

P  W  R  Polarized  wall  receptacle.  This  is  composed  of  two  parts  as 
shown,  the  part  on  the  wall  to  receive  the  supply  wires  from  the  outlet  box, 
and  the  cap  to  connect  with  the  table  switch.  The  metal  connections  of  the 
cap  with  the  receptacle  are  in  planes  at  right  angles  so  that  the  cap  can  be  put 
in  place  only  in  one  .way,  hence  the  polarity  is  always  the  same. 

Arc  Supply  The  wires  connecting  the  cap  of  the  wall  receptacle  and  the 
table  switch. 

Switch  The  double-pole,  knife  switch  for  turning  the  current  on  and  off  the 
arc  lamp. 

Wi     The  wire  extending  from  the  switch  to  the  upper  carbon  of  the  arc 


lamp. 

W2 

W3 
lamp. 


The  wire  extending  from  the  switch  to  the  rheostat. 

The  wire  extending  from  the  rheostat  to  the  lower  carbon  of  the  arc 


1 6  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 

Rheostat     This  is  for  controlling  the  current.     It  is  inserted  in  one  wire. 

Arc  Lamp  ^  The  mechanism  for  holding  and  feeding  the  carbons. 

F  S  Feeding  screws  for  moving  the  carbons  closer  together  or  farther 
apart.  The  carbons  can  be  moved  separately  or  both  at  once. 

V  A     Fine  adjustment  screw  for  moving  the  carbons  up  or  down. 

L  A     Fine  adjustment  screw  for  moving  the  carbons  to  the  right  or  left. 
in  in     Insulation  between  the  carbon  holders  and  the  rest  of  the  lamp.     This 
is  to  prevent  the  current  from  leaving  the  carbons  and  making  a  short  circuit 
through  the  metal  part  of  the  lamp. 

5  s     Set  screws  for  holding  the  carbons  in  place. 

Lamp-House  The  metal  box  enclosing  the  arc  lamp.  The  feeding  screws 
(F  S)  and  the  fine  adjustments  (V  A,  L  A). should  project  through  the  wall  of 
the  lamp-house. 

Condenser  A  condenser  composed  of  three  lenses  with  a  water-cell  in  the 
parallel  beam  between  the  plano-convex  lenses. 

1  The  first  element  of  the  triple  condenser  is  composed  of  a  meniscus  lens 
next  the  arc  lamp,  and  a  plano-convex  lens  next  the  water-cell. 

2  The  second  element  of  this  condenser  is  a  plano-convex  lens.     The  con- 
vex surfaces  of  the  plano-convex  lenses  face  each  other  as  in  the  double  con- 
denser (fig.  i). 

Block  i.  The  block  supporting  the  arc  lamp.  It  is  movable  back  and  forth 
along  the  track  on  the  base-board.  The  socket  and  set  screws  permit  the 
adjustment  of  the  lamp. 

Block  2.  The  block  holding  the  condenser.  It  is  movable  along  the  track 
on  the  base-board.  The  socket  and  set  screw  (S)  enable  one  to  adjust  the 
position  of  the  condenser. 

Base  Board  The  board  on  which  all  the  parts  of  the  projection  apparatus 
rest  (see  fig.  158-159)- 

The  condenser  is  of  two  or  of  three  lenses.  If  of  three  lenses  the 
first  lens,  which  is  nearest  the  arc  lamp,  is  usually  of  meniscus  form, 
with  the  concavity  next  the  lamp.  The  second  lens  is  a  plano- 
convex, as  is  also  the  third  (fig.  2).  If  the  condenser  is  of  two 
lenses  both  are  usually  plano-convex  with  the  convex  surfaces  fac- 
ing each  other  and  the  plane  faces  looking  toward  the  radiant  and 
toward  the  lantern  slide  (fig.  i). 

The  two  condensers  appear  alike  in  form  and  relation  of  the 
lenses  except  that  in  the  three-lens  type  a  meniscus  has  been  added. 

In  the  three-lens  type  the  meniscus  and  first  plano-convex 
together  render  the  diverging  light  from  the  lamp  parallel,  and  the 
third  lens  or  second  element  renders  this  parallel  beam  converging, 
bringing  it  to  a  focus  at  the  center  of  the  projection  objective  when 
the  condenser  and  objective  are  properly  proportioned  to  each 
other  (fig.  1-2). 

With  the  two-lens  condenser  the  usual  practice  is  to  bring  the 
condenser  closer  to  the  lamp  than  the  focal  length  of  the  first  lens. 


CH.  I]        MAGIC  LANTERN  WITH   DIRECT  CURRENT 


FIG.  4.     MAGIC  LANTERN  WITH  INCANDESCENT  LAMP  IN  THE  CIRCUIT 
AFTER  THE  RHEOSTAT  (Compare  fig.  2). 

W  W    Supply  wires. 

F     Fuses  in  the  supply  wires  (see  fig.  3). 

R  R     Rheostat  for  controlling  the  current. 

A     Ammeter  for  indicating  the  amount  of  current. 

p  p  The  two  binding  posts  of  the  knife  switch.  The  two  wires  of  the 
incandescent  lamp  are  connected  at  these  points. 

b  s  The  incandescent  bulb  and  the  key  switch  of  the  lamp  socket.  From 
the  connections  of  the  supply  wires  to  the  incandescent  lamp  it  will  shine 
whenever  the  socket  key  is  closed  whether  the  knife  switch  to  the  arc  lamp  is 
opened  or  closed.  When  the  arc  lamp  is  burning  the  incandescent  lamp  will 
be  very  dim  and  when  the  arc  lamp  is  out  it  will  shine  with  full  brilliance. 

51     The  table,  knife  switch, 

L     The  source  of  light. 

The  +  's,  — 's  and  arrows  indicate  the  polarity  and  course  of  the  electric 
current. 

Condenser     A  two-lens  condenser  with  water-cell  (W). 

L  S     Lantern  slide. 

Axis    The  principal  optic  axis  of  the  condenser  and  of  the  objective. 

Objective  The  objective  for  projecting  an  image  of  the  lantern  slide  upon  a 
screen. 

Screen  Image.     The  image  projected  on  the  screen  by  the  objective. 

This  gives  a  somewhat  diverging  beam  between  the  two  lenses. 
The  second  lens  brings  this  diverging  beam  to  a  focus  beyond  its 
own  principal  focus. 

This  condenser  is  sometimes  placed  so  that  the  crater  of  the  arc 
lamp  is  at  the  principal  focus  of  the  first  lens  and  the  center  of  the 
projection  objective  at  the  focus  of  the  second  lens,  as  in  fig.  2. 

Whatever  the  form  of  the  condenser,  the  lenses  must  be  so 
mounted  that  there  is  freedom  for  expansion;  and  they  must  be 
so  arranged  that  the  proper  lens  is  next  the  radiant  (see  fig.  2,3, 
36  B). 


1 8  MAGIC  LANTERN  WITH  DIRECT  CURRENT  [Cn.  I 

§  18.  Water-cell.  This  is  a  vessel  of  water  with  parallel,  glass 
sides,  placed  in  the  beam  of  light  from  the  lamp,  before  the  light 
reaches  the  lantern  slide  or  other  object.  The  water-cell  absorbs 
most  of  the  radiant  heat  from  the  lamp  and  thus  protects  the 
objects  from  over-heating  (fig.  2-3). 

The  water-cell  is  especially  needed  for  opaque  lantern  slides  like 
those  of  dark  scenes  or  colored  slides  made  by  the  Autochrome 
process.  It  sometimes  happens  that  in  an  exhibition  as  many  as 
10  to  30  per  cent,  of  the  slides  are  cracked  by  the  heat,  if  no  water- 
cell  is  used. 

Unfortunately  the  water-cell  is  oftener  absent  than  present  in 
magic  lanterns.  (For  a  further  discussion  of  the  avoidance  of  heat 
see  §  364,  §  854). 

§  19.  Projection  objective. — This  forms  an  image  of  the  lan- 
tern slide  upon  the  screen.  If  the  instrument  is  in  proper  adjust- 
ment the  objective  will  transmit  to  the  screen  the  rays  of  light  from 
the  condenser  which  pass  through  the  lantern  slide  or  other  semi- 
transparent  object.  These  rays  reflected  from  the  screen  to  the 
eye  give  rise  to  a  picture  with  all  the  gradations  of  light  and  shade 
and  color  of  the  lantern  slide  or  other  object  used  (see  fig.  i,  2,  and 
§8n). 

PERFECTION  AND  BRILLIANCY  OF  THE  SCREEN  IMAGE 

§  20.     The  quality  of  the  screen  image  depends  upon : 

1.  The  accurate  centering  along  one  axis  of  the  source  of  light, 

the  condenser,  and  the  projection  objective  (fig.  1,2). 

2.  The  amount  and  intensity  of  the  light  used. 

3 .  The  excellence  of  the  condenser. 

4.  The  aperture  and  perfection  of  the  objective. 

5.  The  proper  proportion  of  the  objective  and  the  condenser  to 

each  other  and  to  the  size  of  the  room.     (See  fig.  i,  2, 
§  634-636). 

6.  The  perfection  and  transparency  of  the  lantern  slides  or 

other  objects  imaged  on  the  screen. 

7.  The  accuracy  of  the  focus  of  the  image  on  the  screen. 

8.  The  reflecting  qualities  of  the  screen  (see  §  621). 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  19 

9.  The  darkness  of  the  projection  room  (see  §608). 
10.  The  proper  adjustment  of  the  eyes  of  the  spectators  to 
either  daylight  or  twilight  vision  (§  281). 

USE  OF  A  MAGIC  LANTERN  FOR  EXHIBITIONS  AND  FOR 
DEMONSTRATIONS 

SUGGESTIONS  TO  THE  LECTURER  OR  DEMONSTRATOR! 

§  21.  Order  of  the  lantern  slides. — The  lecturer  or  derron- 
strator  should  have  his  slides  in  the  exact  order  in  which  they  are 
to  be  shown.  They  should  not  only  be  in  the  exact  order  of  exhibi 
tion,  but  they  should  all  be  in  the  same  relative  position  so  that 
the  operator  can  insert  them  correctly  without  the  trouble  of 
looking  at  them  individually. 

§  22.  Duplication  of  lantern  slides. — It  frequently  happens 
that  the  same  slide,  for  example,  of  a  map  or  some  other  general 
subject,  should  be  shown  at  two  or  more  stages  of  a  lecture.  There 
is  always  difficulty  in  doing  this  unless  the  operator  is  carefully 
instructed,  and  the  slide  is  marked  to  be  repeated,  and  a  slip  of 
paper  inserted  in  the  pile  of  slides  at  the  proper  level.  With  a 
small  audience,  and  for  an  informal  talk  the  difficulty  is,  perhaps, 
not  great;  but  for  a  large  audience  and  anything  like  a  formal 
presentation,  the  repetition  of  the  same  slide  almost  always  causes 
confusion  and  delay. 

To  avoid  this  confusion,  one  can  have  duplicate  lantern  slides. 
Then  the  slides  can  be  put  exactly  in  order,  and  no  confusion  is 
possible. 

If  a  person  has  ever  exhibited  lantern  slides  for  a  friend,  and  one 
or  more  of  the  slides  had  to  be  shown  two  or  three  times,  he  can 
understand  the  troubles  of  the  operator  when  the  same  slide  must 
be  shown  more  than  once,  and  will  agree  that  it  is  better  to  have 
the  slide  duplicated. 

§  23.  Marking  or  "spotting"  lantern  slides. — In  order  that 
lantern  slides  may  be  inserted  in  the  carrier  by  the  operator 
correctly,  and  without  hesitation  or  worry,  the  slides  must  be 
marked  or  "spotted"  in  some  conspicuous  way  (fig.  7,  8,  13). 


20  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 

If  the  slides  are  not  marked,  and  the  correct  position  must  be 
determined  for  each  individual  slide  during  the  exhibition,  even 
the  most  expert  operator  is  liable  to  make  mistakes,  especially 
when  the  slides  are  shown  rapidly. 

§  24.    Inspection  of  the  room  and  lantern  by  the  lecturer. — It 

is  highly  desirable  that  the  lecturer  make  himself  acquainted  with 
the  room  in  which  he  is  to  speak,  and  inspect  the  lantern  himself 
before  the  lecture  hour.  If  the  operator  is  with  him  it  gives 
opportunity  to  establish  pleasant  relations,  and  to  stimulate  the 
operator  to  make  the  best  exhibition  possible.  It  also  gives  oppor- 
tunity and  time  to  make  any  slight  changes  necessary  to  insure  a 
good  exhibition.  Foresight  is  always  more  satisfactory  in  its 
results  than  hindsight. 

§  25.  Directions  for  the  operator. — The  lecturer  should  in- 
struct the  operator  how  he  wishes  the  slides  shown.  There  must 
be  some  signal  for  changing  the  slides.  Preferably  the  signaling 
device  is  some  form  of  electric  signal  on  the  operator's  table,  then 
he  can  see  or  hear  it,  but  the  audience  will  not  be  distracted  by  it, 
as  when  the  lecturer  has  to  speak  to  the  operator,  or  hammer  on 
the  floor  with  the  pointer,  etc.  (For  signaling  devices  see  the  list 
of  apparatus  in  the  appendix). 

The  lecturer  should  direct  the  operator  to  light  the  lantern 
before  the  room  lights  are  extinguished,  and  give  ample  warning. 
The  operator  should  also  be  told  to  leave  the  lantern  burning 
until  the  room  lights  are  turned  on. 

SUGGESTIONS  TO  THE  OPERATOR 

§  26.  Testing  the  lantern. — Before  every  exhibition  or  demon- 
stration the  operator  should  make  sure  that  the  lantern  is  in  good 
working  order.  This  is  only  fair  to  the  speaker  who  depends  upon 
his  illustrations  which  he  has  taken  so  much  trouble  and  expense  to 
prepare.  If  the  slides  are  not  well  shown  it  injures  the  effect  of  the 
lecture  or  demonstration  and  makes  it  difficult  or  impossible  for 
the  speaker  to  make  clear  the  subject  he  is  treating.  It  also  dis- 
quiets the  audience;  and  should  make  the  operator  uncomfortable. 


CH.  I]  MAGIC  LANTERN  WITH  DIRECT  CURRENT  21 

In  testing  the  lantern  the  following  points  should  be  especially 
looked  to : 

(A)  That  there  is  voltage  in  the  supply  line.     This  is  easily 
determined  by  turning  on  the  incandescent  lamp  (fig.  2),  or  by 
trying  to  light  the  arc  lamp. 

(B)  That  the  arc  lamp  is  in  working  order  and  has  carbons  long 
enough  to  last  during  the  exhibition.     By  closing  the  switch  and 
bringing  the  carbons  in  contact  and  slightly  separating  them  the 
arc  light  should  be  established  almost  instantly  (see  also  §  30). 
It  takes  a  certain  amount  of  experience  to  tell  whether  the  carbons 
are  long  enough  to  last  during  the  exhibition.     If  there  is  any 
doubt,  put  a  new  pair  in  position. 

From  the  high  temperature  of  the  carbons,  and  the  lamp  gener- 
ally, after  the  current  has  been  on  some  time,  it  is  not  easy  to  put 
in  new  carbons  in  the  midst  of  a  demonstration.  It  also  makes  an 
embarrassing  break  in  the  exercises  (see  §  27). 

§  27.  Gloves  with  asbestos  patches. — In  spite  of  all  precau- 
tions it  is  sometimes  necessary  to  work  about  the  arc  lamp  after  it 
has  been  running,  and  is  therefore  very  hot.  By  the  use  of  suitable 
pliers  or  tongs  one  can  usually  manage  to  do  the  things  necessary ; 
but  for  certainty  and  rapidity  one  always  needs  to  be  able  to  use 
the  hands  directly.  This  is  rendered  possible  by  the  use  of  gloves 
with  asbestos  patches  in  the  places  which  come  in  direct  contact 
with  the  hot  metal  or  carbons.  The  gauntlet  form  of  gloves  is  best 
for  then  the  wrists  also  are  protected. 

The  asbestos  patches  may  be  of  asbestos  cloth,  or  preferably  of 
quilted  asbestos  paper.  The  asbestos  cloth  is  very  thick  and 
clumsy.  The  asbestos  paper  of  about  half  a  millimeter  thickness 
(Vso  in.)  quilted  between  thin  cotton  or  linen  cloth  answers  well. 
The  quilting  stitches  should  be  long  and  extend  obliquely  in  two 
directions  (fig.  5).  The  object  of  the  quilting  is  to  overcome  the 
weakness  and  easy  tearing  of  the  asbestos  paper. 

For  most  work  a  patch  on  the  thumb  and  index  finger  is  sufficient 
but  as  it  is  often  convenient  to  grasp  a  hot  carbon  between  the 
index  and  middle  finger,  it  is  well  to  have  a  patch  on  the  middle 
finger  also  (fig.  5). 


22  MAGIC  LANTERN  WITH  DIRECT  CURRENT          ICn    1 


FIG.  5.     GLOVES  WITH  ASBESTOS  PATCHES,  PALM  SIDE  UP. 

Left  glove,  p.  i.  m.  The  pollex  or  thumb  (/>),  the  index  or  fore  finger 
(i),  and  the  medius  or  middle  ringer  (m),  have  the  patches  on  the  palmar  sur- 
face and  sides. 

c     Carbon  held  pincer-like  between  the  index  and  medius. 

Right  glove.  The  asbestos  patches  are  as  in  the  left.  Above  the  correspond- 
ing digits  (i,  2,  3)  are  patterns  of  suitable  patches  drawn  to  the  same  scale  as 
the  gloves. 


With  the  hands  protected  by  such  gloves,  one  can  grasp  the  hot 
carbons  within  two  or  three  centimeters  (i  in.)  of  the  hot  tips  with 
entire  safety.  The  asbestos  being  a  non-conductor  of  electricity 
as  well  as  of  heat,  makes  it  safe  also  to  work  about  the  lantern  when 
the  current  is  on  (§  2ya). 


§  27a.  Old  leather  gloves  answer  very  well  if  one  does  not  wish  to  sacri- 
fice a  new  pair.  New  cloth  gloves  with  gauntlets  can  be  had  for  20  cents. 
These  answer  fairly,  but  are  not  so  good  as  the  leather  gloves,  and  there  is 
no  danger  of  the  leather  gloves  being  motheaten  or  catching  fire.  It  is  easier 
to  sew  the  patches  on  the  cloth  gloves,  however. 

Asbestos  mittens  are  to  be  had  of  dealers  in  chemicals  and  chemical 
apparatus.  They  are  of  asbestos  cloth  but  are  so  thick  and  clumsy  that  they 
are  not  adapted  for  working  about  the  lantern. 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  23 

§  28.  See  if  the  lantern  is  centered. — Make  sure  that  the 
different  elements  of  the  lantern  are  centered  along  one  longitudinal 
axis  (fig.  1,2).  Then  and  then  only  will  a  perfect  screen  image  be 
produced.  If  the  apparatus  was  installed  correctly  in  the  begin- 
ning the  only  part  liable  to  be  out  of  line  is  the  crater  of  the  positive 
carbon.  In  burning  the  carbons  frequently  so  wear  away  that  the 
crater  is  at  one  side  of  the  axis.  Slight  decentering  of  the  crater 
can  be  easily  corrected  by  using  the  fine  adjustment  designed  for 
the  purpose  (§  10,  fig.  3,  see  also  Troubles  §  79). 

§  29.  Slide-carrier. — Be  sure  that  the  slide-carrier  works 
properly  and  easily.  The  "push-through"  form  (fig.  6),  is  very 
convenient,  for  while  one  slide  is  on  exhibition  the  one  previously 
shown  can  be  removed  and  another  put  in  place,  and  it  can  be 
instantly  put  in  front  of  the  condenser  when  the  lecturer  signals. 


S2 

S1 

BUJIJ(-j     BOlBGW      6UJQ"Ve~] 

/^7V                  

2 

i  1  iffi*  QJ«  «jp| 

1 

\^^ 

Ki'.«"8'a"« 

—  P  5  ! 

--.=i_--^-2 

FIG.  6.     "PUSH-THROUGH"  OR  DOUBLE  SLIDE-CARRIER. 

1  The  frame  which  remains  in  one  position  in  front  of  the  condenser  and 
serves  as  a  container  and  guide  for  the  "push-through"  part. 

2  The  movable  slide-holder  or  "push-through"  part  of  the  carrier.     It 
moves  easily  to  the  right  and  to  the  left.     It  contains  two  slides  in  the  proper 
position  for  an  erect  image  on  a  vertical  opaque  screen  (see  fig.  7,  8). 

3  3     Notches  in  the  movable  part  to  enable  one  to  grasp  the  slide  easily. 

4  Elevator  serving  to  lift  the  slide  wrhen  at  either  end.     In  some  forms  the 
elevator  lifts  the  entire  slide  from  the  middle  instead  of  tilting  one  end. 

5  6     Inclined  planes  at  each  end.     These  raise  the  elevator  when  the  carrier 
is  moved  to  either  end  of  the  base. 

5  i     Lantern  slide  in  the  carrier  in  front  of  the  condenser. 

S  2  Lantern  slide  in  the  carrier  at  the  left  end.  It  is  of  the  first  magic 
lantern  (1665)  and  is  in  position  to  be  removed  or  to  be  pushed  to  the  right  for 
exhibition. 

§  30.  To  start  the  arc  light. — Turn  on  the  current  by  closing 
the  switch  (fig.  1-4).  If  the  lamp  is  of  the  automatic  type  the 


24  MAGIC  LANTERN  WITH  DIRECT   CURRENT          [Cn.  I 

magnetic  release  will  allow  the  carbons  to  come  in  contact  and 
separate  slightly  so  that  the  arc  will  be  of  the  correct,  length. 

If  the  lamp  is  of  the  hand-feed  type  the  operator  must  start  it 
by  bringing  the  carbons  in  contact  and  then  separating  them  a 
short  distance  (3  to  4  mm.;  J^  in.).  This  is  done  by  turning  the 
feed  screws  by  hand  (fig.  3,  F.  S.). 

§  31.     Managing  the  arc  lamp  during  the  exhibition. — For  an 

automatic  lamp,  the  operator  has  only  to  close  the  switch  to  start, 
and  to  open  the  switch  to  stop  the  lamp.  The  automatic  mechan- 
ism is  supposed  to  keep  the  lamp  burning  in  the  best  manner. 
From  the  uneven  burning  of  the  carbons  it  is  sometimes  necessary 
to  make  slight  adjustments  by  hand  even  with  automatic  lamps. 
This  is  easily  accomplished  by  turning  the  proper  screws  present 
for  the  purpose  (fig.  3,  F.  S.,  L.  A.,  V.  A.). 

For  the  hand-feed  lamp  the  operator  must  bring  the  carbons 
closer  together  every  four  to  five  minutes  or  oftener  by  turning  the 
feed  screws.  If  this  is  not  done  the  distance  between  the  carbons 
soon  becomes  too  great  for  the  current  to  pass,  and  the  lamp  will  go 
out.  Allowing  the  lamp  to  go  out  when  it  should  not  is  one  of 
the  things  for  the  operator  to  avoid. 

§  32.  Amount  of  current  to  use. — This  depends  upon  the  kind 
of  arc  lamp  used  (Ch.  XIII),  the  screen  distance,  and  the  character 
of  the  lantern  slides.  For  dark  lantern  slides  or  long  distances 
more  current  must  be  used  than  for  clear  lantern  slides  and  short 
distances. 

For  a  screen  distance  up  to  10  meters  (33  ft.)  and  a  right-angled 
arc  lamp  (fig.  1-3)  one  will  rarely  need  more  than  12  amperes. 
For  a  screen  distance  of  15  to  25  meters  (50  to  80  ft.),  15  or  at  most 
20  amperes  should  suffice.  If  more  than  20  amperes  are  needed  to 
give  the  proper  brilliancy  to  the  screen  images  something  is  wrong 
with  the  slides,  the  room,  or  the  lantern  itself,  or  more  probably 
with  the  management  of  the  lantern.  (See  under  Ammeter  §  7). 

§  33.  When  to  light  the  lamp. — The  room  should  never  be 
totally  dark  during  an  exhibition.  The  incandescent  lamp  men- 


CH.  I] 


MAGIC  LANTERN  WITH  DIRECT  CURRENT 


tioned  above  (§  16)  will  avoid  this;  and  furthermore,  the  operator 
should  start  the  arc  lamp  before  the  lecturer  turns  off  the  room 
lights  (§  25). 

§  34.  When  to  put  the  lamp  out. — The  operator  should  not  turn 
out  the  arc  lamp  until  the  lecturer  turns  on  the  room  lights.  The 
intervals  of  total  darkness  so  common  in  exhibitions  can  be  avoided 
by  keeping  in  mind  the  suggestions  in  this  and  the  previous  section. 

It  is  also  a  good  plan  for  the  operator  to  remove  the  last  slide 
when  the  lecturer  is  through  with  it,  and  show  a  blank  disc  of  light. 
This  will  inform  the  lecturer  that  all  the  slides  have  been  exhibited 
and  give  him  the  hint  to  turn  on  the  room  lights. 


To  determine  how  a  lantern  slide 
should  be  placed  in  the  carrier  to  give 
an  erect  image  on  the  screen  : 

Look  through  the  lantern  slide  toward 
something  light.  Turn  it  until  the 
picture  is  right  side  up  and  the  print 
reads  right,  as  in  this  model. 

Then  turn  the  slide  so  that  the  bottom 
edge  is  uppermost  like  the  next  model. 


FIG.  7.     STANDARD  AMERICAN   LANTERN  SLIDE,   FULL  SIZE,  WITH 
DIRECTIONS  FOR    INSERTING  IT   IN    THE  CARRIER  so  THAT 
THE  SCREEN  IMAGE  WILL  BE  ERECT. 


26 


MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 


abel 


SB 


U33JOS 


UQ 


*dn 


3pis 


jnq 


}uud  aq;  puB 


Buuq    HIM    siqj, 
UIQJJ    ABMB    SUIDBJ   si 
SUIOBJ  puB  dn    3q 


'duiB[ 


os 


SB 


ui 


uo 


FIG.  8.     STANDARD  AMERICAN  LANTERN  SLIDE,  FULL  SIZE,   WITH 

DIRECTIONS  FOR    MARKING  IT,    AND   INSERTING    IT    IN  THE 

CARRIER  so  THAT  THE  SCREEN  IMAGE  WILL  BE  ERECT. 

§  35.     Correct  position  of  the  lantern  slide  in  the  carrier. — In 

order  that  the  image  on  the  screen  may  be  right  side  up  and  like  the 
original  in  every  way,  the  lantern  slide  must  be  put  into  the  carrier 
in  the  following  manner  to  counterbalance  the  inverting  effect  of 
the  projection  objective  (fig.  i). 

1 .  A  lantern  slide  with  any  printing  upon  it  must  have  the  side 

which  reads  correctly  face  the  lamp,  if  the  screen  is  of 

ordinary  form. 

If  the  screen  is  translucent  like  ground  glass  and  the  picture  is 
viewed  from  the  back  of  the  screen,  then  the  printing  must  face  the 
screen,  not  the  lamp. 

2.  In  all  cases  the  slide  must  be  put  into  the  holder  with  the 

bottom  edge  up  (fig.  6,  8). 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT 
1O  CENTIMETER  RULE 


27 


0 


The  upper  edge  is  in  millimeters,  the  lower  in  centimeters. 

FIG.  9.     SCREEN  IMAGE  OF  A  LANTERN  SLIDE  CORRECTLY  INSERTED 
IN  THE  CARRIER  (Fie.  6-8). 

ui  si  3§pa  jaddn 


0 


H3X3PUXN3O  ot 

FIG.  10.     LANTERN  SLIDE  IMAGE,  WRONG  EDGE  UP. 

OI 


0 


ni  isv/ol  sdi  .sisJamillim  ni  ai  ygbs  laqqu 
FIG.  ii.     LANTERN  SLIDE  IMAGE,  FACING  IN  THE  WRONG  DIRECTION. 
nbb€L  eq&c  12  ID  rojjjiniGrGig'  rpc  JOMGI.  m 


0 


TO 

FIG.  12.     LANTERN  SLIDE  IMAGE,  WRONG  EDGE  UP  AND  FACING  IN 
THE  WRONG  DIRECTION. 

FIGURES  9-10-11-12.     LANTERN  SLIDES  OF  A  METRIC  RULE  FULL  SIZE. 

The  figures  show  the  image  as  it  appears  on  an  opaque  vertical  screen  in  each 
of  the  four  possible  ways  of  inserting  the  slide  in  the  carrier. 

For  a  translucent  screen,  or  when  a  mirror  is  used,  the  slide  in  fig.  1 1  would 
appear  erect. 


28  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 

3.  In  the  slide-changer  of  the  Spencer  Lens  Co.'s  magic  lan- 
terns (Delineascopes) ,  the  slide  is  laid  flat,  with  the  face 
up,  i.  e.,  so  it  will  be  toward  the  condenser  when  ready  for 
projection.  The  edge  which  is  to  be  uppermost  in  the 
ordinary  vertical  carrier,  is  toward  the  screen.  Now  when 
this  slide-changer  is  used  it  turns  the  slide  up  in  the  ver- 
tical position  so  that  it  is  in  precisely  the  same  position  as 
with  the  ordinary  slide  carrier. 

§  36.  Possible  ways  of  inserting  American  lantern  slides  in  the 
slide  -  carrier. — The  standard  American  lantern  slide  is  oblong 
(10x8.2  cm.;  4x3^  in.),  and  the  carriers  are  constructed  to 
receive  them  lengthwise.  While  they  would  never  be  inserted 
with  the  short  edge  up,  they  can  be  inserted  with  either  long  edge 
up,  and  facing  in  either  direction.  This  gives  four  possible  posi- 
tions in  the  carrier,  only  one  of  which  is  correct.  That  is,  there  are 
three  wrong  ways  of  inserting  the  slide  in  the  carrier  with  the 
corresponding  wrong  images  on  the  screen.  It  is  not  very  uncom- 
mon for  an  audience  to  see  all  possible  images  of  the  same  slide,  and 
occasionally  the  wrong  ones  repeated  once  or  twice.  This  is  as 
inexcusable  as  it  is  unnecessary  (fig.  10-12). 

§  37.    Possible  ways  of  inserting  the  square  English  lantern 

slides.— These  slides  are  8.3  x  8.3  cm.  (3^x3^  in.),  and  being 
square  they  may  be  put  into  the  carrier  with  any  of  the  four  edges 
up,  and  of  course  with  either  face  toward  the  lamp.  This  gives 
eight  possible  ways  of  insertion,  seven  of  which  are  wrong.  Square 
slides  must  have  two  "spots,"  (see  fig.  13). 

§  38.  Focusing  the  image  on  the  screen. — When  the  lantern 
slide  is  in  the  correct  position  before  the  condenser  (fig.  1-2)  the 
objective  must  be  at  such  a  distance  from  the  slide  that  the  screen 
image  will  be  sharp,  and  show  clearly  the  printed  matter  and  all 
the  details  of  the  picture.  With  the  usual  magic  lantern  the 
objective  is  nearly  in  the  right  position  all  of  the  time.  But  for 
any  necessary  final  focusing  there  is  a  rack  and  pinion  on  the 
objective,  or  it  is  mounted  in  a  tube  with  spiral  movement.  By 
turning  the  milled  head  of  the  pinion,  or  by  turning  the  objective 


CH.  I] 


MAGIC  LANTERN  WITH   DIRECT  CURRENT 


29 


FIG.  13.     SQUARE  ENGLISH  LANTERN  SLIDE  FULL  SIZE. 

This  figure  shows  the  method  of  "spotting"  or  marking  by  the  English 
Photographic  Club.  That  is,  there  are  two  marks  on  the  upper  front  margin 
of  the  slide.  Two  marks  are  necessary  for  square  slides,  while  a  single  one 
answers  for  oblong  slides. 

The  picture  on  the  lantern  slide  is  of  a  retouching  frame  to  hold  the  slides 
while  being  colored. 


in  its  spiral  casing  the  image  may  be  made  perfectly  sharp,  provided 
that  the  light  is  good  and  the  objective  also  good.  With  an 
imperfectly  corrected  objective  the  margins  of  the  screen  image  are 
liable  to  be  lacking  in  sharpness  although  the  middle  may  be  good. 

It  may  be  necessary  to  focus  slightly  for  each  individual  slide, 
but  ordinarily  if  one  slide  is  in  perfect  focus  those  following  will 
also  give  good  images. 

If  the  screen  distance  is  small  (three  to  five  meters;  10  to  16  feet) 
it  may  be  necessary  to  focus  slightly  for  each  slide  if  the  sharpest 


30  MAGIC  LANTERN  WITH  DIRECT  CURRENT  [Cn.  I 

images  are  desired.  When,  however,  the  screen  distance  is  10 
meters  (30  ft.)  or  over,  it  is  not  usually  necessary  to  focus  for  each 
slide. 

If  the  screen  distance  is  very  great  (20  meters;  65  ft.  or  more) 
the  operator  cannot  tell  by  his  eye  alone  when  the  screen  image  is 
perfectly  sharp.  In  such  a  case  he  must  have  an  assistant  stand 
near  the  screen  to  tell  him  when  the  image  is  sharp,  or  he  .can  use 
good  opera-glasses  and  determine  for  himself. 

When  the  focus  is  once  found  for  these  long  distances  it  is  well 
to  mark  in  some  way  the  exact  position  of  the  objective ;  then  in 
future  the  operator  can  be  sure  of  good  screen  images  in  the  same 
position  provided  the  lantern  has  not  been  moved. 

§  39.    Hints  on  running  the  lantern  for  a  demonstration  lecture. 

— It  frequently  happens  that  in  a  demonstration  lecture,  slides  are 
to  be  shown  at  several  different  times.  Ordinarily  the  arc  lamp  is 
turned  out  during  the  intervals ;  but  to  make  sure  that  the  desired 
slide  can  be  shown  without  delay,  the  arc  lamp  can  be  left  burning 
all  the  time,  and  to  avoid  lighting  the  screen  a  mask  can  be  put  in 
front  of  the  objective  (fig.  14).  A  "push-through"  carrier  (fig.  6) 
should  be  used,  and  the  next  slide  to  be  shown  put  in  one  of  the 
compartments.  The  other  compartment  is  left  vacant,  and  this 
empty  compartment  is  put  in  front  of  the  condenser.  If  the  slide 
were  left  in  position  all  the  time  it  might  become  over  heated  and 
break. 

Whenever  the  slide  is  called  for  it  is  pushed  into  position  and 
the  mask  turned  aside.  This  will  bring  the  picture  on  the  screen 
almost  instantly. 

A  mask  or  shield  for  the  objective  is  much  more  important  for 
the  slow  starting  lights  like  the  Nernst,  than  for  the  arc  (§  146, 
169,  202,  217). 

§  40.  Collecting  and  arranging  the  lantern  slides  at  the  close 
of  an  exhibition. — After  the  exhibition  is  over  be  sure  to  remove 
the  last  lantern  slide  from  the  slide-carrier.  It  not  infrequently 
happens  that  the  last  slide  is  left  in  the  carrier,  and  the  lecturer's 
set  is  thus  rendered  incomplete. 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  31 

It  should  be  a  part  of  regular  routine  to  look  in  the  slide  carrier 
at  the  close  of  every  exhibition  to  make  sure  that  the  last  lantern 
slide  has  been  removed. 


FIG.  14.     SHIELD  FOR  THE  OBJECTIVE  IN  INTER- 
MITTENT PROJECTION  WITH  SLOW- 
LIGHTING  RADIANTS. 

S1     Shield  raised  to  allow  the  light  to  pass  from  the  objective  to  the  screen. 

S    Shield  down  in  front  of  the  objective  to  cut  off  the  light  from  the  screen. 

The  shield  should  be  of  a  concave  form  and  in  front  of  the  objective  a  short 
distance  to  avoid  heating.  It  should  be  made  of  metal  or  asbestos  and  be 
hinged  so  that  it  can  be  easily  turned  up  or  down. 


This  is  also  the  best  time  to  arrange  the  slides  in  the  box  or  a 
pile  exactly  as  they  were  at  the  beginning  of  the  exhibition;  then 
the  set  will  be  ready  for  use  at  the  next  lecture  or  demonstration. 

§  41 .     Lantern  slides  permanently  fixed  in  individual  carriers. — 

Originally  lantern  slides  were  mounted  in  wooden  frames.  Each 
slide  then  had  its  own  carrier,  which  was  inserted  in  a  special 
opening  for  it  next  the  condenser  (fig.  15,  32).  This  method  of 
mounting  slides  still  prevails  for  some  purposes.  If  one  wishes  to 
use  them  in  the  ordinary  lantern  the  common  slide-carrier  (fig.  6) 
is  removed  entirely ;  then  each  slide  in  its  carrier  is  inserted  in 
order  during  the  exhibition.  This  method  of  mounting  is 
admirable  for  a  small  collection  of  slides,  as  the  wooden  frame  pro- 
tects them,  but  for  a  large  collection  they  take  too  much  space  and 
are  too  expensive. 


32  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 


FIG.  15.     LANTERN  SLIDE  IN  PERMANENT  WOODEN 
CARRIER;    ONE-HALF  SIZE. 

1  Face  view  of  the  carrier  and  its  slide. 

2  Sectional  view  of  the  carrier,  showing  the  shelf  on  which  the  slide  rests, 
and  the  wire  spring  above. 

The  slide  is  usually  cut  in  circular  form,  and  fitted  into  a  circular  opening 
in  the  frame.  A  hole  of  the  desired  size  is  first  made  in  the  middle  of  the 
carrier,  but  not  going  clear  through ;  then  a  slightly  smaller  hole  goes  entirely 
through.  This  leaves  a  narrow  shelf  for  supporting  the  slide.  Above  the 
slide  is  placed  a  cover-glass,  and  then  a  wire  spring  to  hold  the  glass  in  position. 


PROJECTION  OF  HORIZONTAL  OBJECTS 

§  42.  The  ordinary  magic  lantern  is  in  a  horizontal  position 
(fig.  i),  but  the  lantern  slide  must  then  be  vertical.  Objects  in 
liquids,  and  some  other  objects  cannot  be  put  in  a  vertical  position, 
hence  the  necessity  of  a  rearrangement  of  the  lantern  parts  so  that 
the  object  may  be  placed  horizontally.  This  is  accomplished  by 
placing  the  second  or  terminal  part  of  the  condenser,  in  a  horizontal 
position,  and  the  projection  objective  is  made  vertical.  By  means 
of  a  plane  mirror  in  the  path  of  the  beam  of  light  from  the  first  part 
of  the  condenser,  the  light  is  reflected  vertically  upward.  The 
object  is  placed  horizontally  just  above  the  second  element  of  the 
condenser.  The  vertical  projection  objective  would  give  a  picture 


CH.  I]          MAGIC  LANTERN  WITH  DIRECT  CURRENT  33 

on  the  ceiling  above,  but  by  means  of  another  mirror  at  45  degrees 
or  a  prism  this  vertically  directed  light  is  reflected  horizontally  to 
the  ordinary  vertical  screen  (fig.  16,  §  42a).  (For  projection  with 
the  vertical  microscope  see  §  397). 


FIG.  1 6.     ARRANGEMENT  OF  THE  MAGIC  LANTERN  FOR  HORIZONTAL 
OBJECTS. 

(Cut  loaned  by  C.  H.  Stoelting  Co.}. 

Commencing  at  the  left  the  parts  are: 
L     Hand-feed  lamp  with  right-angled  carbons. 
H    Lamp-house  cut  away  to  show  the  lamp  within. 
i  2     Adjusting  screws  to  move  the  carbons, 
j  4     Screws  for  centering  the  crater. 

5     Adjusting  screw  for  moving  the  lamp  toward  and  from  the  condenser. 
C    The  plano-convex  lens  of  the  condenser  next  the  radiant.     It  here  gives 
a  parallel  beam. 

T    Water-cell  in  the  path  of  the  parallel  beam. 


§42a. 


>a.  In  England  and  America  this  is  often  called  vertical  projection  from 
the  position  of  the  objective;  in  Germany  it  is  called  horizontal  projection 
from  the  position  of  the  object. 


34  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [CH.  I 

Mt     45  degree  mirror  to  reflect  the  parallel  beam  vertically. 

C3  Second  element  of  the  condenser  in  a  horizontal  position.  The  lantern 
slide  is  put  just  above  it. 

O     Projection  objective  in  a  vertical  position  for  opaque  projection. 

M  45  degree  mirror  above  the  objective  to  reflect  the  light  horizontally 
to  the  screen. 

G     Vertical  support  for  the  condenser,  objective  and  mirror. 

E     Lantern  front  holding  the  objective. 

§Set  screws  for  holding  the  objective  in  position  when  once  centered. 
Mirror  in  horizontal  position.     When  raised  45°  it  serves  to  reflect  the 
horizontal  beam  down  upon  an  opaque  object. 

C2  Second  element  of  the  condenser  used  in  projection  with  the  microscope 
or  lantern  objective  with  the  object  in  the  ordinary  vertical  position. 

S    Opening  for  the  lantern  slide  carrier. 

D^        Objective  and  its  holder. 

0     Projection  objective  for  lantern  slides. 

FFF    Supports  of  the  condenser,  etc. 

N    Platform  on  which  opaque  objects  are  placed. 

Bx,  2     Legs  or  supports  of  the  prismatic  rod  serving  as  an  optical  bench. 

PROJECTION  WITH  MULTIPLE  LANTERNS 

In  the  period  before  the  common  use  of  the  moving  picture 
machine,  when  the  pictorial  effect  was  dependent  wholly  on  the 
magic  lantern,  two  and  even  more  lanterns  were  run  simultaneously 
i.  e.,  both  were  going  all  the  time. 

§  43.     Composition  of  multiple  lanterns.— 

1 .  Each  lantern  must  be  complete  in  itself. 

2.  The  size  of  image  of  each  lantern  must  be  exactly  the  same. 

3 .  The  lanterns  must  be  so  placed  and  so  inclined  toward  each 

other  that  the  light  discs  on  the  screen  exactly  coincide. 
They  are  now  usually  placed  one  above  the  other  (fig.  17). 

§  44.  Wiring  for  multiple  lanterns. — Each  lantern  must  have 
its  own  electric  lamp.  When  the  supply  is  no  volts  or  less  each 
lamp  must  be  separately  wired,  and  each  lamp  must  also  have  its 
own  rheostat  and  double-pole  knife  switch  (fig.  2,3). 

In  case  the  supply  is  220  volts,  each  lamp  may  be  separately 
wired  as  just  described;  or  both  lamps  may  be  put  in  series,  i.  e., 
along  one  wire,  on  one  system  of  wiring,  and  use  but  a  single 
rheostat. 

§  45.  Use  of  multiple  lanterns. — By  the  use  of  two  lanterns 
there  is  not  shown  first  one  slide  and  then  another  simply,  but  one 


CH.  I]  MAGIC  LANTERN  WITH  DIRECT  CURRENT  35 

slide  seems  to  melt  into  the  other,  hence  the  name  "dissolving 
views."  This  is  brought  about  by  a  shutter  gradually  uncovering 
one  objective  and  at  the  same  time  obscuring  the  other;  or,  as  in 
the  figure  here  shown  (fig.  17),  by  the  closing  of  the  iris  diaphragm 
of  one  objective  while  the  other  opens. 


FIG.  17.     MULTIPLE  LANTERN  FOR  DISSOLVING 
VIEWS. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Company). 

Each  lantern  must  have  its  own  arc  lamp  and  rheostat.  For  dissolving  one 
picture  into  another  the  iris  diaphragm  of  one  objective  is  opened  gradually 
while  the  other  is  gradually  closed.  This  is  accomplished  by  pulling  up  or 
down  on  the  rod  connecting  the  two  iris  diaphragms  in  the  objectives. 

Some  lecture  rooms  are  supplied  with  double  lanterns,  not  so 
much  for  the  dissolving  effect,  as  for  the  rapid  passage  from  one 
slide  to  another.  In  most  cases  the  "push -through"  carrier  with  a 
single  lantern  will  accomplish  this  as  effectively. 

§  46.  Multiple  lanterns  for  "effects." — Formerly  certain 
"effects"  or  striking  appearances  were  produced  by  the  use  of  two 
or  more  lanterns  which  were  in  operation  and  projected  their  light 


36  MAGIC  LANTERN  WITH  DIRECT  CURRENT          [Cn.  I 

upon  the  screen  at  the  same  time.  For  example,  to  show  falling 
snow,  in  one  of  the  lanterns  is  a  slide  showing  a  landscape,  city 
street,  etc..  in  another  is  a  black  band  with  irregular  perforations 
of  minute  size  which  give  the  appearance  of  snow-flakes.  If  now 
the  light  in  the  lanterns  is  properly  regulated,  and  the  black 
perforated  band  is  moved  up  over  the  face  of  the  condenser, 
the  snow-flakes  will  appear  to  fall  either  gently  or  rapidly  in  the 
landscape  or  street  as  one  moves  the  band  slowly  or  rapidly.  One 
can  give  the  appearance  of  a  driving  storm  by  tilting  the  black 
band,  for  this  will  make  the  flakes  seem  to  fall  obliquely. 

For  rain  effects  the  black  band  should  have  slit-like  perforations. 

MOVING  SLIDES  FOR  SINGLE  LANTERNS 

§  47.  "Effects"  with  single  lanterns. — The  appearance  of 
movement  may  also  be  produced  in  a  single  lantern.  For  this  two 
slides  must  be  superposed,  and  one  moved  over  the  other.  By 
this  means  various  combinations  of  designs  may  be  made,  and  also 
appearances  of  relative  movement.  Here,  naturally,  the  two 
slides  must  be  close  together,  or  one  will  be  too  much  out  of  focus. 
Special  slide  carriers  are  constructed  for  showing  these  single- 
lantern  "effects." 

For  simple  experiments  use  a  single  slide-carrier.  The  slides 
should  have  no  cover-glass,  but  may  be  varnished.  Then  one 
slide  is  put  in  place  as  for  an  ordinary  exhibition,  and  another  is 
inserted  over  it  and  pushed  by  the  fingers  into  different  positions 
to  show  various  combinations.  For  this  experiment  the  bellows 
between  the  slide-carrier  and  the  objective  should  be  removed  to 
give  freedom  to  the  hands  in  making  the  various  changes  necessary. 

§  48.  "Slip-slides"  for  optical  deceptions. — Slides  with  lines 
at  various  angles,  etc.,  are  used  to  demonstrate  these.  The  lines 
can  be  shown  separately,  and  then  by  pushing  one  slide  over  the 
other  one  can  get  various  combinations.  For  suggestions  as  to 
slides  the  reader  is  referred  to  works  on  physiology  and  experi- 
mental psychology  under  "optical  deceptions." 

§  49.  Most  of  the  "effects"  produced  by  the  movement  of  two 
slides  over  each  other,  and  the  use  of  multiple  lanterns  are  so  far 


CH.  I]  STEREOSCOPIC  SCREEN  IMAGES  37 

exceeded  in  every  way  by  the  moving  picture  that  it  is  hardly  worth 
while  to  go  to  the  trouble  to  get  together  the  apparatus  and  slides 
to  show  these  small  "effects"  when  such  wonderful  ones  are  shown 
daily  in  every  moving  picture  theater. 

The  moving  picture  was  originally  invented  to  illustrate  scientific 
facts;  and  the  indications  now  are  that  it  is  to  become  a  great 
factor  in  education  by  its  striking  portrayal  of  the  processes  of 
nature.  (SeeCh.  XI). 

STEREOSCOPIC  SCREEN  IMAGES 

§  50.  For  a  stereoscopic  screen  image  the  same  fundamental 
law  must  be  observed  as  for  any  other  stereoscopic  effect.  That 
is,  there  must  be  two  slightly  different  images  corresponding  with 
the  image  seen  by  the  left  eye  and  that  seen  by  the  right  eye. 
These  images  must  be  projected  on  the  screen  so  that  they  nearly 
coincide,  then  by  some  means  the  left  eye  sees  its  left-eye  image, 
but  not  the  right-eye  image;  and  the  right  eye  sees  the  right-eye 
image,  but  not  the  left-eye  image.  The  two  images  are  then 
combined  in  the  brain  and  the  stereoscopic  effect  follows  as  with 
ordinary  naked  eye  binocular  vision  or  when  using  a  stereoscope. 

With  the  magic  lantern  this  effect  has  been  produced  in  three 
principal  ways: 

(1)  By  the  aid  of  prism  spectacles. — Lantern  slides  of  a  stereo- 
scopic pair  are  projected  on  the  screen  so  that  they  nearly  coincide 
by  the  use  of  two  lanterns.     When  this  is  done  some  people  can 
get  the  stereoscopic  effect  by  looking  at  the  pictures  with  the  naked 
eye,  but  for  most  people  it  is  necessary  to  look  through  prism 
spectacles  so  that  the  right  eye  shall  see  only  one  image  and  the 
left  eye  only  one. 

(2)  By  the  aid  of  polarized  light  and  Nicol-prism  spectacles. — 
According  to  this  method  two  lanterns  are  used  and  two  lantern 
slides,  making  a  stereoscopic  pair.     For  one  lantern  there  is  used  a 
Nicol-prism  or  a  glass  pile  and  the  projection  is  made  with  the 
ordinary  polarized  light.     A  similar  prism  or  pile  is  used  for  the 
other  lantern,  but  the  extraordinary  polarized  light  is  used  for 
projecting  its  image.     These  two  images  are  projected  so  that  they 


38  STEREOSCOPIC  SCREEN   IMAGES  [Cn.  I 

nearly  coincide  upon  the  screen.  The  screen  is  covered  with  silver 
foil  to  prevent  the  depolarization  of  the  reflected  light.  Now  to 
look  at  the  screen  image  and  to  make  it  possible  for  each  eye  to  see 
only  its  own  image,  the  observer  must  wear  polarizing  or  analyzing 
spectacles  with  the  prisms  or  piles  corresponding  with  the  one 
supplying  the  light  for  its  own  image.  For  example,  if  the  right 
eye  image  is  made  by  extraordinary  polarized  light,  then  the  right 
eye  of  the  observer  must  have  its  prism  spectacle  so  that  it  trans- 
mits the  extraordinary  polarized  light,  but  extinguishes  the  ordinary 
polarized  light  which  produces  the  left  eye  image.  And  the  left  eye 
must  have  its  prism  so  that  it  will  receive  the  polarized  light  from 
its  image,  but  extinguishes  that  from  the  right  eye  image.  Each 
eye  then  sees  its  own  image,  but  not  the  one  for  the  other  eye,  and 
the  conditions  for  stereoscopic  vision  are  fulfilled. 

(3)     The  two-color  method. — For  this  method  two  complementary 
colors  are  selected — usually  red  and  green. 

(A)  With  two  lanterns  there  are  projected  the  two  images  of  a 
stereoscopic  pair  so  that  they  nearly  coincide.     There  is  put  some- 
where in  the  path  of  the  beam  of  one  lantern  a  plate  of  red  glass  and 
in  that  of  the  other  lantern  a  plate  of  green  glass.     The  observer 
must  have  spectacles  or  viewing  glasses  of  corresponding  colors. 
Then  with  one  eye  he  sees  the  red  image  and  with  the  other  the 
green  image.     The  combination  of  these  colored  images  by  the 
brain  gives  a  stereoscopic  image  in  black  and  white. 

(B)  With  a  single  lantern  the  two-color  stereoscopic  effect  can 
be  produced  as  follows:     The  two  pictures  of  a  stereoscopic  pair 
are  printed  by  one  of  the  color  processes  so  that  one  is  a  red  picture 
and  one  a  green  picture.     These  two  are  placed  together  so  that 
they  nearly   coincide,   then  they  are  projected  by  one  lantern. 
With  the  naked  eye  the  pictures  look  like  any  two-color  picture 
where  the  colors  do  not  register,  and  such  a  screen  picture  is  any- 
thing but  satisfactory ;   but  now  if  spectacles  or  viewing  glasses  of 
corresponding  colors  are  held  before  the  eyes,  one  eye  sees  the  green 
picture  and  one  eye  the  red  picture  and  the  stereoscopic  effect 
comes  out  very  strikingly. 

The  simplest  way  to  determine  which  color  to  put  in  front  of  the 
right  and  which  in  front  of  the  left  eye  is  to  try  first  one  color  then 


CH.  I]  CENTERING  THE  MAGIC  LANTERN  39 

the  other.  In  general  it  will  be  found  that  if  the  red  parts  are  at 
the  right  then  the  red  glass  must  be  over  the  right  eye  and  similarly 
for  the  green.  Presumably  if  one  used  the  wrong  color  then  there 
should  be  a  pseudoscopic  effect,  convex  objects  looking  concave, 
etc. ;  but  this  effect  is  difficult  to  obtain. 

It  is  seen  that  in  all  these  methods  the  observer  must  be  supplied 
with  some  means  by  which  only  one  of  the  projected  images  is  seen 
by  one  eye,  the  other  by  the  other  eye.  Stereoscopic  projection  is 
necessarily,  therefore,  expensive. 

For  most  people  any  good  lantern  slide  shows  perspective  and 
relief  sufficiently. 

CENTERING  THE   VARIOUS   PARTS   OF  THE   LANTERN  AND 
SEPARATING  THEM  THE  PROPER  DISTANCE 

§  51.  Centering. — By  this  is  meant  the  arrangement  of  the 
source  of  light,  the  condenser  and  the  projection  objective  so  that 
the  source  of  light,  and  the  principal  optic  axis  of  the  condenser  and 
of  the  objective  shall  be  in  one  straight  line,  and  each  lens  be 
perpendicular  to  that  straight  line  (fig.  1-4). 

When  the  different  elements  are  once  centered  along  one  straight 
line  the  objective  and  the  condenser  should  be  fixed  in  position  so 
that  they  cannot  be  raised  or  lowered  or  turned  sidewise.  If  the 
source  of  light  gets  slightly  out  of  center  by  the  burning  of  the 
carbons,  it  may  be  recentered  by  bringing  the  carbons  nearer 
together  or  by  regulating  the  position  by  the  fine  adjustments  of 
the  lamp. 

In  the  right-angled  arc  lamp  the  upper  carbon,  which  furnishes 
the  light,  is  constantly  in  the  optic  axis.  With  oblique  carbons 
(fig.  39)  the  source  of  light  constantly  shifts  with  the  burning  away 
of  the  carbons ;  and  with  the  direct  current  lamp  the  source  of  light 
gradually  rises  above  the  axis.  With  the  alternating  current  and 
V-arranged  carbons  one  source  shifts  above  and  one  below  the  axis, 
or  one  to  the  right  and  one  to  the  left  depending  upon  the  arrange- 
ment of  the  V.  In  centering  the  lamp  one  should  start  with  the 
carbons  in  contact  and  take  the  point  of  contact  to  center  from. 


40  CENTERING  THE  MAGIC  LANTERN  [Cn.  I 

Remember  that  one  should  never  change  the  position  of  the 
condenser  or  of  the  objective  to  compensate  for  the  lack  of  center- 
ing of  the  source  of  light. 

§  52.  Mechanical  method  of  centering. — This  is  the  method 
most  satisfactory  for  both  manufacturer  and  user  in  getting  the 
various  parts  properly  aligned. 

Generally  some  form  of  track  (optical  bench)  is  used  on  which 
the  various  parts  are  placed  and  along  which  they  can  slide.  The 
straight  line  or  axis  to  which  all  parts  are  to  be  centered  is  at  a 
selected,  definite  position  above  the  base-board  or  table  supporting 
the  track  (fig.  3,  40). 

The  first  thing,  then,  is  to  decide  upon  the  distance  the  axis  is  to 
be  above  the  base-board  or  table. 

For  all  work  upon  centering,  the  bellows  between  the  condenser 
and  the  objective  should  be  removed  so  that  the  faces  of  all  parts 
can  be  seen. 

The  position  of  the  common  axis  may  be  determined  by  some 
part  of  the  apparatus,  such  as  the  condenser.  Or  one  can  decide 
upon  some  convenient  level  which  will  give  sufficient  room  for  the 
arc  lamp  and  its  carbons,  and  then  adjust  all  parts  to  this  level.  A 
good  way  to  get  all  at  the  proper  height  is  to  make  a  measure  or 
gauge  of  wood  just  the  height  of  the  axis.  If  this  is  a  board  which 
just  fits  between  the  tracks,  and  has  a  peg  indicating  the  middle 
point  between  the  tracks  it  will  help  to  get  the  parts  perpendicular 
to  the  axis  as  well  as  at  the  right  level.  If  the  wooden  gauge  is 
carefully  made  it  will  enable  one  to  center  the  parts  to  within  one 
or  two  mm.  (VIG  to  V24  inch).  Very  slight  variations  from  perfect 
mechanical  centering  can  be  compensated  for  by  using  the  fine 
adjustment  screws  of  the  arc  lamp. 

§  53.  Getting  the  center  of  the  lens  faces. — This  can  be  done 
by  using  a  rule  in  millimeters  or  Vieth's  inch.  Or  it  can  be  done  by 
pressing  some  white  paper  against  the  lens  face  and  creasing  it 
around  the  edges  with  the  finger.  The  center  of  this  circle  of  paper 
can  then  be  found  as  shown  in  fig.  18.  If  the  center  is  marked  and 
the  paper  then  put  over  the  lens  face  one  will  have  a  guide  to  center 
by. 


CH.  I] 


CENTERING  THE  MAGIC  LANTERN 


FIG.   1 8.     FIGURE  SHOWING  HOW  TO 
FIND  THE  CENTER  OF  A  CIRCLE. 

Draw  two  chords  (ch  ch)  and  erect  perpendiculars  at  their  middle  points. 
Where  these  perpendiculars  cross  is  the  center  of  the  circle  (C). 

As  stated  above,  when  once  centered,  the  objective  and  con- 
denser should  be  fixed  in  position. 

§  54.  Avoidance  of  obliquity. — Not  only  must  all  the  parts  be 
at  the  same  level  and  in  one  straight  line,  but  the  lenses  must  be 
perpendicular  to  that  straight  line  and  not  oblique.  Then  the 
straight  line  or  common  axis  passing  from  the  crater  of  the  upper 
carbon  to  the  screen  will  coincide  with  the  principal  axis  of  the 
condenser  and  the  projection  objective,  and  the  arrangement  for 
perfect  projection  will  be  realized  (fig.  1-4,  26). 

One  can  usually  tell  when  the  parts  are  in  line  and  not  oblique  by 
sighting  along  them  with  the  eye,  or  by  the  use  of  a  straight  edge 
like  a  T-square.  To  make  sure  by  measurement  one  can  put  the 
optical  bench  or  base-board  (fig.  158,  1 59),  on  a  level  table  and  next 
a  smooth  wall.  Then  by  measuring  horizontally  the  central  points 
can  be  determined  exactly  as  their  height  was  determined  (§52). 

CORRECT  DISTANCE  APART  OF  THE  DIFFERENT  ELEMENTS 

§  55.  Radiant  and  condenser. — With  the  three-lens  condenser 
the  radiant  is  at  the  right  distance  when  it  is  at  the  principal  focus 
of  the  first  element  of  the  condenser  (fig.  2).  This  will  give  a 


CENTERING  THE  MAGIC  LANTERN 


[CH.  I 


FIG.  19.    CONCENTRIC  CIRCLES  ON  THE 

FACE  OF  THE  CONDENSER,  SHOWING 

THE  SIZE  OF  THE  CIRCLE  OF  LlGHT 

WITH  VARIOUS  POSITIONS  OF 

THE  RADIANT. 

When  the  radiant  is  at  the  proper  distance,  the  entire  face  of  the  condenser 
is  illuminated  (l). 

As  the  radiant  and  condenser  are  separated  the  part  illuminated  becomes 
smaller  and  smaller  (2-4).  (See  also  fig.  20). 

cylinder  of  approximately  parallel  rays  between  the  two  elements 
of  the  condenser,  and  will  fully  light  the  face  of  the  second  element. 
One  can  determine  this  easily  by  putting  a  sheet  of  white  paper 
over  the  face  of  the  condenser  which  is  toward  the  objective.  If 
the  radiant  is  in  the  right  place  the  entire  face  will  be  light.  If  the 
radiant  is  too  far  off,  only  a  part  of  the  face  will  be  illuminated 
(fig.  19).  If  the  radiant  is  too  close  the  face  will  be  lighted, 
but  the  light  will  be  diverging  between  the  condenser  lenses. 
In  this  case  a  part  of  the  light  falls  outside  the  second  element  and 
is  lost.  There  is  liable  also  to  be  a  defective  screen  image  (fig.  28) . 
One  can  get  the  condenser  at  the  right  distance  from  the  lamp  by 
first  separating  the  lamp  and  condenser  a  considerable  distance  and 
then  gradually  bringing  them  closer  and  closer  together  until  the 
condenser  face  is  just  filled  with  light.  Sometimes  the  radiant  is 
put  nearer  than  the  principal  focal  distance  on  purpose,  so  as  to 
correct  in  part  for  the  lack  of  proper  proportion  between  the  con- 
denser and  the  objective  (§  56). 


CH.  I]  CENTERING  THE  MAGIC  -LANTERN  43 

With  the  two-lens  condenser  used  for  lantern  slides  the  lamp  is 
usually  closer  than  the  principal  focal  distance  of  the  first  lens,  this 
makes  the  beam  between  the  lenses  diverging,  hence  it  is  best  to 
have  the  two  lenses  as  close  together  as  possible  to  avoid  loss  of 
light  (fig.  i). 

With  this  condenser  and  diverging  light  between  the  lenses  the 
only  rule  that  can  be  given  is  to  adjust  the  distance  between  the 
lamp  and  the  condenser  until  the  best  light  is  obtained  on  the 
screen.  If  this  brings  the  crater  of  the  arc  lamp  within  8  to  10  cm. 
(4  in.)  of  the  first  lens,  then  it  will  be  necessary  to  substitute  longer 
focus  lenses  for  either  the  first  or  the  second  condenser  lens  or  for 
both.  In  general,  the  first  lens  should  be  of  about  15  cm.  (6  in.) 
focus  and  the  second  lens  should  have  a  somewhat  shorter  focal 
length  than  the  projection  objective.  For  example,  if  the  projec- 
tion objective  is  of  38  cm.  (15  in.)  focus,  the  second  lens  of  the  con- 
denser in  the  two-lens  form  should  be  of  about  25—30  cm.  (10—12  in.) 
focus.  This  will  bring  the  diverging  cone  to  a  focus  near  the  center 

of  the  objective. 
• 

§  56.  Condenser  and  projection  objective. — If  the  projection 
objective  and  the  condenser  are  properly  proportioned  the  conden- 
ser will  focus  the  light  near  the  center  of  the  projection  objective 
when  the  lantern  slide  is  in  focus  on  the  screen  (fig.  1,2). 

If  the  condenser  is  of  so  short  a  focus  that  the  light  from  the 
condenser  comes  to  a  focus  before  reaching  the  objective  the  field  is 
restricted  and  bordered  by  a  red  margin  (fig.  29). 

If,  on  the  other  hand,  the  condenser  is  of  too  long  a  focus  for  the 
objective  the  light  will  not  come  to  a  focus  by  the  time  it  reaches 
the  center  of  the  objective  (fig.  28).  In  this  case  the  field  will  be 
restricted  and  bordered  by  blue. 

OPTICAL  TEST  FOR  CENTERING 

§  57.     Optical  test  for  centering  the  radiant  and  the  condenser. 

— If  these  are  properly  centered  along  one  line,  and  the  two  are 
separated  a  considerable  distance  when  the  lamp  is  burning,  the 
light  spot  on  the  face  of  the  condenser  looking  toward  the  objective 


CENTERING  THE  MAGIC  LANTERN 


[CH.  I 


FIG.  20  A.  CONDENSER  FACE  WITH  THE  SPOT  OF  LIGHT  IN  THE  MIDDLE, 
SHOWING  THAT  THE  LAMP  AND  CONDENSER  ARE  ON  THE  SAME  Axis. 

FIG.  20  B.  CONDENSER  FACE  WITH  THE  SPOT  OF  LIGHT  OUT  OF  THE 

CENTER.  THIS  SHOWS  THAT  THE  CONDENSER  AND  LAMP  ARE  NOT  ON 

ONE  Axis. 

To  get  the  appearance  here  shown  the  lamp  must  be  pulled  back  considerably 
beyond  the  principal  focus  of  the  first  element  of  the  condenser. 

will  appear  in  the  center  (fig.  20).  This  can  be  easily  seen  by  hold- 
ing a  piece  of  paper  against  the  condenser  face.  In  case  the  two 
are  not  properly  aligned,  the  white  spot  on  the  paper  will  appear 
outside  the  center,  at  the  right  or  left,  above  or  below.  On  account 
of  the  inverting  effect  of  lenses  the  arc  light  will  be  too  far  from  the 
center  in  just  the  opposite  direction  from  the  spot  of  light.  For 
example,  in  figure  2oB  the  light  spot  is  too  far  to  the  left, 
consequently  the  crater  of  the  positive  carbon  must  be  too  far  to 
the  right.  One  should  change  it  to  the  left  by  the  adjusting  screws 
until  the  circle  of  light  appears  exactly  in  the  middle  (fig.  20 A). 

§  58.  Optical  test  for  centering  the  condenser  and  the  objec- 
tive.— After  the  condenser  and  the  radiant  are  properly  centered, 
and  the  radiant  put  at  the  principal  focus  of  the  condenser  one  can 
tell  whether  the  objective  is  on  the  same  axis  by  looking  at  both 
ends  of  the  objective  when  it  is  at  the  proper  distance  from  the 
condenser  (fig.  1-2). 

If  the  objective  is  in  line  with  the  lamp  and  the  condenser  the 
spot  of  light  from  the  condenser  can  be  seen  in  the  middle  of  the 


CH.  I]  CENTERING  THE  MAGIC  LANTERN  45 

first  lens  of  the  objective.  The  light  should  strike  the  middle  of 
the  first  lens  and  leave  through  the  middle  of  the  last  lens  of  the 
objective  (fig.  i). 

If  it  is  not  centered  the  cone  of  light  will  strike  at  one  side  of  the 
center  and  leave  at  one  side.  If  it  is  greatly  out  of  center  the  cone 
of  light  may  fall  wholly  outside  the  objective;  this  frequently 
occurs  in  micro-projection. 

To  center  the  objective  it  should  be  moved  up  or  down,  to  the 
right  or  to  the  left,  until  the  cone  of  light  strikes  it  exactly  in  the 
center  and  leaves  the  center.  No  change  of  the  lamp  or  the  con- 
denser should  be  made,  for  that  would  spoil  the  centering  of  those 
two  elements.  After  the  objective  is  centered,  it  should  be  fixed 
firmly  in  position.  Any  slight  variation  from  the  center  by  the 
irregular  burning  of  the  carbons,  can  be  corrected  by  the  fine 
adjusting  screws  of  the  lamp  (fig.  3,  L.  A. ;  V.  A.). 

CENTERING  THE  OBJECTIVE  IN  A  VERTICAL  POSITION 

§  59.  When  the  objective  must  be  made  vertical  in  projecting 
horizontal  objects,  the  radiant  and  the  condenser  should  first  be 
centered  as  described  above  (§  55).  Then  the  second  element  of 
the  condenser  should  be  removed  and  placed  in  a  horizontal  posi- 
tion with  the  convex  face  downward,  and  the  flat  face  upward 
toward  the  objective.  A  plane  mirror  at  45  degrees  is  placed  in 
the  path  of  the  beam  of  light  from  the  first  element  of  the  con- 
denser. The  light  will  be  directed  vertically  upward.  The  hori- 
zontal condenser  lens  must  be  moved  until  it  receives  this  vertical 
cylinder  of  light  and  continues  the  central  or  axial  ray  in  a  vertical 
direction.  One  can  tell  when  this  is  the  condition  by  pulling  the 
arc  lamp  back  from  the  condenser  until  a  small  circle  of  light 
appears  on  the  horizontal  condenser  lens  (fig.  2oA,  B.).  If  it  is 
centered  the  spot  of  light  will  be  in  the  middle.  If  it  is  not  in  the 
middle  move  the  upper  lens  until  it  is,  but  do  not  change  the  posi- 
tion of  the  lamp.  When  the  horizontal  lens  is  centered,  move  the 
arc  lamp  up  toward  the  condenser  until  the  horizontal  lens  is  rilled 
with  light  (§  55). 


46  TROUBLES  WITH  THE  MAGIC  LANTERN  [Cn.  I 

§  60.  Centering  the  vertical  objective. — After  the  horizontally 
placed  condenser  lens  is  centered  the  objective  is  placed  in  a  vertical 
position  over  it  and  moved  sidewise  until  the  cone  of  light  enters 
the  middle  of  the  first  face  and  leaves  the  middle  of  the  last  face  of 
the  objective.  One  proceeds  exactly  as  for  centering  it  in  the 
horizontal  position  (§  55,  58).  Just  over  the  objective  is  placed  a 
45  degree  mirror  silvered  on  the  face,  or  a  right-angled  prism,  to 
direct  the  vertical  rays  horizontally  to  the  screen  (fig.  16).  The 
lower  mirror  may  be  an  ordinary  glass  mirror  silvered  on  the  back, 
but  the  mirror  over  the  objective  must  be  silvered  on  the  face  to 
avoid  a  duplication  of  the  image. 

TROUBLES:    HOW    TO    AVOID    AND    HOW    TO 
OVERCOME  THEM 

THE  LAMP  CANNOT  BE  STARTED 

§  61.  This  may  be  because  there  is  no  voltage  in  the  main  line. 
The  presence  of  current  is  easily  determined  by  using  the  testing 
incandescent  lamp  (fig.  21).  An  incandescent  lamp  in  the  circuit 
as  shown  in  fig.  2  or  4  will  show  whether  the  current  extends  to  the 
lamp  switch. 


FIG.  21.     TESTING  INCANDESCENT  LAMP. 

W^  W2     The  two  supply  wires  for  the  lamp. 

For  this  testing  lamp  a  socket  without  key  switch  is  best.  It  is  also  wise  to 
have  the  lamp  protected  by  a  wire  guard.  The  wires  at  Wj  W2  should  be 
exposed  only  a  short  distance  as  shown. 

To  test  with  the  lamp  put  the  naked  ends  of  the  wires  Wt  W2  upon  metallic 
parts  of  the  circuit  to  be  tested  being  sure  to  make  contact  with  both  conduc- 
tors of  the  circuit.  For  example,  the  two  wires  or  the  two  blades  of  a  knife 
switch,  etc.  If  there  is  voltage  in  the  line  at  that  point  the  lamp  will  light  up. 

§  62.  The  connections  in  the  arc  lamp  may  not  be  good,  that  is, 
the  set  screws  holding  the  connecting  wires  may  have  become 


CH.  I]  TROUBLES  WITH  THE  MAGIC  LANTERN  47 

loosened,  or  a  wire  may  have  become  wholly  separated  from  its 
connections. 

§  63.  A  fuse  may  have  burned  out  somewhere  along  the  line. 
Commencing  with  the  fuse  nearest  the  lamp,  take  each  fuse  out  and 
examine  it.  Use  the  testing  incandescent  lamp  also. 

§  64.  A  fuse  plug  may  not  be  screwed  in  tightly  enough  to  make 
good  contact.  Occasionally  some  one  puts  a  piece  of  paper  or  wood 
in  the  fuse  socket,  thus  preventing  metallic  contact.  Such 
obstructions  should  be  looked  for  and  removed ;  then  the  fuse  plug 
can  be  made  to  produce  metallic  contact. 

§  65.  The  switches  may  not  be  properly  closed,  and  hence  the 
circuit  is  not  complete. 

§  66.  The  carbons  may  be  so  short  that  they  cannot  be  brought 
in  contact,  and  thus  the  circuit  cannot  be  completed.  Put  in  new 
ones. 

§  67.  The  range  of  the  lamp  movement  may  be  at  its  limit,  so 
that  the  carbons  cannot  be  approximated.  This  must  be  corrected 
by  turning  the  screws  back  and  then  setting  the  carbons  by  hand, 
if  long  enough,  or  by  putting  in  new  carbons. 

§  68.  If  one  uses  an  automatic  arc  lamp,  it  may  be  that  the 
mechanism  does  not  work.  Before  looking  elsewhere  for  the 
trouble,  one  should  try  the  hand-feed  device  present  in  all  auto- 
matic lamps  and  make  sure  that  the  carbons  are  brought  in  con- 
tact and  then  slightly  separated  to  establish  the  arc. 

§  69.  Of  course,  if  one  uses  a  hand-feed  lamp  it  will  not  start 
until  one  brings  the  carbons  in  contact  by  the  proper  device  for  the 
purpose.  As  soon -as  the  carbons  touch  there  will  be  a  flash  of 
light;  then  the  carbons  should  be  slightly  separated. 

§  70.  There  may  be  a  short  circuit  in  the  lamp  itself  due  to  a 
burning  out  of  the  insulation.  This  may  be  detected  by  opening 
the  double-pole  knife  switch  slowly.  If  there  is  a  big  spark  when 
the  switch  finally  opens,  a  short  circuit  in  the  lamp  is  strongly 
indicated. 


48       TROUBLES  WITH  THE  MAGIC  LANTERN      lCn.  I 

Unless  one  has  considerable  knowledge  of  arc  lamps  it  is  advis- 
able to  get  an  electrician  to  repair  the  lamp. 

Short  circuiting  in  the  lamp  is  a  rare  trouble  and  less  liable  to 
occur  than  almost  anything  else. 

GOING  OUT  OF  THE  LAMP 

§  71.     This  may  be  due  to  the  stopping  of  the  dynamo. 

§  72.     A  fuse  may  burn  out  somewhere  along  the  line. 

§  73.  Some  connection  may  burn  out  or  one  or  both  wires  may 
be  disconnected. 

§  74.  The  carbons  may  have  burned  off  so  that  the  interval 
between  the  ends  is  too  great  for  the  current  to  pass.  This  is  a 
very  common  cause,  and  is,  of  course,  easily  remedied  by  the  use 
of  the  feeding  screws  of  the  lamp  to  bring  them  closer  together. 
If  the  carbons  are  so  short  that  they  cannot  be  brought  together, 
new  carbons  must  be  inserted.  Always  open  the  table  switch 
before  putting  in  new  carbons. 

Sometimes  the  screw  holding  the  lower  carbon  is  not  set  up 
enough  and  the  carbon  falls  down.  If  this  is  the  trouble  open  the 
table  switch  and  replace  the  lower  carbon  in  its  proper  position  and 
tighten  well  the  set  screw  holding  it. 

Always  look  at  the  carbons  first  in  case  the  lamp  goes  out 
unexpectedly  (see  also  above  §  66-67,  7°  and  a^  the  causes  for  no 
current  §  61-70). 

NOT  ENOUGH  CURRENT 

§  75.     There  may  not  be  enough  in  the  line. 

§  76.  The  line  may  be  grounded.  Test  for  this  with  the  testing 
incandescent  by  touching  one  of  the  terminal  wires  of  the  incan- 
descent to  some  metal  object  connected  with  the  ground,  like  the 
metal  tube  enclosing  the  wires,  a  water  or  gas  pipe  or  radiator,  and 
the  other  to  one  of  the  exposed  metal  parts  of  the  conductors,  first 
on  one  side  and  then  on  the  other.  If  there  is  a  connection  of 
either  wire  with  the  ground  the  testing  lamp  will  light  when  its  two 
wires  are  connected,  one  with  the  radiator,  etc.,  and  the  other  with 


CH.  I] 


TROUBLES   WITH  THE  MAGIC  LANTERN 


49 


the  line  wire  which  is  not  grounded.  In  some  cases  one  wire  is 
purposely  grounded.  In  such  cases  great  care  must  be  taken  not 
to  ground  the  other  wire  (see  also  fig,  266-267  §  689). 

§  77.  There  may  be  too  much  resistance  in  the  circuit.  Open 
the  rheostat  wider,  if  it  is  adjustable  (fig.  281),  keeping  an  eye  on 
the  ammeter  to  see  when  the  current  is  of  the  desired  amperage. 


FIG.  22.     INCLINED   AND  VERTICAL   CARBONS 
IN  THE  CORRECT  RELATIVE 
POSITION. 

The  upper  carbon  is  positive  and  supplies  the  light  in  both  cases. 


FIG.  23.     CARBONS  IN  THE  CORRECT  RELATIVE  POSITION  FOR  BOTH 
DIRECT  AND  ALTERNATING  CURRENTS. 

A     Inclined  carbons  in  the  correct  position  for  alternating  current. 

B     Inclined  carbons  in  the  correct  position  for  direct  current. 

C  Carbons  at  right  angles  in  the  correct  position  for  either  direct  or 
alternating  current.  Direct  current  is  indicated. 

D  Carbons  arranged  in  a  V-shaped  position.  For  this  position  alternating 
current  only  is  employed;  and  the  crater  on  each  carbon  contributes  to  the 
light.  The  V  may  be  either  in  a  vertical  or  in  a  horizontal  plane.  The  ver- 
tical arrangement  is  the  more  common. 


TROUBLES  WITH  THE  MAGIC  LANTERN  lCn.  I 


FIG.  24.     CARBONS  IN  BAD  POSITION;    THE  UPPER  CARBON  CUTTING 

OFF  THE  LIGHT  FROM  THE  UPPER  PART  OF  THE  CONDENSER,  AND 

HENCE  CASTING  A  SHADOW  ON  THE  LOWER  PART  OF  THE  SCREEN. 

A  Carbons  at  an  inclination  of  about  25  degrees,  with  the  upper  or  positive 
carbon  too  far  forward. 

B     Carbons  at  right  angles,  with  the  upper  carbon  too  far  forward. 

S  Screen  image  of  the  condenser  face.  As  the  upper  carbon  is  in  the  way, 
the  upper  part  of  the  condenser  is  partly  in  shadow,  and  hence  the  screen  image 
will  be  shaded  on  its  lower  part  due  to  the  inverting  action  of  the  objective. 


FIG.  25.     CARBONS  IN  BAD  RELATIVE  POSITION,  THE  LOWER  OR  NEGATIVE 
CARBON  EXTENDING  UP  IN  FRONT  OF  THE  POSITIVE  CARBON. 

A     Carbons  at  right  angles,  with  the  lower  carbon  too  high. 

B  Both  carbons  vertical,  but  the  lower  or  negative  one  standing  in  front 
of  the  upper  one. 

S  Screen  image  of  the  condenser  face.  As  the  condenser  is  not  well  lighted 
on  its  lower  part  due  to  the  shading  action  by  the  lower  carbon,  the  screen  image 
will  be  shaded  correspondingly  on  its  upper  part  due  to  the  inverting  action  of 
the  objective. 


CH.  I]  TROUBLES  WITH  THE  MAGIC  LANTERN  51 

IRREGULAR  OR  INSUFFICIENT  LIGHT  ON  THE  SCREEN 

§  78.  There  may  be  an  insufficient  current  flowing  through  the 
lamp.  Consult  the  ammeter  (§  7,  75-77). 

§  79.  Improper  relative  position  of  the  carbons. — Look  at  them 
occasionally  through  the  window  in  the  lamp  house.  They  should 
be  in  the  relative  position  shown  in  fig.  23 .  If  they  are  in  a  wrong 
position  (fig.  24,  25),  one  cannot  expect  to  get  a  good  screen  light. 

It  sometimes  happens  that  one  or  both  of  the  carbons  has  no  soft 
core,  although  the  hole  in  the  carbon  is  present.  In  such  a  case  the 
crater  is  liable  to  jump  around  as  with  a  solid  carbon.  Easily 
corrected  by  substituting  a  properly  cored  carbon. 

§  80.  Wrong  polarity  of  the  supply  wires. — As  stated  above 
(§5)  the  positive  supply  wire  should  be  connected  with  the  lamp 
so  that  the  current  passes  along  the  upper  carbon  and  from  its  tip 
over  to  the  lower  carbon,  whence  by  means  of  the  negative  wire,  it 
passes  back  to  the  generator  or  dynamo.  In  case  the  wires  were 
reversed  in  position,  the  lower  carbon  would  be  positive  and  the 
bright  crater  would  be  on  it.  This  would  give  a  poor  light,  for  the 
crater  would  not  face  the  condenser,  and  as  this  carbon  would  burn 
away  more  rapidly  than  the  upper  carbon  the  upper  one  would  soon 
be  in  the  position  shown  in  fig.  246.  There  would  then  be  a 
double  reason  for  a  poor  screen  image,  viz.,  the  crater  would  not 
face  the  condenser,  and  the  upper  carbon  would  act  as  a  shield  to 
cut  the  light  off  the  condenser.  To  determine  whether  the  wires 
are  connected  to  the  lamp  properly,  insert  carbons,  turn  on  the 
current,  and  let  the  lamp  burn  a  minute  or  two.  Then  turn  off 
the  lamp  and  watch  the  hot  ends  of  the  carbons.  The  positive  one 
will  remain  red  hot  longest.  (See  also  Ch.  XIII,  §  701-703  for 
determining  the  polarity).  In  case  the  lower  carbon  remains 
glowing  longer  than  the  upper,  the  polarity  is  wrong  (fig.  271). 

Open  the  switch  and  remove  both  wires  from  their  binding  posts 
and  insert  them  in  the  reverse  position.  Then  repeat  the  experi- 
ment and  the  upper  carbon  should  remain  glowing  longest. 

After  one  has  had  some  experience  it  is  easy  to  tell  whether  or  not 
the  wires  are  properly  connected  by  watching  the  carbons  through 


52  TROUBLES  WITH  THE  MAGIC  LANTERN  [Cn.  I 

the  lamp-house  window  when  the  lamp  is  burning.  The -upper 
carbon  should  always  be  considerably  brighter  than  the  lower  one. 
When  one  has  found  the  correct  polarity  it  is  wise  to  mark  the 
positive  wire  red  and  the  negative  wire  black.  It  is  also  a  good 
plan  to  mark  the  positive  switch  connections  plus  with  red  and  the 
negative  connections  minus  with  black.  But  one  must  not  forget 
that  the  polarity  is  liable  to  be  changed  by  the  changing  of  the 
wires  in  the  main  line  when  repairs  are  made,  so  one  must  be  on  the 
alert  to  detect  polarity  change. 

§  81.  Non-registering  of  the  direct  current  ammeter. — In  first 
installing  an  ammeter  if  the  hand  does  not  register  on  the  dial  when 
the  current  is  turned  on  and,  the  arc  lamp  started,  either  the 
instrument  is  out  of  order,  or  more  likely  the  wires  are  wrongly  con- 
nected. Remember  that  the  ammeter  must  be  inserted  in  one 
wire,  then  if  it  does  not  register  when  the  lamp  is  burning  the  wires 
were  inserted  wrong.  Turn  off  the  current  and  reverse  the  wires 
in  the  binding  posts  of  the  ammeter.  If  now  the  wires  are  properly 
connected  both  to  the  ammeter  and  the  arc  lamp,  the  polarity  in 
both  will  be  changed  by  a  change  in  polarity  in  the  main  line,  and 
the  wires  must  be  changed  around  in  the  binding  posts  in  the 
ammeter  and  in  the  arc  lamp  to  get  the  polarity  correct  in  both. 
As  the  lamp  and  the  ammeter  are  wholly  independent  instruments, 
the  polarity  may  be  correct  in  both  or  wrong  in  both,  or  correct  in 
one  and  wrong  in  the  other.  (See  also  Ch.  XIII,  yo2a  for  ammeter 
which  can  be  used  with  both  alternating  and  direct  current) . 

DEFECTIVE  OPTICAL  RESULTS 

§  82.  There  may  be  direct  light  falling  on  the  screen  from  some 
window  or  some  lighted  lamp  in  the  room.  This  will  make  the  disc 
of  light,  or  the  lantern  picture  on  that  part  of  the  screen  receiving 
the  adventitious  light,  look  faded  or  gray  instead  of  brilliant.  It 
will  look  as  if  that  part  of  the  screen  were  not  so  brilliantly  illumin- 
ated, when,  in  fact,  more  light  may  be  falling  on  it.  To  be  effec- 
tive the  light  must  reach  the  screen  from  the  lantern  and  from  no 
other  source. 


CH.  I] 


TROUBLES  WITH  THE  MAGIC  LANTERN 


S3 


SHADOWS  AND  RESTRICTION  IN  THE  Disc  OF  LIGHT  ON  THE  SCREEN 
§  83.  The  radiant,  i.  e.,  the  crater  of  the  upper  carbon  (fig.  27) 
may  be  outside  the  main  axis  (above,  below,  to  the  right  or  to  the 
left  of  it).  If  sufficiently  outside  the  center  there  will  be  only  an 
elliptical  light  area  present.  On  the  side  toward  which  the  crater 
is  displaced  there  will  be  a  blue  crescent  or  spot,  and  on  the  oppo- 
site side  a  dark  crescent,  bordered,  in  extreme  cases,  by  red. 
Remedy:  get  the  crater  back  in  the  axis. 

§  84.    The  condenser  may  be  out  of  center. — This  will  give  the 
same  defective  light  on  the  screen  as  when  the  light  source  is  off 


FIG.  26  (A).     DIAGRAM  OF  A  MAGIC  LANTERN  AND  A  SCREEN  IMAGE  WHEN 
ALL  THE  PARTS  ARE  IN  CORRECT  PROPORTION  AND  ON  ONE  Axis. 

Axis  The  common  axis  passing  from  the  radiant  along  the  principal  axis 
of  the  condenser  and  the  objective  to  the  screen. 

C  Condenser  of  three  lenses,  the  first  element  (LJ  composed  of  a  meniscus 
and  a  plano-convex;  the  second  element  (L2),  is  a  plano-convex.  The  con- 
vex surfaces  face  each  other  as  usual. 

F    Principal  focal  distance  of  the  condenser. 

0     Projection  objective. 

R     Radiant  giving  the  light. 

5    The  screen  fully  and  perfectly  lighted. 

FIG.  27  (B).     DIAGRAM  SHOWING  THE  EFFECT  OF  HAVING  THE  RADIANT 
BELOW  THE  Axis. 

There  appears  a  blue  shadow  on  the  lower  part  of  the  screen  (5). 

Whenever  the  radiant  is  off  the  axis  the  dark  blue  shadow  will  be  on  the 
corresponding  side  of  the  screen.  In  this  'case  the  radiant  would  have  to  be 
raised  to  get  rid  of  the  shadow.  If  the  shadow  were  on  the  left  it  would  be 
necessary  to  move  the  radiant  to  the  right  and  so  on. 


54 


TROUBLES  WITH  THE  MAGIC  LANTERN  lCn    1 


B 


FIG.  28  (A).     DIAGRAM  SHOWING  THE  EFFECT  ON  THE  SCREEN  IMAGE 
WHEN  THE  RADIANT  is  TOO  NEAR  THE  CONDENSER. 

In  this  case  the  conjugate  focus  of  the  condenser  (/)  is  considerably  farther 
off,  i.  e.,  beyond  the  objective,  the  screen  image  is  made  smaller,  and  the  light 
disc  on  the  screen  is  bordered  with  blue.  With  some  condensers  there  is  a 
dark  or  blue  disc  in  the  center  also.  (Lettering  as  in  fig.  26). 

.  FIG.  29  (B).     DIAGRAM  SHOWING  THE  EFFECT  ON  THE  SCREEN  IMAGE 

WHEN  THE  RADIANT  is  BEYOND  THE  PRINCIPAL  Focus 

OF  THE  CONDENSER. 

This  brings  the  conjugate  focus  of  the  condenser  (/)  nearer  the  condenser, 
and  in  this  case  just  before  the  light  reaches  the  objective.  It  narrows  the 
screen  image  and  the  light  disc  is  bordered  with  red.  (Lettering  as  in  fig.  26). 

the  axis,  but  the  blue  spot  or  disc  will  be  on  the  side  away  from 
which  the  condenser  is  displaced,  being  just  the  reverse  of  the 
position  when  the  light  source  is  off  the  axis  (§  83). 

If  the  condenser  is  too  high  the  blue  spot  or  disc  will  be  on  the 
lower  part  of  the  screen;  and  if  the  condenser  is  too  low  the  blue 
edge  will  appear  on  the  upper  part  of  the  screen;  if  to  the  right  the 
blue  disc  will  be  at  the  left,  etc.  That  is  the  condenser  inverts  the 
position  (fig.  27). 

The  condenser  should  be  correctly  centered  once  for  all  and 
firmly  fixed  in  position  so  that  it  need  never  be  changed. 

§  85.    The  projection  objective  may  be  off  the  main  axis. — The 

effect  will  be  the  same  as  when  the  source  of  light  is  off  the  axis. 
This  is  due  to  the  fact  that  while  the  condenser  inverts  the  rays, 


CH.  I]  TROUBLES  WITH  THE  MAGIC  LANTERN  55 

they  are  re-inverted  or  erected  by  the  objective.  If  the  condenser 
and  source  of  light  are  on  one  axis  and  the  objective  off  that  axis, 
it  must  be  recentered;  but  as  stated  above  (§  54)  when  the  objec- 
tive and  condenser  are  once  centered  they  should  be  fixed  in  posi- 
tion, then  the  only  element  of  the  lantern  to  become  decentered  is 
the  crater  of  the  arc  lamp,  i.  e.,  the  source  of  light.  The  fine 
adjustment  screws  of  the  lamp  will  enable  one  to  center  the  light. 
By  limiting  the  changes  to  one  element,  viz.,  the  source  of  light, 
corrections  can  be  made  quickly  and  accurately.  If  one  tries  to 
get  the  light  centered  by  changing  two  or  all  three  of  the  elements  it 
leads  only  to  chaos. 

§  86.  For  the  effects  of  spherical  aberration  and  for  a  ghost,  a 
white  or  black  spot  in  the  center  of  the  field,  see  also  §  828. 

§  87.  The  radiant  (i.  e.,  crater  of  the  upper  carbon)  may  be  too 
close  to  the  condenser.  This  will  give  a  restricted  field  with  a  blue 
margin  or  there  may  be  a  blue  circle  in  the  center  of  the  disc  (fig. 
29,  30). 

§  88.  The  radiant  may  be  too  far  from  the  condenser.  This 
will  produce  a  restricted  screen  disc  with  the  edge  bordered  with 
red  (fig.  29).  It  is  easily  corrected  by  bringing  the  radiant  and 
condenser  closer  together. 

§  89.  The  condenser  may  be  of  too  short  focus,  so  that  the 
light  comes  to  a  focus  before  reaching  the  objective  when  the 
lantern  slide  is  in  focus  (see  §  56,  fig.  29,  30).  Correct  the  defect 
by  using  a  lens  of  longer  focus  for  the  second  element  of  the  con- 
denser. It  may  be  less  satisfactorily  compensated  for  by  putting 
the  radiant  nearer  the  condenser. 

§  90.  The  condenser  may  be  of  too  long  a  focus  (see  §  56, 
fig.  28) .  Correct  by  using  a  shorter  focus  condenser.  It  may  also 
be  compensated  for  in  part  by  removing  the  radiant  farther  from 
the  condenser,  but  this  lessens  the  available  light. 

§  91.  There  may  be  dirt,  mist  or  opacities  on  some  of  the  glass 
surfaces.  This  is  easily  remedied  by  cleaning  the  glass. 


TROUBLES  WITH  THE  MAGIC  LANTERN 


[Co,  I 


FIG.  30.  ARRANGEMENT  AND  CENTERING  OF  THE  RADIANT. 

(From  the  catalogue  of  Fuess). 

(j)     The  Radiant,  i.  e.,  the  crater  is  too  far  to  the  right. 
(2)     The  crater  is  too  far  to  the  left, 
(j)     The  crater  is  too  high. 
(4)     The  crater  is  too  low. 

(5}     The  crater  is  too  far  from  the  lamp  condenser. 
(6-7}     The  crater  is  too  near  the  condenser. 
(8}     The  crater  is  in  the  correct  position. 

One  of  the  condenser  lenses  may  be  cracked.  If  a  new  lens  can- 
not be  inserted,  but  the  cracked  one  must  be  used  at  the  time, 
rotate  it  around  until  the  crack  is  least  noticeable. 

There  may  be  strings  or  wires  hanging  down  in  the  path  of  the 
beam  of  light.  They  will  give  sharp  shadows  on  the  screen. 
Remove  them. 

§  92.  Defective  or  too  opaque  lantern  slide. — The  lantern 
slides  may  be  cracked,  producing  a  dark  streak  through  the  picture. 
There  may  be  dirt  or  mist  on  one  or  more  of  the  glass  surfaces. 

The  slide  may  be  too  opaque.  There  is  a  tendency  to  make 
lantern  slides  so  opaque  that  only  the  most  powerful  radiants  can 
give  anything  like  satisfactory  screen  images.  This  is  a  great 
mistake.  Lantern  slides  properly  made  are  very  transparent  and 
show  all  the  delicate  shading,  from  the  densest  to  pure  transparency 
(clear  glass).  Probably  99  slides  are  too  dense  where  one  is  not 
dense  enough.  The  opacity  of  the  slides  made  by  the  autochrome 


CH.  I]  TROUBLES  WITH  THE  MAGIC  LANTERN  57 

or  starch  process  is  one  of  their  great  drawbacks.     Only  powerful 
radiants  give  satisfactory  screen  images. 

§  93.  Shadow  on  the  screen  with  water-cell.  In  case  the  water 
in  the  water  cell  has  evaporated  in  part  there  will  be  a  very  dis- 
agreeable shadow  on  the  lower  part  of  the  screen  (fig.  31).  It  is  on 
the  lower  part  of  the  screen  although  it  is  the  upper  part  of  the  water 
cell  that  will  be  empty.  This  is  due  to  the  inverting  action  of  the 
objective. 


FIG.  31.     SHADOW  ON  THE  LOWER  PART  OF  THE  SCREEN  WHEN  THE 
WATER-CELL  is  BUT  PARTLY  FILLED. 

S  Screen  image  with  shadow  on  the  lower  side.  The  water  is  of  course 
present  in  the  lower  part  of  the  water  cell,  and  absent  from  the  upper  part; 
but,  owing  to  the  inversion  produced  by  the  objective,  the  screen  image  shows 
the  shadow  on  the  lower  part. 

Occasionally  the  water  is  entirely  absent  from  the  water-cell. 
Then  there  will  be  a  very  poor  screen  image,  the  entire  screen  being 
affected  by  the  obscurities  on  the  dry  surfaces  of  the  water-cell. 

BREAKING  OF  CONDENSER  LENSES 

§  94.  It  is  usually  the  lens  next  the  radiant  that  crocks  or 
becomes  shattered.  This  is  due  to  the  too  rapid  heating  or  cooling 
of  the  condenser  lens,  or  to  the  mounting,  which  may  be  too  rigid 
to  allow  of  free  expansion  of  the  lens  as  it  becomes  hot. 

Condenser  lenses  are  especially  liable  to  break:  (i)  When  too 
heavy  currents  are  used;  (2)  when  the  lamp-house  is  not  well  and 
evenly  ventilated;  (3)  when  currents  of  cold  air  strike  the  hot 
condenser;  (4)  when  the  lens  mounting  is  not  provided  with 
ventilating  openings  for  free  circulation  of  air  between  the  lenses ; 


5.8  TROUBLES  WITH  THE  MAGIC  LANTERN  [Cn.  I 

(5)  when  the  lens  next  the  radiant  is  of  such  a  focus  that  the 
lamp  must  be  put  very  close  to  it. 

§  95.  Unequal  heating. — Breakage  often  occurs  from  unequal 
heating  of  the  lens.  This  is  perhaps  as  common  with  large  flame 
sources  such  as  the  kerosene  flame,  the  alco-radiant  or  Welsbach 
mantle  gas  flame  as  with  the  electric  arc.  With  the  electric  arc, 
if  the  crater  is  too  close  to  the  lens  the  thick  central  part  of  the  lens 
expands  rapidly  before  the  edge  is  heated  enough  to  expand  with 
the  middle  part.  Separating  the  lamp  and  condenser  somewhat, 
for  a  few  minutes  after  starting  the  lamp  would  give  the  condenser 
a  chance  to  expand  uniformly. 

§  96.  Mounting  of  the  lenses. — This  may  not  give  the  lenses 
sufficient  freedom  of  expansion.  In  all  forms  of  condensers  as  now 
constructed  there  is  almost  invariably  provision  for  this  expansion, 
and  for  free  circulation  of  air  between  the  lenses.  The  lens  next 
the  radiant  is  usually  held  by  a  few  obliquely  extending  springs, 
(fig.  36  B),  thus  giving  the  greatest  freedom.  To  prevent  break- 
age some  operators  avoid  all  direct  contact  of  the  condenser  with 
the  metal  mounting  by  the  use  of  asbestos  paper.  Others  think 
that  a  heavy  metal  ring  around  the  edge  of  the  condenser  will 
lessen  breakage  by  preventing  the  too  rapid  cooling. 

The  final  solution  of  condenser  breakage  will  come  when  the 
glass  makers  produce  heat-resisting,  optical  glass. 

§  97.    Breakage  due  to  reversing  the  ends  of  the  condenser. — 

That  is,  the  condenser  lens  which  should  be  next  the  projection 
objective  is  put  next  the  lamp.  The  lens  which  should  be  next  the 
lamp  is  specially  mounted  for  expansion  (§  96).  Furthermore,  the 
condenser  is  not  designed  optically  in  most  cases  so  that  it  will  give 
equally  good  results  if  reversed.  In  the  magic  lantern  the  lens  next 
the  objective  has  frequently  a  longer  focus  than  the  one  next  the 
radiant,  so  that  a  reversal  injures  the  optical  effect  as  well  as 
endangers  the  condenser. 

If  the  makers  of  projection  apparatus  would  so  construct  their 
condenser  mountings  that  they  could  not  be  reversed,  they  would 
be  doing  a  friendly  service  to  many. 


CH.  I] 


SOME  AMERICAN  MAGIC  LANTERNS 


59 


§  98.  If  the  lantern  table  is  on  a  concrete  floor  which  is  damp 
the  operator  is  liable  to  get  a  shock  unless  he  stands  on  a  mat  or 
board  or  other  insulating  material,  provided  some  part  of  the  cir- 
cuit is  grounded  (see  §  689). 


SOME  EXAMPLES  OF  AMERICAN  MAGIC  LANTERNS  FOR  THE  DIRECT 
CURRENT  ARC  LAMP 

§  99.  The  following  examples  of  American  Magic  Lanterns  are 
introduced  to  give  the  reader  some  notion  of  the  lanterns  on  the 
market  which  can  be  obtained  at  any  time  and  at  a  very  moderate 
cost. 

In  subsequent  chapters  will  be  found  pictures  of  lanterns  for 
the  different  forms  of  radiants,  and  for  two  or  more  kinds  of 
projection  (combination  apparatus). 

In  the  appendix  at  the  end  of  the  book  will  be  found  the  addresses 
of  some  of  the  great  manufacturers  in  all  countries  with  the  prices 
for  the  different  complete  outfits  for  the  various  forms  of  projection. 


FIG.  32.     MAGIC  LANTERN  IN  OUTLINE  TO  SHOW  THE  PARTS. 

(Cut  loaned  by  Williams,  Brown  &  Earle). 

At  the  left,  the  side  of  the  lamp-house  is  removed  to  show  the  hand-feed, 
right-angled  arc  lamp  with  the  supply-wires  and  the  carbons  in  position. 

C  D  The  condenser  composed  of  two  plano-convex  lenses.  In  the  space 
(o)  a  water-cell  may  be  inserted. 

G  The  oblong  opening,  just  in  front  of  the  condenser,  into  which  the  slide 
carrier  is  inserted. 

A  The  projection  objective  fastened  to  the  end  piece  B,  which  also  holds 
the  bellows. 

E  F    Set  screws  serving  to  fix  the  apparatus  on  the  guide  rods. 


6o 


SOME  AMERICAN  MAGIC  LANTERNS 


[CH.  I 


FIG.  33.     SIMPLE  MAGIC  LANTERN  WITH  A  TWO-LENS  CONDENSER. 
(Model  C,  Balopticon;  Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 


FIG.  34.     MAGIC  LANTERN  OF  THE  LATHE-BED  TYPE  WITH  A  THREE- 
LENS  CONDENSER  AND  WATER-CELL. 
(Model  D,  Balopticon;   Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 


CH.  I] 


SOME  AMERICAN  MAGIC  LANTERNS 


61 


I       Good  2 


FIG.  35. 


SECTIONAL  VIEW  OF  AN  ARC  LAMP  AND  A  TRIPLE-LENS 
CONDENSER  WITH  WATER-CELL. 


-f  W    Wire  going  to  the  positive  carbon. 

—     W    Wire  from  the  negative  carbon. 

He     Horizontal  or  upper  carbon;    it  is  positive. 

Vc    Vertical  or  lower  carbon ;   it  is  negative. 

L     The  crater  of  the  positive  carbon;  it  is  the  source  of  light. 

Cond  i  The  first  element  of  the  triple-lens  condenser.  The  meniscus  is 
always  placed  with  the  concavity  next  the  source  of  light. 

Cond  2  The  second  element  of  the  triple-lens  condenser.  It  is  a  plano- 
convex lens  and  should  be  of  the  same  focus  as  the  projection  objective.  The 
different  lenses  should  be  in  the  position  shown  in  this  diagram.  Between  the 
two  convex  lenses  in  the  parallel  beam  of  light  is  placed  the  water-cell. 

Bt  Ba     Blocks  supporting  the  arc  lamp  and  the  condenser. 

Base  The  base-board  with  the  track  along  which  the  different  parts  move 
(see  fig.  40). 

Axis  The  principal  optic  axis  of  the  condenser  and  continuous  with  that 
of  the  projection  objective. 


FIG.  36  A.      MAGIC  LANTERN  WITH  AN  AUTOMATIC  LAMP  AND  INCLINED 

CARBONS. 
(Cut  loaned  by  P.  Keller  &  Co.,  successors  to  the  J.  B.  Colt  Co.}. 

This  lantern  is  very  widely  used.  It  has  a  two-lens  condenser  (see  fig.  i). 
Its  main  defect  is  that  every  part,  lamp,  condenser  lantern-slide  holder  and 
objective  can  be  separately  raised  or  lowered. 


62 


SOME  AMERICAN  MAGIC  LANTERNS 


[CH.  I 


FIG.  36  B.     CONDENSER  LENS  NEXT  THE 
RADIANT  IN  ITS  MOUNT. 


This  is  a'picture  of  the  end  of  the  condenser  next  the  radiant  of  the  lantern 
shown  in  fig.  36  A. 

The  lens  is  held  in  place  by  four  thin  metal  supports,  fastened  at  one  end  to 
the  condenser  mount,  and  hooked  over  the  edge  of  the  condenser  at  the 
other.  The  lens  is  considerably  smaller  than  the  condenser  mount,  thus 
giving  abundant  room  for  expansion. 

i,  2, 3,  4.  The  four  thin  metal  strips  for  holding  the  lens  in  position.  They 
are  white  where  they  hook  over  the  edge  of  the  lens. 

c     End  view  of  the  metal  tube  supporting  the  condenser. 

(The  white  spots  in  the  condenser  face  are  mirror  images  of  the  windows  near 
where  the  picture  was  taken). 


FIG.  37.     MAGIC  LANTERN  WITH  TWO-LENS 
CONDENSER,  AND  HAND-FEED  ARC  LAMP. 

(Portable  Sciopticon.     Cut  loaned  by  the  Mclntosh  Stereopticon  Co.). 


CH.  I] 


SOME  AMERICAN  MAGIC  LANTERNS 


FIG.  38  A.     SIMPLE  MAGIC  LANTERN  WITH  TWO-LENS  CONDENSER  AND 
A  HAND-FEED  ARC  LAMP  WITH  RIGHT-ANGLED  CARBONS 

AND  WATER-CELL. 
(Model  2,  Delineascope.     Cut  loaned  by  the  Spencer  Lens  Co.}. 


FIG.  38  B.     DETAILS  OF  MODEL  2,  DELINEASCOPE. 
(Cut  loaned  by  the  Spencer  Lens  Co.}. 

The  entire  instrument  is  in  one  metal  box. 

At  the  left  is  the  right-angled  arc  lamp  with  the  feeding  and  fine  adjustment 
screws. 

The  condenser  is  of  the  two-lens  type  with  a  water  cell  (W  C}  between  the 
lenses. 

S  P  Sl  The  slide- carrier  is  a  flat  frame  on  which  the  slides  are  laid  and 
turned  to  a  vertical  position  by  the  crank  L. 

S  When  the  crank  L  turns  a  slide  into  position  the  one  already  in  position 
is  released  and  it  falls  down  the  curved  incline  to  S  where  it  can  be  removed. 

L  0  The  projection  objective.  Its  conical  holder  is  hinged  so  that  it  can 
be  readily  turned  aside  to  give  place  to  the  projection  microscope,  which,  in 
the  figure,  is  turned  over  on  the  top  of  the  lantern  box. 


DO  AND  DO  NOT  WITH  THE  MAGIC  LANTERN   [Cn.  I 


§  99i.     Summary  of  Chapter  I : 

Do 

1 .  Connect  both  supply  wires 
to  the  arc  lamp  as  indicated  in 
fig.  3,  i.  e.,  connect  the  positive 
wire  with  the  binding  post  of 
the  upper  carbon  and  the  nega- 
tive wire  with  the  binding  post 
of  the  lower  carbon. 

Make   sure   of   the   polarity 

(§  so). 

2.  Always  use  a  rheostat  or 
other  balancing  device  with  an 
arc  lamp  (§  6). 

3.  Insert  the  rheostat  along 
one  wire  (fig.  1-4). 


4.  Insert  the  ammeter  along 
one  wire  (fig.  2,  4). 

5.  Always  have  a  double-pole 
switch  on  the  lantern  table  (fig. 
i-3). 

6.  Insert    the    switch    along 
both    wires,    and    before    the 
rheostat,  so  that  all  the  appara- 
tus on  the  lantern  table  has  no 
current  when  the  switch  is  open 

(fig.  3). 

7.  Always   open   the   switch 
before    changing    any    of    the 
wires. 


Do  NOT 

i .  Do  not  connect  the  nega- 
tive wire  to  the  upper  carbon 
and  thus  make  the  polarity 
wrong. 


2.  Never  try  to  use  an  arc 
lamp  without  a  balancing  device 
— (rheostat,  etc.). 

3 .  Do  not  connect  both  wires 
with  the  binding  posts  of  the 
rheostat,   but  insert  it  in  one 
wire. 

4.  Do  not  connect  the  am- 
meter with  both  wires.     Insert 
it  in  one  wire. 

5.  Do  not  try  to  get  along 
without    a    double-pole    table 
switch. 

6.  Do  not  insert  the  switch 
along  one  wire,  but  connect  it 
with  both  wires.     Do  not  put 
the  switch  after  the  rheostat, 
etc.,  but  before. 

7 .  Never  change  wires  on  the 
apparatus  until  the  current  is 
turned     off    by    opening     the 
switch. 


CH.  I]        DO  AND  DO  NOT  WITH  THE  MAGIC  LANTERN 


8.  Open    the    switch    before 
inserting  or  changing  carbons. 

9.  Center   the   parts   of   the 
lantern  when  it  is  first  installed 
(§  5i-6o). 

10.  When  the  condenser  and 
objective  are  once  centered  they 
should    be    fixed    in    position 

(§  51). 

1 1 .  Use  the  fine  adjustments 
on  the  arc  lamp  (fig.  3)  for  cen- 
tering the  light  on  the  screen 
after  the  first  centering.     Look 
at    the    carbons    through    the 
lamp-house  window  occasionally 
to  make  sure  that  they  are  in 
the    correct    relative    position 
(§  79). 

12.  Make  sure  that  the  arc 
lamp  and  condenser,  the  con- 
denser and  objective  are  separ- 
ated the  right  distance  (§  55- 

56). 

13.  For  the  triple  condenser 
select  a  condenser  lens  to  go 
next   the   lantern   slide   which 
shall  be  of  approximately  the 
same   focus   as   the   projection 
objective,  then  the  light  from 
the  condenser  will  cross  at  the 
center  of  the  objective  (fig.  1-2). 


8.  Do  not  try  to  insert  car- 
bons when  the  current  is  on. 
Open  the  switch. 

9-10.  After  the  parts  of  the 
lantern  are  once  centered,  never 
change  the  position  of  the  con- 
denser or  objective  for  center- 
ing. 


ii.  Do  not  fail  to  keep  the 
light  centered  by  the  use  of  the 
fine  adjustments  on  the  lamp 
and  by  keeping  the  carbons  in 
the  correct  relative  position. 


12.  Do  not   try  to  use  the 
lantern  when  the  arc  lamp  and 
condenser  are  too  near  together 
or  too  far  apart. 

The  same  for  the  condenser 
and  objective. 

13.  Do   not   try   to   use   an 
objective  with  a  condenser  that 
does  not  cross  its  rays  at  the 
center  of  the  objective.    .Objec- 
tive and  condenser  should  have 
the  same  focal  length  approxi- 
mately. 


66 


DO  AND  DO  NOT  WITH  THE  MAGIC  LANTERN         [Cn.  I 


14.  Make  sure  that  the  con- 
denser   is    arranged    with    the 
proper  lens  next  the  radiant. 
If  a  three-lens  condenser,    the 
meniscus  should  face  the  source 
of  light ;  if  a  two-lens  condenser, 
it  is  the  lens  in  a  special  mount- 
ing (fig.  36  B),  or  if  there  is  no 
special  mounting,  it  is  the  one 
of  shorter  focus  usually,  i.  e.,  of 
15  to  19  cm.  (6-7 %  in.),  while 
the  one  next  the  objective  is 
often  of  longer  focus. 

15.  Mark  or  ''spot"  the  lan- 
tern slides  so  that  they  may  be 
inserted  in  the  lantern  correctly 
(§  23,  fig.  7,  8,  13)  and  arrange 
the  slides  as  desired  before  the 
exhibition  (§  21). 

1 6.  Make    sure    that    every- 
thing is  in  working  order,  the 
room   properly   darkened,    and 
the  proper  amount  of  current 
available  (10  to  15  amperes). 

17.  Light  the  arc  lamp  before 
the  room  lights  are  turned  off 
(§  33). 

18.  Keep  the  arc  lamp  burn- 
ing until  the  room  lights  are 
turned  on  (§34). 

19.  After  the  last  slide,  show 
simply  a  lighted  screen  (§34). 


14.  Do  not  reverse  the  ends 
of  the  condenser  and  thus  have 
the  wrong  lens  next  the  light 
and  the  wrong  one  next  the 
objective. 


15.  Do  not  try  to  exhibit 
slides  that  are  not  in  order  and 
not  marked  for  insertion  in  he 
carrier. 


1 6 .  Do  not  attempt  an  exhibi- 
tion unless  the  room  is  properly 
darkened,  and  the  apparatus  in 
working  order. 

17.  Do  not  let  the  room  get 
dark,  but  turn  on  the  arc  lamp 
before  the  room  lights  are  out. 

1 8.  Do  not  turn  out  the  arc 
lamp  until  the  room  lights  are 
turned  on. 

19.  Do    not    keep    the    last 
slide  in  the  holder  too  long,  but 
show  a  light  screen  to  indicate 
that   the   last   slide   has   been 
shown. 


CH.  I]        DO  AND  DO  NOT  WITH  THE  MAGIC  LANTERN 


67 


20.  Study    the    "Troubles," 
their  causes  and  remedies  (§62- 
98). 

21.  Focus  the  screen  image 
sharply,  using  opera-glasses,  if 
necessary  (§38). 


20.  Do  not  fail  to  study  the 
"Troubles"  and  their  remedies. 

21.  Do    not   let    the    screen 
image  appear  vague  and  out  of 
focus.     Do  not  forget  the  aid 
opera-glasses  will  give,    if   the 
screen  distance  is  great. 


CHAPTER  II. 

THE  MAGIC  LANTERN  WITH  AN   ALTERNATING 
CURRENT  ARC  LAMP  AND  ITS  USE 

§  100.     Apparatus  and  Material  for  Chapter  II: 

Suitable  room  with  screen  (Ch.  XII) ;  Magic  lantern  with  lan- 
tern table  (§  102);  Arc  lamp  for  alternating  current  with 'suitable 
carbons  (§  108);  Alternating  current  supply ;  Rheostat,  choke-coil 
or  other  balancing  device  (§  105-106);  Ammeter  for  alternating 
current  (§  in);  Incandescent  lamp,  flash-light,  gloves  with  asbes- 
tos patches,  testing  lamp,  fuses,  extra  condenser  lenses,  screw 
driver,  pliers,  opera-glasses,  lantern  slides  as  in  Ch.  I  (§  i). 

§  101.  For  the  historical  development  of  the  alternating  cur- 
rent arc  lamp  see  the  Appendix ;  and  for  the  character  and  advan- 
tages and  disadvantages  of  alternating  current  see  §  652-653,  and 
modern  works  on  the  subject. 

The  same  books  of  reference  given  in  §  2,  Ch.  I,  are  available  for 
this  chapter. 

COMPARISON  OF  ALTERNATING  AND  DIRECT  ELECTRIC 
CURRENTS  AND  LANTERNS 

§  102.  A  magic  lantern  for  alternating  current  may  be  pre- 
cisely like  one  for  direct  current,  the  only  essential  difference  being 
that  the  arc  lamp  must  be  of  the  hand-feed  type  and  the  mechanism 
for  feeding  the  carbons  gives  equal  movement  to  the  upper  and  to 
the  lower  one,  both  carbons  being  of  the  same  size. 

One  would  never  use  an  alternating  current  with  the  magic 
lantern  if  direct  current  were  available.  It  frequently  happens, 
however,  that  the  lighting  system  of  a  place  is  of  the  alternating 
current  type,  and  no  direct  current  is  available.  In  such  a  case 
one  must  make  the  best  of  it,  or  use  a  motor-generator  set  or  a 
rectifier  (see  §  682-683). 

The  objections  to  an  alternating  current  for  the  arc  lamp  in 
projection  are:  (i)  The  lamp  is  noisy;  (2)  It  requires  about  two 
and  one-half  times  as  much  current  for  the  same  effective  light. 

68 


CH.  II]     ALTERNATING  AND  DIRECT  CURRENT  LANTERNS     69 

That  is,  if  10  to  12  amperes  of  direct  current  give  satisfactory 
illumination  in  a  given  case,  it  would  require  from  25  to  30 
amperes  of  alternating  current  to  give  the  same  brilliancy  of 
screen  image.  Naturally  also  the  heating  with  the  larger  alter- 
nating current  is  greater  than  with  the  smaller  direct  current 
(see  also  §  768). 

§  103.  The  difference  between  direct  and  alternating  current  is, 
in  general  terms,  this:  the  direct  current  has  a  constant  polarity 
and  one  carbon  is  always  positive;  while  the  alternating  current 
has  an  alternation  of  polarity,  as  the  current  flows  in  one  direction 
for  an  instant  and  then  in  the  opposite  direction.  The  result  is 
that  each  carbon  is  positive  half  the  time  and  negative  half  the 
time,  hence  both  carbons  have  brilliant  craters  from  which  light 
for  the  screen  image  might  be  obtained.  Sometimes  an  effort  is 
made  to  utilize  the  light  from  both  craters  by  the  arrangement  of 
the  carbons  in  the  form  of  a  V,  the  apex  of  the  V  pointing  toward 
the  condenser  (fig.  230). 


INSTALLATION   OF   A   MAGIC   LANTERN   WITH   AN  ALTERNATING 
CURRENT  ARC  LIGHT 

§  104.  Wiring  from  the  supply  to  the  lantern. — This  is  pre- 
cisely as  for  the  direct  current  lamp.  If  the  lantern  is  to  be  used 
for  experimental  purposes  it  is  advantageous  to  have  an  incandes- 
cent lamp  inserted  in  the  circuit  as  shown  in  fig.  2. 

§  105.  Rheostat  or  other  regulating  device. — There  must  be 
introduced  along  one  of  the  supply  wires  to  the  lantern  some  form 
of  balancing  device.  This  may  be  in  the  form  of  a  rheostat  like 
that  used  for  the  direct  current  (§  6) ;  an  inductor  or  choke-coil, 
a  transformer,  or  a  mercury  arc  rectifier  may  be  used.  For  the 
special  advantages  and  disadvantages  of  the  different  balancing 
devices  (see  §  736-738)- 

§  106.  Wiring  the  lamp. — For  the  alternating  current  it  makes 
no  difference  which  supply  wire  is  connected  with  the  upper  carbon, 
as  each  carbon  has  an  approximately  equally  brilliant  crater. 


70     ALTERNATING  AND  DIRECT  CURRENT  LANTERNS     [Cn.  II 

But  in  installing  a  magic  lantern  for  either  current,  it  must 
never  be  forgotten  that  the  arc  lamp  must  not  be  connected  with  the 
main  line  without  some  form  of  rheostat  or  regulating  device  in  the 
circuit  (fig.  3,  40,  and  §  744}. 


FIG.  39.     MAGIC  LANTERN  WITH  INCLINED  CARBONS. 

U  C,  L  C  The  upper  and  the  lower  carbon.  Only  the  carbons  of  the  arc 
lamp  are  shown. 

A   C    Alternating  current  supply  wires. 

F    Fuses  at  the  outlet  box  (see  fig.  40). 

L     Incandescent  lamp  for  use  in  working  around  the  magic  lantern. 

S    Double-pole,  knife  switch. 

R     Rheostat  in  one  wire. 

A     Ammeter  for  indicating  the  amount  of  current. 

Condenser  A  two-lens  condenser.  The  light  is  shown  as  a  parallel  beam 
between  the  lenses.  It  is  usually  diverging  (see  fig.  i). 

L  S    Lantern  slide  next  the  condenser. 

Axis  Axis  The  principal  optic  axis  of  the  condenser  and  the  projection 
objective. 

Objective     The  projection  objective  for  forming  the  screen  image. 

c  Center  of  the  projection  objective.  The  objective  and  condenser  should 
be  so  related  that  the  light  from  the  condenser  crosses  at  the  center  when  the 
image  is  in  focus  on  the  screen. 

Screen  Image     The  image  of  the  lantern  slide  on  the  screen. 

§  107.  Double-pole  table  switch. — This  is  especially  necessary 
when  using  an  alternating  current,  because  with  it  the  current  can 
be  turned  completely  off  the  lamp  whenever  desired.  Any  changes 
in  the  carbons  or  in  the  lamp  mechanism  can  then  be  made  with 
safety,  as  the  lamp  is  completely  cut  off  from  the  electric  supply, 
which  would  not  be  the  case  if  a  single-pole  switch  were  used. 
The  shock  from  an  alternating  ciirrent  supply  of  no  volts  is  much 


CH.  II]     ALTERNATING  AND  DIRECT  CURRENT  LANTERNS      71 

more  disagreeable  than  from  a  direct  current  supply  of  the  same 
voltage. 


n 

| 

Outlet 

4 

Box 

Lamp  House 


Supply  Wires 


FIG.  40. 


MAGIC  LANTERN  SHOWING  THE  WIRING  AND  THE  RELATION 
OF  THE  PARTS. 


Supply  Wires     Wires  from  the  electric  supply  to  the  outlet  box. 

Outlet  box  The  iron  box  receiving  the  supply  wires  and  containing  fuses  of 
the  cartridge  form,  a  double-pole  knife  switch  and  the  wires  extending  to  the 
wall  receptacle. 

P  W  R  Polarized  wall  receptacle  from  which  is  taken  the  current  to  supply 
the  arc  lamp  of  the  magic  lantern.  As  this  receptacle  is  polarized  the  cap  can 
be  put  on  but  one  way,  and  hence  the  polarity  will  always  be  the  same  if 
the  current  is  direct.  With  alternating  current  this  form  of  connection  is 
also  good. 

Arc  Supply  The  wires  extending  from  the  wall  receptacle  to  the  table 
switch  and  the  arc  lamp. 

Switch     The  double-pole,  knife  switch  on  the  lantern  table. 

Wi     The  wire  extending  from  the  switch  to  the  upper  carbon. 

W2  W3  Wire  from  the  table  switch  through  the  rheostat  to  the  lower 
carbon. 

Arc  Lamp     Hand-feed,  right-angle  carbon  arc  lamp. 

F  S      Feeding  screws  for  the  carbons. 

V  A     Fine  adjustment  for  moving  the  source  of  light  vertically. 

L  A     Fine  adjustment  for  moving  the  source  of  light  laterally. 

in  in  Insulation  between  the  carbon  holder  and  the  rest  of  the  arc  lamp  so 
that  the  current  will  keep  to  the  carbons  instead  of  short  circuiting  through  the 
lamp. 

^  5     Set  screws  for  holding  the  carbons  in  place,  etc. 

Lamp-House  The  metal  box  enclosing  the  arc  lamp.  The  feeding  and  fine 
adjustment  screws  project  through  the  back  end  of  the  lamp-house. 

V    Ventilator  of  the  lamp-house. 

Condenser    The  three-lens  condenser. 

Water  Cell    The  vessel  of  water  in  the  path  of  the  beam. 


72       MAGIC  LANTERN  WITH  ALTERNATING  CURRENT      [CH.  II 

1  The  first  element  of  the  condenser  consisting  in  a  meniscus  lens  next  the 
arc  lamp  and  a  plano-convex  lens. 

2  Plano-convex  lens  toward  the  lantern  slide.     The  lenses  of  this  condenser 
should  be  arranged  as  here  shown. 

Objective     The  projection  objective. 

c  The  optic  center  where  the  rays  from  the  condenser  should  cross  when  the 
objective  is  in  focus. 

Base  Board  The  board  bearing  the  track  and  the  blocks  for  supporting  the 
different  parts. 

Block  i,  Block  2,  Block  3  The  blocks  supporting  the  arc  lamp,  condenser 
and  objective. 

Rods  The  rods  or  tubes  on  the  base-board  and  serving  as  a  track  for  the 
blocks  to  move  upon. 

§  108.  Arc  lamps  for  alternating  current. — These  are  almost 
invariably  of  the  hand-feed  type.  Lamps  are  made  to  hold  the 
carbons:  (i)  at  right  angles  (fig.  1-3);  (2)  inclined  backward  30 
degrees  (fig.  23,  39);  (3)  converging  in  the  form  of  a  V  (fig.  23 
D);  or  (4)  even  in  a  vertical  position  (fig.  22).  Each  form  is 
best  adapted  to  some  special  purpose. 

With  carbons  of  the  same  size  and  composition  both  carbons 
burn  away  at  the  same  rate,  and  therefore  must  be  fed  forward  at 
the  same  rate.  If  the  carbons  are  of  different  size  or  material,  then 
the  mechanism  must  be  adjusted  to  move  the  two  at  a  rate  which 
shall  hold  the  ends  at  the  same  level. 

§  109.  Fine  adjustments  for  the  lamp. — As  indicated  for  the 
direct  current  arc  lamp  (§  10),  there  should  be  some  means  of 
moving  one  or  both  carbons  separately  to  compensate  for  any 
unequal  burning.  There  must  also  be  some  means  of  raising  and 
lowering  the  lamp  and  moving  it  sidewise  so  that  any  slight  varia- 
tions of  the  source  of  light  from  the  axis  may  be  corrected 
(§  10,  fig.  3). 

§  110.  Lamp-House. — There  should  be  a  well  ventilated  metal 
lamp-house  of  good  size  and  with  large  doors,  so  that  all  the 
apparatus  within  can  be  easily  got  at.  There  should  also  be  a  good 
sized  window  (say  5  cm.,  2  in.  square)  glazed  with  smoky  mica 
or  a  combination  of  green  and  red  glass  or  some  smoked  glass  of 
sufficient  depth  of  tint  for  the  protection  of  the  eyes.  This  window 


CH.  II]     MAGIC  LANTERN  WITH  ALTERNATING  CURRENT       73 

should  be  opposite  the  craters  of  the  electrodes,  so  that  the  position 
of  the  carbons  can  be  readily  seen  (fig.  133,  145). 

§  111.  Ammeter  for  alternating  current. — The  ammeter  serves 
the  same  purpose  for  the  alternating  as  for  the  direct  current; 
that  is,  it  indicates  the  amount  of  current  (§7).  The  construction 
for  the  alternating  current  is  somewhat  different,  so  that  the  one 
for  direct  current  cannot  be  used  for  alternating.  On  the  other 
hand  excellent  ammeters  are  now  constructed  which  can  be  used 
for  both  alternating  and  direct  currents  (§  664,  702a). 

§  112.  Mechanical  centering  in  a  horizontal  axis. — This  is  done 
precisely  as  for  the  direct  current  lantern  (§51,  fig.  i ,  2  and  40) . 

§  113.  Amount  of  current  necessary. — In  genera]  it  requires 
from  two  and  one-half  to  three  times  as  many  amperes  of  alter- 
nating current  to  get  the  same  brilliancy  of  image  as  of  the  direct 
current  (see  §  755-768).  Then  for  a  screen  distance  of  10  meters 
(30  feet)  one  should  have  a  current  of  about  25-30  amperes;  and 
for  a  distance  of  15  to  25  meters  (50-75  ft.)  one  should  use  from  30 
to  45  amperes.  If  one  can  be  satisfied  with  less  brilliant  screen 
images,  of  course  the  amount  of  current  may  be  somewhat  less. 

For  a  further  discussion  of  the  comparative  merits  of  direct  and 
alternating  currents,  and  means  of  changing  alternating  to  direct 
current  see  Ch.  XIII,  §  755-756,  682-683. 

USE    OF   THE  MAGIC  LANTERN   WITH    ALTERNATING   CURRENT 
FOR  EXHIBITIONS  AND  LECTURE  DEMONSTRATIONS 

§  114.  The  suggestions  for  the  lecturer  are  as  in  Chapter  I 
(§21-40). 

§  115.  Suggestions  for  the  operator. — These  are  the  same  as 
when  using  the  direct  current  arc  lamp  (§  26-42),  except  that  in 
using  the  alternating  current  arc  lamp  more  care  is  required  to  get 
good  results. 

(i)  The  carbons  must  be  properly  proportioned  to  each  other. 
If  they  are  of  the  same  composition  they  should  be  of  the  same  size. 
If  one  is  solid  and  the  other  cored,  the  solid  one  is  smaller  (§  7  53  a) . 


74       MAGIC  LANTERN  WITH  ALTERNATING  CURRENT     [Cn.  II 

(2)  As  there  are  two  sources  of  light  it  is  necessary  to  take 
special  pains  to  focus  the  lantern  slide  very  sharply  on  the  screen, 
or,  when  the  carbons  burn  away  so  that  the  sources  of  light  are 
relatively  far  apart,  the  image  on  the  screen  will  appear  partly 
double  like  print  that  has  slipped  on  the  press,  or  like  color  printing 
when  the  impressions  do  not  register,  thus  giving  two  partly  super- 
imposed images,  especially  if  the  carbons  are  arranged  like  a  V. 

If  the  image  is  sharply  focused  and  the  carbons  kept  close 
together  this  trouble  will  be  avoided. 

(3)  The  carbons  must  not  be  allowed  to  burn  away  too  far 
before  they  are  fed  up,  or  the  lantern  will  become  very  noisy.     The 
carbons  should  be  kept  about  three  mm.  (3^  in.)  apart.     This  will 
involve  feeding  them  toward  each  other  every  five  minutes  (see 
also  §  131,  753a)- 

A  pair  of  gloves  with  asbestos  patches  (fig.  5)  should  be  at  hand 
when  working  about  the  alternating  current  lamp. 

Practically  all  of  the  magic  lanterns  found  in  the  open  market 
may  be  used  with  an  alternating  lighting  system,  provided  a  lamp 
designed  for  the  alternating  current  is  used  (§  102,  fig.  3). 

TROUBLES  WITH  A  MAGIC  LANTERN  WITH  ALTERNATING 
CURRENT  ARC  LAMP 

§  116.  Noisy  arc. — There  is  no  way  of  entirely  obviating  the 
noise  in  an  arc  lamp  with  alternating  current.  It  may  be  kept  at  a 
minimum  by  using  carbons  of  the  proper  size  for  the  amperage  used 
(§  753a)  and  by  keeping  them  relatively  close  together.  As  the 
carbons  burn  away,  increasing  the  length  of  the  arc,  the  noise 
increases.  If  a  heavy  current  (much  amperage)  is  used  the  noise 
becomes  very  loud  and  disagreeable. 

The  noise  is  also  increased  if  there  is  any  loose  part  around  the 
rheostat  or  lamp  which  can  vibrate  in  unison  with  the  alternations 
of  the  current. 

§  117.  Managing  the  arc  lamp. — Practically  all  of  the  arc 
lamps  used  for  the  magic  lantern  with  alternating  current  are  of  the 
hand-feed  type,  hence  besides  all  the  other  things  the  operator 


CH.  II]     MAGIC  LANTERN  WITH  ALTERNATING  CURRENT        75 

must  see  to  it  that  the  carbons  are  brought  toward  each  other 
occasionally  by  turning  the  proper  screws.  With  moderate  cur- 
rents the  lamp  will  run  from  five  to  ten  minutes  without  feeding, 
but  the  greater  the  amount  of  current  the  oftener  must  the  carbons 
be  fed  together.  As  stated  above,  the  noise  increases  with  the 
length  of  the  arc ;  therefore  the  carbons  should  be  brought  nearer 
together  every  two  to  four  minutes. 

§  118.  Shadows  on  the  screen. — All  the  defects  indicated  under 
"troubles"  in  chapter  i  (§83)  for  the  direct  current  light  are  liable 
to  appear  when  using  alternating  current.  This  is  somewhat 
complicated  by  the  presence  of  an  equally  brilliant  crater  on  both 
the  upper  and  the  lower  carbons.  As  with  direct  current,  there  is 
less  trouble  with  right-angled  carbons  than  with  vertical  or  inclined 
ones.  With  right-angled  carbons  the  defect  is  greatest  when  the 
lower  carbon  is  too  high,  thus  shading  the  upper  carbon,  as  in  fig. 
25  A  (for  the  shadows  see  fig.  24—25,  27-29).  As  with  the  direct 
current,  the  greater  the  aperture  of  the  projection  objective,  the 
less  marked  is  the  screen  defect  of  a  slight  mal-position  of  the  car- 
bons. (See  also  Ch.  Ill,  §  127,  Ch.  IX,  §  417,  and  Ch.  X,  §  488  for 
the  arc  lamp  with  small  carbons  to  be  used  on  the  house  lighting 
system). 


76  SUMMARY  FOR  ALTERNATING  CURRENT  LANTERNS   [Cn.  II 


§  119.     Summary  of  Chapter  II: 

Do 

i.  Connect  both  supply  wires 
with  the  lamp;  and  remember 
that  with  the  alternating  cur- 
rent lamp  it  makes  no  difference 
which  supply  wire  goes  to  the 
binding  post  of  the  upper  and 
which  to  the  post  for  the  lower 
carbon  (§  106). 


2.  Insert  a  rheostat  or  other 
balancing  device  along  one  of 
the  supply  wires  (fig.  3). 


3.  Insert  the  ammeter  along 
one  wire  (fig.  2). 

4 .  Install  a  double-pole  switch 
before  the  rheostat  (fig.  3). 


5.  If  the  lantern  table  is  on  a 
concrete  floor,  use  a  board  or 
insulating  mat  to  stand  on  and 
thus  avoid  possibility  of  a  shock 
if  the  metal  part  of  the  lantern 
is  touched  (§98,  689). 

6.  Feed  the   carbons   nearer 
together    every    three    to    five 
minutes  so  that  the  lamp  will 
not  be  noisy  or  go  out  or  give 
double  screen  images. 


Do  NOT 

i .  Do  not  fail  to  connect  both 
supply  wires  to  the  arc  lamp. 


2.  Never  try  to  use  an  arc 
lamp    without    a    rheostat    or 
balance.     Do  not  connect  the 
rheostat  with  both,  but  with  a 
single  wire. 

3.  Do  not  connect  the  am- 
meter with  both  supply  wires, 
but  with  one. 

4.  Do  not  install  a  lantern 
without    a    double-pole,    table 
switch  which  will  cut  off  the 
current  from  all  the  apparatus 
on  the  lantern  table  (fig.  40) . 

5.  Do  not  stand  directly  on  a 
moist  concrete  floor  when  oper- 
ating a  magic  lantern  with  an 
alternating  current  lamp. 


6.  Do  not  let  the  lamp  go  too 
long  before  feeding  up  the  car- 
bons. 


CH.  II]    SUMMARY  FOR  ALTERNATING  CURRENT  LANTERNS    77 


7.  Focus    the    screen    image 
with   special   care  when   using 
alternating  current  lest  the  two 
sources  of  light  produce  a  doub- 
ling of  the  screen  image. 

8.  Use  opera-glasses,  if  neces- 
sary,   for    focusing    sharply    a 
distant  screen  image  (§38). 


7.  Do  not  forget  the  greater 
need  for  accurate  focusing  with 
an  alternating  current  lamp,  on 
account  of  the  double  source  of 
light. 

8.  Do  not  forget  the  advan- 
tage in  using  opera-glasses  for 
focusing  if  the  screen  distance 
is  great. 


9.  Look  out  for  shadows  on         9.  Do  not  permit  any  defect 

the    screen.     Center    carefully  in  the  management  of  the  lan- 

and  remove  all  causes  for  shad-  tern,  suspended  strings,  etc.,  to 

ows  (§  83-93).  give  shadows  on  the  screen. 


10.  Study  the  "Troubles"  in          10.  Do  not  neglect  any  of  the 
§  116-118,  and  62-98.  causes  for  "Troubles." 


CHAPTER  III. 

MAGIC  LANTERN  TO  BE  USED  ON  THE  HOUSE 
ELECTRIC  LIGHTING  SYSTEM 

§  120.    Apparatus  and  Material  for  Chapter  III : 

Suitable  room  and  screen  (Ch.  XII) ;  Magic  lantern  with  lamp- 
house  and  lantern  table;  Arc  lamp  for  small  carbons  (§  127); 
Rheostat  (§  129);  Flexible  cable  for  connecting  the  lamp  and 
rheostat  with  the  house  lighting  system  (fig.  40) ;  Separable  plugs 
and  extension  plugs  (fig.  49-50);  Polarized  plugs  (fig.  48-49); 
Nernst  lamps  (fig.  54-55);  Objective  shield  (fig.  14);  Concen- 
trated filament,  Mazda  lamps  (§  136);  Flash-light;  testing  lamp, 
screw  drivers  and  pliers;  lantern  slides,  etc.,  as  in  Ch.  I. 

§  121.  For  the  historical  summary  of  the  use  of  the  house, 
electric  lighting  system  for  the  magic  lantern,  see  the  Appendix. 

For  works  of  reference  see  §  2 .  Consult  also  the  Microscopical 
Journals,  and  the  catalogues  of  manufacturers  of  projection 
apparatus. 


MAGIC  LANTERN  WITH  SMALL  CURRENT  ELECTRIC  LIGHTS  FOR 
LABORATORY  AND  HOME  USE 

§  122.  For  public  exhibitions  and  large  lecture  rooms  special 
electric  wiring  and  large  current  arc  lamps  are  necessary,  as 
described  in  Ch.  I,  II  and  XIII.  For  small  audiences  as  in  labora- 
tories and  for  home  use,  where  less  than  100  people  are  usually 
present,  very  satisfactory  results  may  be  obtained  by  means  of 
lighting  apparatus  drawing  current  from  the  ordinary  house  light- 
ing system ;  and  the  electric  current  may  be  direct  or  alternating. 

§  123.  Kinds  of  lamps  to  be  used  with  small  currents. — There 
are  three  forms  of  lamps  which  have  been  successful  for  use  with 
the  magic  lantern  drawing  current  from  an  ordinary  lighting 
system : 

(i)  An  arc  lamp  of  small  size  using  small  carbons,  i.  e.  carbons 
of  6  to  8  mm.  (%  to  5/ie  in.)  in  diameter.  A  large  arc  lamp 
is  equally  available  if  it  has  long  clamping  screws,  bushings  or 

78 


CH.  Ill]         MAGIC  LANTERN  WITH  SMALL  CURRENTS  79 


FIG.  41.     THE  LILIPUT  ARC  LAMP  OF  LEITZ. 

This  lamp  was  designed  to  use  with  the  Edinger  drawing  apparatus  and  with 
the  condenser  for  dark  ground  illumination,  etc.  Both  carbons  are  moved 
equally  by  means  of  the  rack  and  pinion  movement.  For  direct  current  the 
horizontal  or  positive  carbon  is  larger  than  the  vertical  or  negative  carbon  in 
the  proportion  of  8  to  6. 

The  condensing  lens  in  the  tube  is  mounted  in  a  telescoping  sleeve.  When 
the  sleeve  is  in,  the  lens  is  at  its  principal  focal  distance  from  the  crater,  and 
gives  a  parallel  beam  of  light.  When  the  sleeve  is  pulled  out  more  or  less  the 
condenser  gives  a  converging  beam  of  light. 

For  use  with  the  magic  lantern  the  tube  and  special  condenser  are  removed, 
as  shown  in  fig.  47. 

adapters  for  the  small  carbons.     Such  carbons  require  from  three 
to  six  amperes  of  current  for  the  best  effect  (fig.  41-44). 

(2)  A  Nernst  lamp  with  one  or  more  filaments  (fig.  54-55). 

(3)  A  Mazda  lamp  with  concentrated  filament  (fig.  52). 

The  arc  lamp  is  permanent.  One  has  simply  to  renew  the 
carbons  when  they  are  burned  out. 

If  alternating  current  is  used,  carbons  150  mm.  (6  in.)  long  and 
8  mm.  (5/4e  in.)  in  diameter  last  about  three  hours. 

If  direct  current  is  used  the  upper  carbon  is  8  mm.  (5/ie  in.) 
and  the  lower  carbon  6  mm.  (y$  in.)  in  diameter.  Both  are  150 
mm.  (6  in.)  long,  and  they  last  about  three  hours  (§  753a). 

The  Nernst  and  Mazda  lamps  are  fragile  and  must  be  handled 
carefully.  They  have  a  working  life  of  500  hours,  more  or  less. 
then  a  new  lamp  must  be  obtained. 


8o  MAGIC  LANTERN  WITH  SMALL  CURRENTS        [Cn.  Ill 

§  124.  Room  for  projection. — Any  room  may  be  used  at  night, 
and  this  makes  these  magic  lanterns  especially  adapted  for  the 
home. 

In  the  daytime,  of  course,  the  room  where  they  are  used  must 
have  shutters  or  curtains  so  that  it  can  be  darkened. 

§  125.  Screen  for  the  image. — The  screen  need  not  be  over 
three  or  four  meters  square  (9—1 2  feet) .  For  many  purposes  a  large 
sheet  of  cardboard,  72x120  cm.  (28x44  m-)  makes  the  best 
possible  screen  (see  Ch.  XII). 

For  home  use  a  white  wall  or  a  well  stretched  sheet  will  serve. 
If  the  screen  is  to  be  used  frequently  in  the  same  place  in  the 
laboratory  or  home  it  is  desirable  to  use  a  white  wall  or  a  regularly 
painted  screen  (see  §  621-630). 

§  126.  The  magic  lantern  and  its  support. — Any  of  the  good 
modern  forms  of  magic  lantern  can  be  used.  Special  small  and 
compact  lanterns  have  been  constructed  for  this  purpose,  and  they 
are  excellent  and  cheap  (see  prices  in  the  appendix)  (fig.  51—52). 

For  a  lantern  support  any  table  of  sufficient  height  may  be  used. 
A  pile  of  books  or  an  empty  box  on  an  ordinary  table  will  serve 
to  raise  the  lantern  sufficiently. 

ARC  LAMPS  FOR  THE  HOUSE  CIRCUIT 

§  127.  Small  arc  lamps,  using  small  carbons  only,  are  con- 
venient ;  but  the  ordinary  large  arc  lamp  can  be  used  if  the  screws 
for  clamping  the  carbons  are  long  enough,  or  by  means  of  bushings 
or  adapters  for  the  small  carbons  (for  wiring  and  rheostat,  see 
§  128-129). 

The  small  carbon  arc  lamps  are  easily  managed,  and  the  amount 
of  light  they  give  (see  §  756)  much  more  than  offsets  the  attention 
they  require  over  the  other  lamps  used  on  the  house  circuit. 

If  a  lamp  must  be  purchased  for  use  on  the  house  circuit,  one  of 
small  size  is  preferable.  They  are  designed  for  the  small  carbons 
only.  They  are  nearly  always  of  the  hand-feed  type,  but  when 
direct  current  is  available  there  are  automatic  lamps  to  be  had. 
The  Thompson  automatic  arc  lamp,  and  the  Bausch  &  Lomb 


CH.  Ill]        MAGIC  LANTERN  WITH  SMALL  CURRENTS 


81 


FIG.  42.     THE  SMALL  ARC  LAMP  OF  THE  SPENCER  LENS  Co. 

With  this  small  arc  lamp  the  two  carbons  may  be  moved  separately  or 
together,  as  the  carbon  movement  is  like  that  of  the  larger  lamps,  i.  e.,  one  shaft 
within  the  other,  and  the  corresponding  milled  heads  are  placed  close  together, 
so  that  either  can  be  turned  separately  or  both  together. 

It  is  arranged  for  giving  parallel  or  converging  light.  When  used  with  the 
magic  lantern  the  special  condenser  and  its  tube  are  removed  (fig.  47). 

automatic  lamp  are  so  adjusted,  or  may  be  so  adjusted  if  desired, 
that  they  will  work  with  currents  ranging  from  5  to  25  amperes 
(fig.  4i-44). 

The  small  lamps  (from  their  size,  called  "Liliput  or  baby"  arc 
lamps)  are  largely  used  for  darkground  illumination  and  ultra- 
microscopy  and  for  drawing.  For  these  purposes  they  have  a  tube 
attached  with  a  condensing  lens  (fig.  41).  For  use  with  the  magic 
lantern  the  tube  and  condensing  lens  are  removed  (fig.  i). 


82 


ARC  LAMPS  WITH  SMALL  CURRENTS  [Cn.  Ill 


n 


FIG.  43.     THE  SMALL  ARC  LAMP  OF  REICHERT. 

This  is  arranged  in  the  figure  for  giving  a  parallel  beam  of  light  from  the 
small  condenser;  and  the  mechanism  for  feeding  the  carbons  can  be  actuated 
at  a  distance  by  means  of  a  Hooke's  joint  and  rod. 

a     The  horizontal  or  positive  carbon. 

b     Clamp  for  holding  the  lamp  to  the  upright  at  any  desired  height. 

c     Milled  head  of  the  feeding  mechanism  for  the  carbons. 

d     Rod  extending  from  the  Hooke's  joint. 

e-  S-j  f-  £•     Holders  and  clamping  screws  for  the  carbons. 

/     Terminal  points  of  the  carbons  where  the  arc  is  formed. 

ra  The  tube  holding  the  condensing  lens.  It  is  cut  away  on  one  side  to 
show  the  carbons. 

pn  The  condensing  lens  in  the  end  of  the  tube.  It  is  at  the  principal  focal 
distance  from  the  crater  and  the  diverging  beam  is  made  parallel;  by  pulling 
it  to  the  right  the  beam  will  be  converging. 


WIRING   AND   CONNECTING  THE   ARC   LAMP   WITH   THE   HOUSE 

CIRCUIT 

§  128.  Wiring. — The  wiring  is  in  principle  exactly  as  for  the 
large  current  arc  lamp  (fig.  i,  2,  45). 

One  end  of  a  double,  flexible  cable  of  sufficient  length  (2  meters, 
6  ft.  at  least)  is  connected  with  a  separable  attachment  plug  (fig. 
49) .  The  two  wires  near  the  other  end  of  the  cable  are  separated 
for  a  short  distance,  and'one  wire  is  cut.  The  cut  ends  of  this  wire 


CH.  Ill] 


ARC  LAMPS  WITH  SMALL  CURRENTS 


are  then  inserted  into  the  binding  posts  of  the  rheostat  (fig.  45). 
This  puts  the  rheostat  along  one  supply  wire  (in  series). 

The  cut  ends  of  the  cable  are  then  connected  with  the  binding 
posts  of  the  arc  lamp  (fig.  45).  For  polarity  see  §  701. 

§  .129.  Rheostat  or  other  balancing  device. — As  with  the  arc 
lamp  for  heavy  currents,  those  to  be  used  on  the  house  circuit  must 
also  have  a  balancing  device  of  some  sort  like  a  rheostat.  It  must 
be  in  one  wire  (fig.  45). 

Never  try  to  use  an  arc  lamp  on  any  circuit  without  a  rheostat 
or  other  balancing  device.  If  one  is  not  used  the  fuses  will  be 
burned  out. 


FIG.  44.     REICHERT'S  AUTOMATIC  ARC  LAMP  FOR  USE  ON  THE  HOUSE 
LIGHTING  SYSTEM  IF  DIRECT  CURRENT  is  AVAILABLE. 

At  the  bottom  are  screws  for  fine  adjustment,  laterally  or  vertically. 


§  128a.  In  modern  wiring  for  incandescent  lamps  each  group  of  not  over 
1 6,  or  in  special  cases  not  over  32,  lamp  sockets  must  be  protected  by  a  fuse 
or  cut-out.  The  wire  must  be  equivalent  to  a  copper  wire  No.  14  or  No.  18 
B.  &  S.  gauge,  and  the  fuse  or  cut-out  must  be  for  not  over  10  amperes  (usually 
6  amperes)  for  a  1 10  volt  circuit.  This  is  sufficient  for  the  small  arc  lamp. 

In  the  older  constructions  where  only  one  to  three  lamps  were  on  a  single 
line,  very  weak  fuses  were  used  which  would  melt  if  over  two  or  three  amperes 
were  drawn  from  the  line.  Naturally,  on  a  house  circuit  thus  wired  and  fused, 
the  fuses  would  be  burned  out  if  one  tried  to  use  the  small  arc  lamp  upon  it,  for 
that  rarely  draws  less  than  four  amperes  and  often  as  many  as  six. 

In  using  the  arc  lamp  on  the  house  circuit  it  is  therefore  necessary  to  make 
sure  that  the  wiring  and  fuses  are  of  sufficient  capacity  for  the  current  needed. 


84  ARC  LAMPS  WITH  SMALL  CURRENTS  [Cn.  Ill 

The  rheostat  needed  for  the  small-current,  arc  lamp  is  small  and 
inexpensive.  It  need  not  be  adjustable.  One  has  only  to  be  cer- 
tain that  it  will  not  deliver  a  current  above  five  or  six  amperes. 

In  purchasing  a  rheostat  for  the  house  circuit,  tell  the  manufac- 
turer the  kind  of  current  (direct  or  alternating)  and  the  voltage 
(no  or  220).  If  one  does  not  know  the  character  and  voltage  of 
his  house  circuit  the  information  can  be  obtained  at  the  office  of 
the  company  furnishing  the  current. 


FIG.  45.     WIRING  AND  CONNECTIONS  OF  THE  ARC  LAMP  USED  ON  THE 
HOUSE  LIGHTING  SYSTEM. 

§  130.  Polarity  with  the  arc  lamp. — With  alternating  current 
both  wires  are  the  same  (see  §  103  and  653),  but  with  direct  current 
one  of  the  wires  is  positive  and  one  negative,  and  the  positive  wire 
must  be  connected  with  the  binding  post  for  the  upper  carbon. 
The  most  practical  ways  of  determining  the  polarity  are  described  in 
Ch.  I,  §  80;  Ch.  XIII,  §  702. 

In  case  the  lower  carbon  shows  the  brightest  crater  it  is  positive 
and  hence  the  polarity  wrong.  If  the  separable  attachment  plug  is 
of  the  polarized  form,  separate  the  two  parts  thus  turning  off  the 
current.  Then  reverse  the  position  of  the  wires  in  the  binding 
posts  of  the  lamp.  This  will  connect  the  positive  wire  with  the 
upper  carbon  as  it  should  be.  A  simple  way,  if  non-polarized  plugs 


CH.  Ill] 


ARC  LAMPS  WITH  SMALL  CURRENTS 


are  used  (fig.  496),  is  to  leave  the  wires  as  they  are  in  the  lamp,  but 
pull  the  separable  plug  apart  and  turn  it  half  way  round.  This  will 
reverse  the  position  of  the  connections  so  that  the  polarity  will  be 
found  correct  on  lighting  the  lamp  again. 

When  the  correct  polarity  has  been  obtained  at  one  particular 
lamp  socket  it  is  well  to  make  a  straight  line  with  a  glass  pencil,  a 
pen  or  a  brush  across  the  socket,  and  the  two  parts  of  the  separable 
plug,  then  the  correct  connections  can  be  made  with  that  socket  at 
anv  time  without  trouble. 


Condenser 


Objective 


S— -P 


(!$) 


FIG.  46.     THE  MAGIC  LANTERN  FOR  USE  ox  THE  HOUSE  LIGHTING 

SYSTEM. 

SW    Supply  wires  to  the  lamp  socket  (So). 

So,  K     The  lamp  socket  with  the  key  switch. 

5 — P  Separable  attachment  plug.  The  cap  has  been  removed  to  show 
the  metal  prongs  serving  to  make  the  contact. 

L  W  Wires  connecting  the  cap  of  the  separable  plug  with  the  knife  switch. 
(K  5).  As  shown  in  fig.  45,  47,  the  knife  switch  is  more  frequently  omitted. 

K  S     Double-pole  knife  switch  for  opening  and  closing  the  circuit. 

Rheostat     For  controlling  the  current.     It  is  in  one  wire. 

Arc  Lamp     This  is  one  of  the  small  forms. 

5  5     Set  screws  for  holding  the  carbons  in  place. 

h  c     Horizontal  or  upper  carbon. 

v  c     Vertical  or  lower  carbon. 

In  In  Insulation  between  the  carbon  holders  and  the  rest  of  the  lamp  to 
compel  the  current  to  follow  the  carbons,  and  not  to  short  circuit. 

fs     Feeding  screws  for  moving  the  carbons. 

cl     Clamp  to  fix  the  lamp  at  any  desired  position  on  the  vertical  rod. 

Condenser     The  two-lens  condenser  for  illuminating  the  lantern  slide. 

i  2  The  two  plano-convex  lenses  with  their  curved  surfaces  facing  each 
other. 

L  S     Lantern  slide  close  to  the  condenser. 

Axis  Axis     The  principal  optic  axis  of  the  condenser  and  the  objective. 

Objective     The  projection  objective  for  giving  the  screen  image. 

Image  Screen     The  white  screen  on  which  the  image  is  projected. 


86 


ARC  LAMPS  WITH  SMALL  CURRENTS 


[On.  Ill 


Condenser 


FIG.  47.     THE  MAGIC  LANTERN  WITH^A  THREE-LENS  CONDENSER  AND  A 
WATER-CELL  FOR  USE  ON  THE  HOUSE  LIGHTING  SYSTEM. 

This  is  the  same  as  fig.  46  except  that  no  double-pole  knife  switch  is  used,  and 
there  is  a  triple-lens  condenser  and  water-cell  in  place  of  a  double-lens  condenser. 

It  is  well  also,  when  one  has  the  lamp  properly  connected,  to  turn 
off  the  current  by  opening  the  separable  plug,  and  then  paint  the 
positive  wire  red  where  it  is  inserted  into  the  binding  post  for  the 
upper  carbon.  The  negative  wire  can  be  painted  black  also.  If 


FIG.  48.  WALL  RECEPTACLES  WITH  SEPARABLE  CAP. 
(Cuts  loaned  by  H.  Hubbell,  Inc.). 

A  Wall  receptacle  with  the  connecting  prongs  polarized  so  that  the  cap 
can  be  put  on  only  one  way,  thus  avoiding  change  of  polarity  with  direct  cur- 
rent. 

B     Wall  receptacle  in  which  the  cap  can  be  put  in  place  either  way  around. 

Either  form  can  be  used  with  both  direct  and  alternating  current. 


CH.  Ill] 


ARC  LAMPS  WITH  SMALL  CURRENTS 


these  precautions  are  taken,  it  will  be  very  simple  to  connect  up 
the  lamp  correctly  at  any  time. 

"Polarized  attachment  and  extension  plugs"  are  made  (fig.  48A, 
4gA).  These  can  only  be  put  together  one  way.  They  are  very 
convenient  for  direct  current  connections;  they  are  also  equally 
adapted  for  alternating  current. 


FIG.  49.     SEPARABLE  ATTACHMENT  PLUGS. 
(Cuts  loaned  by  H.  Hubbell,  Inc.}. 

A  Polarized,  separable  plug  for  a  lamp  socket.  The  metal  prongs  are  in 
planes  at  right  angles  and  hence  can  be  inserted  in  only  one  way,  thus  avoiding 
change  of  polarity  with  direct  current. 

B  Non-polarized  attachment  plug.  The  connection  can  be  made  either 
way  around  as  the  prongs  are  in  the  same  plane. 


FIG.  50.     SEPARABLE  EXTENSION  CONNECTOR. 
(Cut  loaned  by  H.  Hubbell,  Inc.). 

This  is  to  enable  one  to  extend  the  line  by  joining  separate  cables.     These 
extension  connectors  can  be  had  with  polarized  or  non-polarized  prongs  to  the 


§  131.     Carbons  for  small  currents;  feeding  the  carbons. — For 

the  small  currents  used  with,  the  house  circuit,  the  carbons  should 
be  small.  For  alternating  current  of  five  to  six  amperes,  8  mm. 
carbons  answer  well.  For  three  to  four  amperes  the  carbons 
should  not  be  over  6  mm.  in  diameter. 


88  ARC  LAMPS  WITH  SMALL  CURRENTS  [Cn.  Ill 

For  direct  current  the  two  carbons  must  be  of  different  size  if  the 
feeding  mechanism  of  the  lamp  moves  the  carbons  equally.  With 
an  equal  feeding  mechanism,  the  upper  or  positive  carbon  can  be 
7  mm.,  the  lower  one  5  mm.,  or  the  upper  8  mm.  and  the  lower 
one  6  mm. 

One  could  use  carbons  of  the  same  diameter  for  direct  current, 
but  it  would  be  necessary  to  feed  the  upper  or  positive  one  more 
rapidly  than  the  lower  one  on  account  of  the  unequal  rate  of  burn- 
ing, otherwise  the  correct  relative  position  of  the  carbons  would 
not  be  maintained  (fig.  24-25).  On  a  no  volt,  direct  current  cir- 
cuit, the  lamp  will  burn  about  six  minutes  without  going  out. 
The  carbons  should  be  fed  up  every  three  to  five  minutes. 

For  alternating  current  of  no  volts,  the  small  lamps  will  burn 
from  eight  to  ten  minutes,  sometimes  longer.  It  is  well  to  feed  the 
carbons  every  five  to  seven  minutes. 

In  case  a  choke-coil  is  used  (Ch.  XIII,  §  736),  the  lamp  burns 
more  quietly  and  will  burn  longer  without  being  fed.  If  a  step- 
down  transformer  is  used,  then  the  right-angled  lamp  will  not  burn 
so  long — only  one  to  two  minutes — while  a  lamp  with  inclined 
carbons  will  burn  three  minutes,  because  it  takes  a  higher  voltage 
to  maintain  the  right-angled  than  the  inclined  carbon  arc  (see  Ch. 
XIII,  §  753,  768). 


TURNING  THE  ARC  LAMP  ON  AND  OFF 

§  132.  Lighting  the  small  arc  lamp. — For  this,  make  sure  that 
the  carbons  are  not  in  contact.  Now  turn  the  switch  for  the  room 
lights  and  the  snap  switch  in  the  socket  where  the  separable  attach- 
ment plug  for  the  lamp  wiring  is  screwed  in.  Feed  the  carbons 
together  until  they  touch.  There  should  be  a  flash  of  light. 
Separate  the  carbons  two  or  three  millimeters  as  soon  as  the  flash 
is  seen  and  the  arc  will  be  established  and  the  light  will  be  at  full 
brilliance.  Sometimes  it  is  necessary  to  keep  the  carbons  almost 
in  contact  for  a  half  minute  or  so,  until  the  tips  are  well  heated, 
before  the  arc  will  burn.  If  on  separating  the  carbons  the  light 
goes  out,  they  must  be  brought  together  again  as  at  first. 


CH.  Ill]  ARC  LAMPS  WITH  SMALL  CURRENTS  89 

§  133.  Turning  off  the  small  arc  lamp. — The  snap  or  key  switch 
in  the  usual  incandescent  lamp  socket  is  designed  to  break  the  cir- 
cuit where,  at  most,  two  amperes  are  used.  These  key  switches,  if 
used  to  interrupt  a  relatively  large  current,  like  that  used  for  the 
small  arc  lamp,  are  liable  to  start  an  arc  within  the  socket.  If  such 
an  arc  is  started,  the  socket  will  be  short  circuited,  resulting  either 
in  the  burning  out  of  a  fuse,  the  burning  out  of  the  socket  or  some- 
thing more  serious. 

The  liability  of  a  socket  to  arc  is  much  greater  with  direct  than 
with  alternating  current.  The  liability  to  arc  is  also  much  greater 
if  the  key  switch  is  turned  slowly  than  when  it  is  turned  quickly. 

By  observing  the  following  directions  the  current  may  be  turned 
off  with  perfect  safety : 

(1)  Turn  off  the  current  by  separating  the  carbons  until  the 
lamp  goes  out,  then  the  key  switch  may  be  used,  or  a  plug  or  exten- 
sion pulled  apart. 

(2)  Turn  off  the  current  by  pulling  the  separable  plug  or  the 
separable  extension  apart  (fig.  49-50). 

(3)  Make  use  of  a  knife-  or  snap-switch  (fig.  1,2,  40). 

(4)  Do  not  turn  off  the  current  by  the  key  switch  in  the  bulb 
socket.     When  the  lamp  is  out,  it  is  safe  to  turn  the  key  switch  in 
the  socket. 

(5)  Do  not  unscrew  a    plug   to   turn   off  the    light,    for  the 
break  in  the  circuit  is  so  slow  that  an  arc  will  almost  certainly 
be  formed. 

§  134.  What  to  do  in  case  the  key  switch  is  used  and  an  arc 
is  formed  in  the  socket: 

(1)  Turn  the  key  on  again  as  quickly  as  possible. 

(2)  If  the  arc  lamp  is  still  burning  after  turning  on  the  key 
switch,  turn  the  lamp  off  by  method  i  to  3  (§  133). 

(3)  Go  to  the  nearest  room  switch  and  turn  off  the  current. 

In  case  a  fuse  is  blown  out — which  is  almost  sure  to  occur  if  an 
arc  is  formed  in  the  socket — or  if  the  lamp  socket  is  burned  out,  it 
is  wise  to  call  in  an  electrician  to  make  the  necessary  repairs. 
This,  of  course,  assumes  that  the  user  has  not  the  technical  knowl- 
edge necessarv  to  make  the  corrections  himself.  It  is  further 


go  MAGIC  LANTERNS  WITH  MAZDA  LAMPS          [Cn.  Ill 

assumed  that  if  he  had  possessed  the  technical  knowledge  no  mis- 
takes, and  hence  no  accident  would  have  happened. 

§  135.  Use  of  the  small  arc  lamp  for  demonstrations  and 
exhibitions. — The  centering  of  the  apparatus  to  one  axis,  and 
using  the  correctly  proportioned  condenser  and  projection  objec- 
tive, the  lighting  and  putting  out  the  lamp,  arrangement  and 
insertion  of  lantern  slides,  etc.,  are  all  exactly  as  described  in  Ch. 
I,  II  (§  26-41,  52,  112). 


FIG.  51.     MAGIC  LANTERN  WITH  SMALL  ARC  LAMP. 
(Balopticon  B.;   Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

MAGIC    LANTERN    WITH    A    MAZDA,    CONCENTRATED    FILAMENT 
INCANDESCENT  LAMP 

§  136.  Next  to  the  arc  lamp  the  Mazda  concentrated  filament 
lamp  is  perhaps  the  best  electric  light  at  present  available.  They 
are  as  simple  to  use  as  an  ordinary  incandescent  bulb.  No  rheo- 
stat is  necessary.  The  lamp  is  on  a  stand  by  which  it  may  be 
raised  and  lowered  and  brought  the  proper  distance  from  the  con- 
denser (fig.  52-53). 

§  137.  Connections  with  the  house  circuit. — This  is  made  by  a 
double  flexible  cable,  one  end  of  which  is  connected  with  a  separable 
plug,  and  the  other  with  the  lamp  socket  of  the  Mazda  lamp.  As 
no  rheostat  is  used,  and  as  the  light  is  turned  on  and  off  exactly  as 
for  any  incandescent  bulb,  this  light  is  absolutely  simple  in  use. 
It  gives  a  light  sufficient  for  a  small  room,  where  not  over  50  to 
100  people  are  to  watch  the  exhibition. 


CH.  Ill]  MAGIC  LANTERNS  WITH  MAZDA  LAMPS  91 


FIG.    52.     SIMPLE    MAGIC    LANTERN    WITH    INCANDESCENT 

LAMP    AS  RADIANT. 
(Cut  loaned  by  Williams,  Brown  &  Earle). 

This  is  known  as  the  "Society  Incandescent  Lantern  No.  3  G.'VjIt  is 
especially  designed  for  use  with  permanently  mounted  lantern  slides  (fig.  15). 

§  138.     Centering   and   distance   from   the    condenser. — The 

centering  along  one  axis  is  as  with  the  arc  lamp  (§  51). 

In  general  the  concentrated  filament  should  be  at  the  principal 
focal  distance  from  the  condenser.  One  can  determine  the  best 
position  by  the  use  of  a  good  lantern  slide  and  changing  the  dis- 
tance of  light  and  condenser  until  the  best  position  is  found.  It  is 
well  to  mark  that  position  for  future  use. 


FIG.  53.     MAGIC  LANTERN  WITH  MAZDA  LAMP. 
(Balopticon  B.;    Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

This  lantern  can  be  used  with  the  small  arc  lamp  on  the  house  lighting  sys- 
tem, with  the  Mazda  incandescent  lamp  or  with  acetylene. 


92  MAGIC  LANTERN  WITH  NERNST  LIGHT  [Cn.  Ill 

§Jl39.     Management  of  an  exhibition  with  the  Mazda  lamp. — 

The  exhibition  should  be  managed  as  for  the  arc  light  (§  26-41). 

One  must  remember  that  with  this  relatively  weak  light  only  a 
small  screen  image  should  be  attempted,  and  that  the  room  must 
be  relatively  darker  than  for  the  arc  light.  In  brilliancy  the  screen 
images  will  be  more  like  that  of  the  old  lanternists  with  their  weak 
lights.  Clear  lantern  slides  are  especially  desirable.  The  very 
opaque  lantern  slides  sometimes  met  with  can  only  be  well  shown 
by  a  large  arc  lamp. 


MAGIC  LANTERN  WITH  A  NERNST  AUTOMATIC  LAMP 

§  140.  This  is  also  an  excellent  lamp  to  use  with  a  magic  lantern 
in  a  small  room.  Some  forms  are  automatic  in  starting  when  the 
current  is  turned  on,  and  some  have  to  be  specially  heated.  The 
automatic  form  is  to  be  preferred,  for  it  is  no  more  trouble  to  run 
than  an  ordinary  incandescent  lamp.  It  takes  some  time,  usually 
one  to  three  minutes,  for  the  glowers  to  come  to  full  brilliancy  after 
the  current  is  turned  on.  They  are  made  for  the  lantern  with  one, 
two,  three  and  four  filaments  or  glowers.  The  single  glower 
approximates  most  closely  to  the  arc  lamp  in  the  smallness  of  the 
source  of  light.  Of  course,  with  the  multiple  glower  lamps  a  greater 
amount  of  light  is  given  off,  but  they  make  an  extended  source. 
Whether  the  lamp  has  one  or  more  filaments  it  can  be  attached 
directly  to  the  house  lighting  system  through  any  incandescent 
bulb  socket  as  described  for  the  Mazda  lamp  (§  137). 

§  141.  Rheostat  or  ballast  for  the  Nernst  lamp. — This  lamp 
like  the  arc  lamp  is  always  used  with  a  balancing  device,  but  unlike 
the  arc  lamp,  the  ballast  is  an  integral  part  of  the  lamp  as  pur- 
chased, and  not  a  separate  apparatus  as  with  the  arc  light  (fig.  54, 
55  and  i). 

The  glowers  and  the  ballast  must  be  adapted  to  each  other 
and  both  must  be  adapted  to  the  line  voltage. 

The  ballasts,  which  are  enclosed  in  a  vacuum  glass,  as  with  an 
incandescent  bulb,  sometimes  burn  out.  The  filaments  will  not 
then  glow  when  the  current  is  turned  on.  If  a  ballast  burns  out  it 
must  be  replaced  by  a  perfect  one. 


CH.  Ill] 


MAGIC  LANTERN  WITH  NERNST  LIGHT 


93 


W2 


Gl 


FIG.  54.    NERNST  LAMP  FOR  THE  MAGIC  LANTERN  (REICHERT). 

This  is  on  a  support  and  has  a  rack  and  pinion  for  raising  and  lowering  the 
lamp.  It  is  automatic. 

Gl    The  three  filaments  or  glowers  with  this  lamp. 
5  5X     The  two  supply  wires  from  the  house  circuit. 
Tt     Pinion  and  milled  head  of  the  rack  work. 
Wt,  W2,  W3     The  three  ballast  tubes. 

§  142.  Centering  and  distance  from  the  condenser  with  the 
Nernst  lamp. — The  lamp  must  be  centered  with  the  condenser  and 
objective  as  described  in  Ch.  I  (§  51+).  It  must  have  some  form 
of  support  with  means  of  raising  and  lowering  the  lamp.  The 
distance  of  the  lamp  from  the  condenser  which  gives  the  best 
illumination  can  be  determined  as  follows:  Light  the  lamp,  put  a 
good  lantern  slide  in  the  lantern,  then  move  the  lamp  up  toward 
the  condenser,  shifting  it  back  and  forth  until  the  best  screen  image 
is  produced.  In  general,  it  will  be  found  that  this  results  when  the 
glower  is  at  about  the  principal  focal  distance  from  the  condenser. 
When  the  best  position  is  found  the  place  should  be  clearly  marked, 
then  the  lamp  can  be  put  in  this  position  quickly  at  any  time. 

§  143.  Connecting  the  lamp  with  the  house  circuit;  alternating 
current. — This  is  done  by  means  of  a  flexible  conductor  connected 


94 


MAGIC  LANTERN  WITH  NERNST  LIGHT 


[CH.  Ill 


with  the  lamp  at  one  end,  and  a  separable  plug  at  the  other  (fig. 
4gA).  The  plug  is  of  standard  size  and  can  be  screwed  into  the 
socket  or  receptacle  of  any  incandescent  lamp. 

To  light  the  lamp  turn  the  key  switch  of  the  socket  as  for  an 
incandescent  lamp,  and  in  a  minute  or  longer  the  glower  or  glowers 
will  attain  their  full  brilliancy,  and  one  can  use  the  lamp  as  long 
as  desired  without  further  attention. 


FIG.  55.     NERNST  LAMP  OR  SCHWANN-LIGHT. 

(Cut  loaned  by  the  Chas.  Besler  Co.). 

If  one  uses  a  three  or  four  glower  lamp  drawing  about  four 
amperes  of  current  there  might  be  a  short  circuit  in  the  incandes- 
cent lamp  socket  if  the  snap  switch  were  turned  off  slowly.  If  that 
is  used,  turn  it  as  quickly  as  possible  (see  §  133).  Pulling  the 
separable  attachment  plug  apart  will  avoid  all  danger  (§  133). 

§  144.  Nernst  lamp  and  direct  current. — If  one  has  a  direct 
current  lighting  system,  then  the  Nernst  lamp  must  be  adapted  to 
that,  and  must  be  connected  properly  as  with  the  direct  current  arc 
lamp.  The  two  connections  with  the  lamp  are  marked,  plus  (+) 
and  minus  ( — ) ;  or  positive  (P)  and  negative  (N) ;  and  the  corre- 


CH.  Ill]  MAGIC  LANTERN  WITH  NERNST  LIGHT  95 

spending  wires  must  be  attached  to  these  lamp  binding  posts  or 
connections,  or  the  lamp  will  soon  burn  out.  Unfortunately,  one 
cannot  tell  by  simple  observation  when  the  wires  are  connected 
properly,  as  for  the  arc  lamp ;  but  he  must  determine  the  polarity 
of  the  wires  before  connecting  them  with  the  lamp  (see  Ch.  XIII, 
§  701-703). 

§  145.  Marking  the  wires  and  attachments  after  determining 
the  polarity  with  the  direct  current  system. — When  the  polarity  of 
the  wires  is  determined,  if  one  is  to  use  the  same  place  for  current 
repeatedly,  it  is  a  good  plan  to  mark  the  position  of  the  socket  and 
the  two  parts  of  the  separable  plug  by  a  straight  line  of  colored 
paint  when  all  are  in  position.  Then  it  will  be  easy  to  connect  up 
the  parts  correctly  at  any  future  time.  Then  if  the  positive  wire 
has  its  insulation  material  colored  red,  at  least  at  the  lamp  end,  it 
will  enable  one  to  connect  up  with  that  particular  socket  correctly 
at  any  future  time.  It  is  also  a  convenience  to  have  polarized 
separable  plugs  (fig.  4pA),  then  the  two  parts  of  the  plug  cannot  be 
reversed  if  they  should  become  separated.  On  the  other  hand,  if 
the  attached  part  is  left  in  place  and  the  cap  or  removable  part 
pulled  off,  one  can  make  the  connection  correctly  at  any  time  with- 
out trouble,  as  it  cannot  be  put  together  wrong. 

Unfortunately,  one  cannot  be  sure  that  a  separable  plug  and  the 
lead  wires  connected  to  the  lamp  properly  for  one  incandescent 
socket  will  be  so  for  any  other,  and  one  must  determine  the  polarity 
for  each  socket. 

An  alternating  current  is  more  satisfactory  for  the  Nernst  lamp 
than  a  direct  current,  for  with  the  alternating  current  one  does  not 
have  to  trouble  about  the  polarity  of  the  two  wires,  since  both  are 
alike. 

§  146.     Management  of  an  exhibition  with  the  Nernst  light.— 

This  is  precisely  as  for  any  other  magic  lantern  radiant  except  that 
the  lamp  must  be  started  three  to  four  minutes  before  it  is  needed, 
for  it  may  take  that  time  to  get  good  illumination.  Furthermore, 
it  is  better  to  leave  the  light  burning  during  the  entire  lecture,  so 
that  there  will  be  no  delays.  The  light  can  be  shut  off  the  screen 
during  the  intervals  with  the  objective  shield  (fig.  14). 


96  TROUBLES  WITH  SMALL  CURRENT  LANTERNS      [Cn.  Ill 

§  147.  Troubles  with  the  magic  lantern  on  the  house  lighting 
system. — With  the  arc  lamp  these  are  the  same  as  those  indicated 
in  Ch.  I,  §  62-98;  Ch.  II,  §  116-118.  See  also  §  i28a  for  fuses. 
There  is  also  the  danger  of  smarting  an  arc  in  the  incandescent 
socket  from  which  the  current  is  drawn  unless  the  precautions  given 
in  §  133  for  turning  out  the  light  are  observed. 

For  all  the  lights  the  management  of  the  exhibition,  centering  of 
apparatus,  etc.,  are  the  same  as  for  the  lanterns  in  Ch.  I,  II. 

The  most  striking  difficulty  will  probably  be  the  comparatively 
dim  screen  pictures  as  compared  with  the  brilliant  screen  images 
given  by  the  large  current  arc  lamp. 

The  room  must  be  darker  and  the  screen  picture  smaller  with 
these  lights. 

The  Mazda  lamp  may  go  out  on  account  of  the  breakage  of  some 
of  the  connections  within  the  bulb.  If  this  happens  the  only  thing 
to  do  is  to  use  a  new  lamp.  It  is  wise  to  have  several  on  hand. 

With  the  Nernst  lamp  also  some  of  the  connections  are  liable  to 
break,  or  the  ballast  may  burn  out,  or  the  glower  be  broken. 
Usually  only  the  defective  parts  must  be  renewed,  and  not  an 
entirely  new  lamp  obtained. 


CH.  Ill]          SUMMARY  FOR  SMALL  CURRENT  LIGHTS 


97 


§  148.     Summary  of  Chapter  HI: 


Do 

1.  Find  out  the  kind  of  cur- 
rent used  in  the  house  lighting 
system  and  the  voltage   (alter- 
nating or  direct  current;    vol- 
tage 1 10  or  220). 

2.  Wire  the  small  arc  lamp 
exactly  as  the  large  arc  lamp  is 
wired  (fig.  40). 

3.  Always  use  a  rheostat  or 
some    other    balancing    device 
with  the  arc  lamp  (§  129). 

4.  Use  small  carbons  for  the 
arc  lamp  on  the  house  circuit 

(§131). 

5.  Make  sure  of  the  polarity 
if  direct  current  is  used  (§701- 
703). 


6.  Follow  carefully  the  direc- 
tions for  lighting  the  arc  lamp 

(§  132). 

7.  Be  very  careful  to  turn  off 
the  arc  lamp  by  one  of  the  safe 
methods  (§  133). 

8.  Make  the  room  darker  for 
the  small  arc  lamp  than  for  the 
large  one,  and  have  a  smaller 
screen  picture  (§139). 


Do    NOT 

1.  Do  not  try  to  use  an  arc 
lamp  on  the  house  circuit  with- 
out knowing  the  kind  of  current 
and  the  voltage. 

2 .  Do  not  wire  the  small  lamp 
differently  from  the  large  lamp 
except  that  smaller  wire  can  be 
used. 

3.  Never  use  the  arc  lamp 
without     a    proper    balancing 
device. 

4.  Do  not  use  large  carbons 
for  the  lamp  on  the  house  cir- 
cuit,    they    would    not    heat 
enough  to  give  a  good  light. 

5.  Do  not  worry  about  the 
polarity   if  alternating  current 
is   used.     If   direct    current   is 
used    the    polarity    must     be 
attended  to  so  that  the  upper 
carbon  is  positive. 

6.  Do  not  have  the  carbons 
in  contact  when  turning  on  the 
current. 

7.  Do  not  turn  off  the  arc 
lamp  by  the  socket  switch. 

8.  Do  not  expect  so  much  of 
the  small  as  of  the  large  current 
arc   lamp.     Do   not   have   the 
room   too  light   for   the   small 
lamp. 


98 


SUMMARY  FOR  SMALL  CURRENT  LIGHTS          [Cn.  Ill 


Do 

1.  Wire  for  the  Mazda  lamp 
exactly  as  for  any  incandescent 
bulb  lamp. 

2.  Turn  the  lamp  on  and  off 
by  the  key  switch  as  for  any 
incandescent  lamp. 

3.  As  this  light  is  relatively 
dim,  make  the  room  dark  and 
project  a  small  picture. 

It  is  also  wise  to  have  one  or 
more  extra  bulbs  in  case  one 
burns  out. 


Do  NOT 

1 .  Do  not  use  a  rheostat  with 
a  Mazda  lamp. 

2.  Do    not    take    any   more 
trouble  with  the  concentrated 
filament  Mazda  than  for  any 
bulb  lamp. 

3.  Do  not  try  to  make  too 
large  a  screen  picture;    and  do 
not  have  the  room  as  light  as 
for  the  arc  lamp. 


4.  Turn  the  lamp  on  and  off 
whenever  desired  as  it  gives  full 
brilliancy  in  an  instant. 


4.  Do  not  let  the  lamp  burn 
all  the  time  during  an  inter- 
mittent exhibition  any  more 
than  with  the  arc  lamp. 


Do 

i.  Find  out  the  kind  of  cur- 
rent and  the  voltage  wherever  a 
Nernst  lamp  is  to  be  used. 


Do  NOT 

i .  Do  not  use  a  Nernst  lamp 
with  a  current  and  voltage  for 
which  it  was  not  constructed. 


2.  Purchase  a  Nernst  lamp 
adapted  to  the  current  with 
which  it  must  be  used. 


2.  Do  not  purchase  a  Nernst 
lamp  for  direct  current  if  it 
must  be  used  on  an  alternating 
current  line. 


3.  A  special  rheostat  or  bal- 
last forms  a  part  of  every 
Nernst  lamp  for  projection. 


3.  Do  not  insert  a  separate 
rheostat  in  the  wiring  for  a 
Nernst  lamp. 


CH.  Ill]          SUMMARY  FOR  SMALL  CURRENT  LIGHTS 


99 


4.  Wire  the  Nernst  lamp  just 
as  the  arc  lamp  is  wired  except 
that    no    separate    rheostat  is 
inserted.      Wire     the     Nernst 
lamp    for    alternating    current 
just  as  a  Mazda  incandescent 
lamp  is  wired. 

5.  Wire  the  Nernst  lamp  for 
direct  current  with  the  positive 
wire  in  the  binding  post  marked 
-f   or  P,  i.  e.,  the  same  as  a 
direct  current  arc  lamp  is  wired, 
except  that  no  separate  rheostat 
is  included  (§  141). 

6.  Determine  the  polarity  of 
the  supply  wires  with  precision 
and  care  (§  701-703). 

7.  Let  the  Nernst  lamp  burn 
during  the  entire  exhibition,  as 
it    takes    from    one    to    three 
minutes  for  the  light  to  reach 
full  brilliancy. 

8.  Shut    the    light    off    the 
screen  when  not  needed,  by  the 
objective  shield  (fig.  14). 

9.  Handle  the  Nernst  lamp 
carefully,  as  it  is  easily  injured. 

10.  Manage    the    exhibition 
with  a  Nernst  lamp  as  with  any 
other   light,    remembering    the 
need  of  a  dark  room  and  a  screen 
picture  of  moderate  size  for  this 
relatively  weak  light. 


4.  Do  not  worry  about  polar- 
ity in  wiring  the  Nernst  lamp 
for  an  alternating  current  sys- 
tem. 


5.  For  a  direct  current  cir- 
cuit, do  not  put  the  positive 
wire  in  the  negative  binding 
post  of  the  Nernst  lamp. 


6.  Do  not  neglect  the  polarity 
of  the  two  wires  on  a  direct 
current  circuit. 

7.  Do  not  turn  the  Nernst 
lamp  out  during  an  exhibition 
for  it  takes  too  long  to  light  it. 


8.  Do  not  forget  to  use  the 
objective  shield  for  shutting  the 
light  off  the  screen  when  it  is 
not  needed. 

9.  Do  not  handle  the  Nernst 
lamp  roughly.     It  is   delicate. 

10.  Do  not  expect  too  much 
of  a  Nernst  lamp  with  the  magic 
lantern.     One  cannot  have  the 
room  so  light,  nor  project  such 
large  screen  pictures,   nor  use 
such  dark  lantern  slides  as  with 
the  arc  lamp. 


CHAPTER  IV 

THE  MAGIC  LANTERN  WITH  THE  LIME  LIGHT 
AND  ITS  USE 

§  150.    Apparatus  and  material  for  Chapter  IV : 

Suitable  room  with  screen  (Ch.  XII) ;  Magic  lantern  with  a  suit- 
able lamp-house  and  a  lime-light  burner  (fig.  56-59) ;  Cylinders 
of  compressed  Oxygen  and  Hydrogen  (§  154-155) ;  Lime  or  other 
refractory  substance  for  giving  the  light  (§  153,  157);  Oxygen 
generator  and  ether  saturator  (§  177-179);  Objective  shield  (fig. 
14,  62,  §  169);  Tubes  for  making  the  connections  (§  159,  i59a); 
Flash-light,  screw  drivers  and  pliers,  asbestos-patch  gloves  (fig.  61) ; 
lantern  slides,  etc.;  Matches  or  gas  lighters  (§  160). 

§  151.  For  the  discovery  that  oxygen  and  hydrogen  burning 
together  give  a  very  hot  flame,  and  that  dazzling  light  is  produced 
by  directing  the  flame  against  lime,  etc.,  and  the  application  to  the 
magic  lantern,  see  the  Appendix. 

For  works  of  reference  see  Chapter  I,  §  2. 

THE  LIME  LIGHT  FOR  THE  MAGIC  LANTERN 

§  152.  The  Magic  Lantern  used  with  the  lime  light  is  in  every 
way  like  the  standard  magic  lantern  with  the  direct  current  arc 
lamp  with  the  single  difference  of  the  source  of  light. 

§  153.  The  lime  light. — This  is  one  of  the  most  brilliant  avail- 
able lights  for  projection  purposes.  It  is  produced  by  directing 
the  exceedingly  hot  flame  of  hydrogen  burning  in  oxygen  against  a 
piece  of  unslaked  lime.  The  oxy-hydrogen  flame  in  itself  is  not 
brilliant,  but  the  heated  lime  gives  a  light  of  dazzling  brilliancy 
from  a  very  small  area ;  hence  it  is  especially  well  adapted  for  pro- 
jection with  the  magic  lantern  and  the  projection  microscope. 

If  the  candle-power  of  a  lime  light  is  compared  with  the  other 
lights  used  for  projection  it  will  be  seen  that  it  stands  third,  sun- 
light being  first  and  the  arc  light  second. 

Hydrogen  is  not  always  used,  but  illuminating  gas,  the  vapor  of 
alcohol,  ether  or  gasoline  sometimes  takes  its  place. 

Unslaked  lime  is  not  the  only  refractory  substance  which  gives 
great  incandescence.  Zirconium  discs  and  discs  made  of  the  mix- 

100 


CH.  IV]  MAGIC  LANTERN  WITH  THE  LIME  LIGHT  101 

ture  of  thorium  and  cereum  such  as  is  used  in  Welsbach  mantles 
have  been  employed.  Nothing  gives  a  more  brilliant  incandescence 
than  the  unslaked  lime,  but  it  deteriorates  rapidly  by  absorbing 
moisture  when  exposed  to  the  air.  This  is  not  the  case  with  zircon 
and  thorium ;  the  discs  of  these  may  be  used  over  and  over,  some- 
times hundreds  of  times,  while  with  the  limes  one  usually  has  to  put 
a  new  one  in  place  every  time  the  lantern  is  used  (§  153 a). 


FIG.  56.     MAGIC  LANTERN  WITH  THE 

LIME  LIGHT. 
(From  the  Catalogue  of  the  Enterprise  Opt.  Mfg.  Co.). 

The  door  of  the  lamp-house  is  open,  showing  the  burner  with  the  lime  in 
position. 

H    The  hydrogen  supply  tube,  extending  to  the  burner. 
0     The  oxygen  supply  tube,  extending  to  the  burner.. 

§  154.  Oxygen  gas  in  steel  cylinders. — This  is  now  a  great 
article  of  commerce.  Nearly  every  large  drug  store  keeps  one  or 
more  of  them  in  stock  for  the  use  of  physicians.  The  steel  cylin- 
ders for  containing  oxygen  were  formerly  large  and  contained 
oxygen  under  a  pressure  of  about  17  atmospheres  (250  pounds  per 
square  inch).  Such  cylinders  are  still  used;  but  at  the  present 

§  153a.  There  has  lately  been  introduced  a  substitute  for  limes,  known  as 
Guil  Pastils.  These  are  rather  soft  white  cylinders  of  a  substance  giving 
great  brilliancy  when  used  in  place  of  lime.  The  Guil  pastil  is  put  into  the 
holder  so  that  the  end  is  heated,  hence  the  lamp  should  be  in  the  form  shown 
in  fig.  57  K,  not  as  in  fig.  56  or  59  L.  The  Guil  pastil  serves  for  10  to  20 
exhibitions.  It  is  composed  mostly  of  a  zirconium  compound  and  is  not  hurt 
by  exposure  to  the  air.  It  should  be  heated  up  gradually  as  directed  for  the 
limes  (§  162). — Moving  Picture  World,  June  13,  1914,  p.  1539. 


102 


MAGIC  LANTERN  WITH  THE  LIME  LIGHT          [Cn.  IV 


time  smaller  cylinders  with  the  gas  at  a  much  higher  pressure  (100 
to  120  atmospheres)  are  employed  (see  also  §  156).  In  using  the 
gas  it  is  drawn  off  through  a  reducing  valve  by  which  it  can  be 
delivered  at  any  pressure  desired,  and  of  course  in  any  volume 
desired. 

One  should  never  try  to  use  the  gas  without  drawing  it  through 
the  reducing  valve.  The  cylinders  have  special  junctions  for  the 
reducing  valve,  so  that  it  is  easy  to  make  the  connections. 


FIG.  57.     OXYGEN  CYLINDER 
WITH  COMPRESSED  OXYGEN, 
THE  PRESSURE  GAUGES  AND 
THE  MIXED  JET  OR  BURNER. 
(Catalogue    of    Schmidt    and 

Haensch], 

B     Tip  of   the    nozzle    of 
the  mixed  jet. 

K  Holder  for  the  lime. 
The  end  of  the  lime  is  used, 
not  the  side  as  in  fig.  56,  59. 
G  Handle  of  the  stop-cock 
for  hydrogen  in  the  tube  of 
the  burner. 

S    Stop-cock  for  oxygen. 
H    The    tube    conveying 
hydrogen  to  the  burner,  steel 
cylinder  not  shown. 

0     Tube  conveying  oxygen  from  the  steel  cylinder  to  the  burner. 
J    The  high  pressure  gauge  giving  the  number  of  atmospheres  under  which 
the  gas  in  the  cylinder  is  compressed. 

M    The  low  pressure  gauge  to  show  the  pressure  of  the  gas  after  it  has 
passed  the  pressure  reducing  valve  (St.). 

St    The  handle  of  the  valve  serving  to  open  the  pressure  reducing  apparatus. 
V    The  valve  of  the  cylinder.     This  must  be  opened  to  allow  the  compressed 
gas  to  escape  into  the  tube  passing  to  the  reducing  valve  and  to  the  high  pres- 
sure gauge.     It  must  be  closed  after  every  exhibition. 


CH.  IV]          MAGIC  LANTERN  WITH  THE  LIME  LIGHT  103 

In  Great  Britain  and  on  the  Continent  oxygen  cylinders  are,  by 
common  usage,  painted  black,  and  the  screw  threads  are  right- 
handed. 

Hydrogen  cylinders  are  painted  red,  and  their  screw  threads 
are  left-handed. 

In  the  United  States  of  America  this  uniformity  of  color  and  dis- 
tinction of  screw  threads  is  not  always  found. 

§  155.  Hydrogen  in  steel  cylinders. — Hydrogen  gas  is  also 
compressed  in  steel  cylinders,  and  forms  an  article  of  commerce. 
It  must  also  be  drawn  off  through  a  pressure  reducing  valve. 

Every  precaution  should  be  taken  to  avoid  mixing  the  two  gases 
in  large  quantities.  Safety  lies  in  mixing  the  gases  only f  at  the 
moment  of  exit  from  the  two  tubes  of  the  blow-through  jet  or  in 
the  small  mixing  chamber  of  the  mixed  jet. 

§  156.  Pressure  gauges  for  gas  cylinders. — While  a  pressure 
reducing  valve  is  a  practical  necessity,  the  pressure  gauges  are 
highly  desirable. 

The  one  beyond  the  pressure  reducing  valve  is  a  low  pressure 
gauge  and  may  indicate  the  pressure  in  millimeters  of  mercury  or 
in  centimeters  of  water  (or,  of  course,  in  inches  of  water  or  mercury). 
This  shows  the  pressure  under  which  the  gas  is  actually  being  used. 

The  gauge  next  the  cylinder  registers  the  full  pressure  within. 
The  figures  on  the  dial  usually  represent  atmospheres  of  pressure, 
one  atmosphere  being  760  mm.  of  mercury.  The  special  purpose 
of  this  gauge  is  to  enable  one  to  determine  the  amount  of  gas  in  the 
cylinder  at  any  given  time,  hence  it  is  sometimes  called  a  "capacity 
meter"  or  a  "finimeter." 

If  the  pressure  gauge  does  not  indicate  directly  the  atmospheric 
pressure,  it  may  give  the  number  of  pounds  per  square  inch  or  the 
number  of  kilograms  per  square  centimeter.  To  change  these  to 
atmospheres  one  can  use  the  approximate  values:  15  Ibs.  per 
square  inch  =  i  atmosphere;  or  i  kilo  per  square  centimeter  =  i 
atmosphere  (§  i56a). 

§  156a.     The  exact  values  are: 

One  atmosphere  equals  14.73  pounds  per  square  inch. 

One  atmosphere  equals  i  .033  kilograms  per  square  centimeter. 


104  MAGIC  LANTERN  WITH  THE  LIME  LIGHT          [Cn.  IV 

For  example,  suppose  the  pressure  gauge  indicates  1800  Ibs.  per 
square  inch,  then  it  would  be  under  a  pressure  of  1800  -f-  15  =  120 
atmospheres. 

If  the  pressure  gauge  should  read  100  kilograms  per  square 
centimeter,  then  it  would  be  under  approximately  100  atmospheres 
pressure. 

From  Boyle's  law  of  the  relation  between  the  volume  of  a  gas 
and  the  pressure  to  which  it  is  subjected  it  is  known  that  if  one 
starts  with  a  cylinder  holding  five  liters  of  oxygen  or  hydrogen,  or 
indeed  of  any  other  gas,  under  one  atmosphere  pressure,  it  will  hold 
twice  as  much  under  two  atmospheres,  etc.,  so  that  a  cylinder  of 
five  liters  capacity  at  one  atmosphere,  will  hold  500  liters  at  100 
atmospheres  pressure.  Now  to  determine  the  amount  of  gas 
present  in  a  given  cylinder  with  the  high  pressure  gauge  one  must 
know  the  capacity  of  the  cylinder  under  the  ordinary  atmospheric 
pressure,  and  multiply  this  amount  by  the  number  of  atmospheres 
of  pressure  indicated  on  the  gauge.  For  example,  if  the  capacity  of 
the  cylinder  is  10  liters  at  one  atmosphere  (often  called  no  pressure) 
and  the  high  pressure  gauge  indicates  that  the  gas  in  the  cylinder 
is  under  a  pressure  of  25  atmospheres,  then  the  amount  of  gas  is 
I0  X  25  =  250  liters  of  gas;  and  so  in  like  manner  with  any  other 
pressure.  For  example,  in  England,  the  cylinders  are  filled  under 
120  atmospheres  pressure;  this  would  give  in  the  above  case 
10  X  120  =  1200  liters  to  the  full  cylinder. 

On  the  Continent,  the  filling  pressure  is  often  100  atmospheres 
and  the  cylinder  of  the  same  capacity  would  then  contain  10  X 100 
=  1000  liters  of  the  gas. 

The  practical  application  of  this  knowledge  is  to  determine  in  a 
given  case  whether  there  is  sufficient  of  the  gases  present  for  the 
exhibition.  Authors  differ  somewhat  in  estimating  the  amount 
of  gas  used  per  hour  with  the  lime  light  lantern.  A  conservative 
estimate  would  be,  for  oxygen,  85  liters  (about  three  cubic  feet) 
and,  for  hydrogen,  something  over  twice  that  volume,  as,  in  prac- 
tice, there  is  an  excess  of  hydrogen  (§  161). 

§  157.  Limes. — The  masses  of  unslaked  lime  (calcium  oxid) 
used  for  the  lime  light  are  usually  cylindrical  in  form.  For  some 


CH.  IV]          MAGIC  LANTERN  WITH  THE  LIME  LIGHT  105 

burners  they  are  placed  on  a  pin  or  axle,  and  then  must  have  a 
corresponding  central  hole.  With  other  burners  they  are  pressed 
into  place  between  surrounding  springs,  somewhat  as  a  lamp 
chimney  is  put  on  its  burner  (fig.  57,  59). 

The  limes  are  sealed  hermetically  in  glass  tubes,  or  are  packed  in 
powdered  unslaked  lime,  in  air-tight  tin  cans,  to  prevent  the  access 
of  moisture. 

If  moisture  reaches  the  limes  they  will  slake  and  become  powdery 
and  useless  for  the  light.  To  avoid  any  moisture  reaching  them 
they  should  not  be  removed  from  their  protective  covering  until  a 
few  minutes  before  they  are  to  be  used. 

§  158.  Lamp  for  the  lime  light.  —  This  consists  of  a  burner  or  jet 
for  conducting  the  two  gases,  oxygen  and  hydrogen,  to  a  point 
where  they  can  be  mixed  and  burned  ;  and  a  device  for  holding  the 
lime  in  a  proper  position,  and  raising,  lowering,  rotating  and  adjust- 
ing the  lime  with  reference  to  the  burner. 

There  are  two  principal  forms  of  burner  or  jet  : 

(1)  The  blow-through  jet.  —  In  this  a  stream  of  oxygen  is  blown 
into  a  flame  of  hydrogen  on  the  principle  of  the  gas  or  alcohol  blow- 
pipe (fig.  58). 

(2)  Mixed  jet.  —  In  this  form  the  two  gases  (oxygen  and  hydro- 
gen) meet  and  mix  in  a  common  chamber  just  before  the  nozzle 


o  HO 

FIG.  58.     FORMS  OF  BLOW-THROUGH  JETS  (Lewis  Wright}. 

The  form  c  shows  best  that  the  principle  is  that  of  a  blowpipe. 

The  form  d  approaches  the  mixed  jet  somewhat. 

With  all  of  them  the  hydrogen,  or  hydrogen  substitute  (illuminating  gas, 
ether  or  gasoline  vapor)  passes  out  from  the  supply  through  the  tube  marked 
H  at  the  left.  The  oxygen  is  then  blown  through  the  flame  from  the  tube  at 
the  right  marked  0.  Not  so  much  light  can  be  got  with  these  jets  as  with  the 
mixed  jet,  but  for  illuminating  gas  or  ether  vapor,  etc.,  this  form,  especially 
a,  b,  c  is  safer  in  the  hands  of  amateurs  than  the  mixed  jet. 


io6  MANAGEMENT  OF  THE  LIME  LIGHT  [Cn.  IV 

opens;  then  the  mixed  gases  burn  on  emergence  from  the  nozzle 
(fig-  SQ)-  This  form  of  jet  gives  the  greater  amount  of  light  but 
the  two  gases  should  be  under  considerable  pressure.  The  tip  of 
the  nozzle  (fig.  59  N)  makes  an  angle  of  40  or  45  degrees  with  the 
lime.  This  gives  a  source  of  light  above  the  tip  of  the  nozzle,  and 
hence  there  is  free  passage  for  the  light  to  the  condenser. 

The  blow-through  jet  is  usually  10  to  15  mm.  (y£  inch)  from  the 
lime  while  for  the  mixed  jet  the  nozzle  is  within  about  3  mm.  (}/% 
inch)  of  the  lime. 

MANAGEMENT  OF  THE  LIME  LIGHT 

§  159.  Connecting  the  gases  with  the  burner. — This  is  accom- 
plished by  means  of  rubber  tubes  of  thick  walls,  and  the  ends  of 
the  tubes  should  be  tied  or  wired  to  the  supply  pipes  and  to  the 
burner  (§  isga). 

It  is  a  great  advantage  to  have  the  two  parts  of  conducting  tubes 
of  the  burner  of  the  same  color  as  the  gas  tanks,  viz.,  red  for  hydro- 
gen and  black  for  oxygen,  then  there  will  be  less  liability  to  error 
in  connecting  the  gas  supply. 

It  is  only  while  using  the  gas  that  the  cylinder  valve  (fig.  57  V) 
should  be  opened.  And  in  opening  it  care  should  be  taken  to  open 
slowly  so  that  the  sudden  rush  of  the  compressed  gas  may  not  injure 
the  pressure  gauges  or  the  reducing  valve. 

When  through  with  the  cylinder  at  any  time  the  cylinder  valve  V 
should  be  closed. 

The  pressure  of  the  two  gases  should  be  about  equal.  This  can 
be  arranged  by  the  pressure  reducing  valve.  Set  this  to  give  the 
desired  pressure,  which  ordinarily  is  equal  to  a  column  of  water 
about  28  to  50  cm.  high  (n  to  22  inches)  or  2  to  4  cm.  of  mercury 
(^4  to  i}4  in.  Hg)  a  pressure  of  .03  to  .06  kilos  per  sq.  cm.  (.4  to  .8 
Ibs.  per  sq.  in.). 

§  160.  Lighting  the  jet. — Turn  on  the  hydrogen  slightly  and 
light  it  with  a  match  or  a  cerium-iron  gas  lighter,  then  continue  to 

§  159a.  Flexible  metallic  tubes. — There  is  now  available  flexible  metallic 
tubing  with  rubber  connections  at  the  ends  to  use  in  place  of  rubber  tubes  for 
conducting  gases  (fig.  60). 


CH.  IV]  MANAGEMENT  OF  THE  LIME  LIGHT  107 

N 


H 


M 
FIG.  59.     MIXED  BURNER  OR  JET  FOR  THE  LIME  LIGHT. 

(From  the  Catalogue  of  Williams  Brown  &  Earle). 

H  H  The  metal  tube  of  the  burner  conveying  the  hydrogen  to  the  mixing 
chamber  (M).  It  should  be  painted  red  to  correspond  with  the  color  of  the 
hydrogen  cylinder  of  compressed  gas. 

0  0  The  metal  tube  conveying  oxygen  to  the  mixing  chamber  (Jlf).  It 
should  be  painted  black  to  correspond  with  the  color  of  the  oxygen  cylinder  of 
compressed  gas. 

M  The  common  chamber  into  which  open  the  oxygen  and  hydrogen  tubes. 
Here  the  gases  mix  before  passing  out  through  the  nozzle  (N). 

N  The  nozzle  or  outlet  tube  from  the  mixing  chamber.  It  is  at  an  inclina- 
tion of  about  40  to  45  degrees  with  the  vertically  standing  lime  face;  and 
when  the  burner  is  in  action  the  nozzle  and  lime  are  about  3  mm.  (^th  in.) 
apart. 

L    The  support  and  springs  for  holding  the  lime. 

5  The  milled  heads  of  the  pinions  by  which  the  lime  is  rotated  or  raised  and 
lowered.  The  lime  support  slides  back  and  forth  on  the  supply  tubes  0  and  H 
so  that  the  lime  may  be  withdrawn  from  or  made  to  approach  the  tip  of  the 
nozzle  (N). 

open  the  stop-cock  until  the  flarr.e  is  from  8  to  15  cm.  (3-6  in.)  long. 
Then  turn  on  the  oxygen  slowly  until  the  flame  just  commences  to 
hiss.  After  the  lamp  has  been  going  some  minutes  the  operator 
can  slightly  increase  or  decrease  the  oxygen  until  the  most  brilliant 
light  is  obtained.  One  must  learn  by  experience.  The  flame  will 
become  very  small  as  the  oxygen  is  turned  on,  and  this  small, 
intensely  hot  flame  heats  a  very  small  part  of  the  lime,  hence  the 
source  of  light  is  very  small,  something  like  the  crater  of  the  posi- 
tive carbon  in  the  direct  current  arc  lamp. 

Caution. — Always  turn  on  the  hydrogen  first  and  light  it  before 
turning  on  the  oxygen. 

Never  turn  on  the  oxygen  first,  and  never  until  the  hydrogen 
has  been  lighted,  and  then  turn  it  on  slowly. 

If  both  were  turned  on  before  lighting  the  hydrogen,  there  would 
be  a  greater  or  less  explosion.  This  might  not  be  very  dangerous, 


io8  MANAGEMENT  OF  THE  LIME  LIGHT  [Cn.  IV 

but  it  has  a  dangerous  sound ;  and  the  purpose  of  the  exhibition  is 
to  instruct  or  entertain,  not  to  scare  the  audience.  To  insure  the 
correct  use  of  the  gases  it  is  a  good  plan  to  have  the  stop-cock 
handles  of  the  two  gases  so  different  that  one  can  tell  by  feeling 
which  one  is  being  turned  on. 


FIG.  60.     FLEXIBLE  METALLIC  TUBING  WITH  RUBBER  CONNECTORS  AT 

THE  ENDS. 

(Cut  loaned  by  the  Pennsylvania  Metallic  Tubing  Co.). 

§  161.  Regulating  the  flame. — Theoretically  the  proportion  of 
the  two  gases  should  be  their  combining  quantities  (H2  O) ;  but 
experience  has  shown  that  better  results  are  gained  when  the 
hydrogen  is  in  excess.  When  the  oxygen  is  in  exactly  the  combin- 
ing proportion  there  is  liable  to  be  a  snap  and  the  light  goes  out. 
If  there  is  an  excess  of  hydrogen  this  does  not  happen.  As  stated 
above,  the  oxygen  should  be  added  until  the  flame  just  begins  to 
hiss. 

§  162.  Putting  a  lime  in  position. — A  fresh  lime  from  the  box 
should  be  put  in  position  in  the  burner  (fig.  59  L)  before  lighting 
the  hydrogen,  but  the  lime  should  at  first  be  3  to  5  cm.  (i  to  2  in.) 
distant  from  the  tip  of  the  nozzle  (fig.  59  N),  and  it  should  be 
rotated,  raised  and  lowered  until  it  is  warmed.  If  the  full  heat  of 
the  O-H  flame  were  directed  against  one  point  of  the  cold  lime  for 
too  long  a  time  the  lime  would  be  liable  to  break.  After  it  is  well 


CH.  IV] 


MANAGEMENT  OF  THE  LIME  LIGHT 


109 


warmed  up  the  lime  is  not  liable  to  break.  Some  operators  warm 
the  lime  by  means  of  the  hydrogen  flame  only.  When  the  lime  is 
warm  the  oxygen  is  turned  on  slowly  until  the  most  brilliant  light 
is  obtained. 

§  163.    Arranging  the  lime  and  the  burner;  rotating  the  lime.— 

After  warming  the  lime  for  half  a  minute  or  so  it  should  be  grad- 
ually brought  toward  the  nozzle  until  it  is  only  about  5  mm.  (^4 
inch)  distant.  If  now  one  watches  the  disc  of  light  on  the  screen 
and  slowly  moves  the  lime  slightly  closer  to  and  farther  from  the 
tip  of  the  nozzle  it  is  easy  to  tell  when  one  gets  the  most  light.  It 
is  to  be  remembered  that  the  best  light  is  not  practically  instan- 
taneous, as  with  the  arc  lamp,  but  is  produced  after  the  lime  has 
been  half  a  minute  or  so  in  one  position. 

§  164.  Changing  the  position  of  the  lime. — The  intense  heat  of 
the  oxy-hydrogen  jet  makes  a  little  pit  in  the  surface  of  the  lime. 
In  about  two  minutes  this  pit  gets  so  deep  that  the  light  is  greatly 


FIG.  61.     GLOVES  WITH  ASBESTOS  PATCHES  ON  THE  THUMB,  INDEX  AND 
MIDDLE  FINGERS  FOR  USE  IN  WORKING  ABOUT  THE  HOT  LIME- 
LIGHT LANTERN. 

Right  hand,  palm  up:  p,  Pollex  or  thumb;  i,  Index  or  fore  finger;  m, 
Medius  or  middle  finger;  c,  the  index  and  medius  used  as  pincers  to  grasp  a 
hot  lime  or  a  hot  carbon. 

Left  hand,  palm  up:  i,  2,  3  The  first,  second  and  third  digits,  but  num- 
bered instead  of  named. 


no  MANAGEMENT  OF  THE  LIME  LIGHT  [Cn.  IV 

lessened,  and  one  must  move  the  lime  a  little  so  that  a  new  surface 
may  be  acted  upon. 

In  practically  all  the  modern  burners  there  is  a  screw  mechanism 
for  rotating  the  limes  and  for  raising  and  lowering  them  (fig.  59  S). 
With  a  little  experience  one  learns  by  the  looks  of  the  screen 
light  when  to  turn  the  lime.  If  the  limes  must  be  handled,  use 
tongs  or  asbestos-patch  gloves  (fig.  61). 

§  165.  Turning  out  the  light. — Always  turn  off  the  oxygen  first, 
then  the  hydrogen.  Never  turn  off  the  hydrogen  until  after  the 
oxygen  is  turned  off. 

Perhaps  it  will  help  to  remember  the  order  by  keeping  in  mind 
that  (i)  the  Hydrogen  is  ihe  first  to  come  and  the  last  to  go.  (2) 
And  the  Oxygen,  like  the  best  in  human  nature,  is  last  to  come  and 
first  to  go. 

MANAGEMENT   OF  THE   LIME   LIGHT   MAGIC   LANTERN   FOR  AN 
EXHIBITION  OR  DEMONSTRATION 

§  166.  Preparation  for  an  Exhibition. — Before  the  exhibition 
the  operator  should  see  that  everything  is  in  perfect  order  and 
readiness.  The  gas  cylinders  should  be  connected  with  the  burner, 
and  a  perfect,  fresh  lime  should  be  in  position  in  the  burner.  The 
box  of  limes  should  also  be  at  hand  in  case  anything  goes  wrong 
with  the  one  in  the  burner. 

§  167.  To  start  the  light. — It  takes  much  longer  than  for  the 
arc  lamp.  It  is  usually  about  half  a  minute  before  the  brightest 
light  possible  is  produced,  and  one  must  not  forget  the  precaution 
to  warm  the  lime  before  subjecting  one  spot  to  the  full  power  of  the 
O-H  jet. 

Light  up  as  directed  above  (§  160). 

If  there  is  a  snap  and  the  light  goes  out,  turn  off  the  oxygen,  and 
relight  the  hydrogen.  Turn  on  the  oxygen  slowly  until  the  best 
light  is  obtained  (§  160). 

§  168.  To  put  out  the  light. — Turn  off  the  oxygen  first,  then  the 
hydrogen  (§  165). 


CH.  IV] 


MANAGEMENT  OF  THE  LIME  LIGHT 


in 


§  169.     Shield  for  cutting  off  the  light  from  the  screen. — As  it 

takes  considerable  time  to  start  the  lamp  after  it  has  been  put  out 
it  is  not  so  easy  to  use  the  lime  light  intermittently  as  the  arc  lamp, 
hence  in  a  lecture  or  demonstration  in  which  the  lantern  slides  are 
to  be  shown  at  several  different  times,  it  is  best  to  leave  the  lamp 
burning  all  the  time.  But  the  screen  should  not  be  lighted  all  the 
time,  and  to  avoid  this  the  objective  shield  (fig.  62)  may  be  used. 


FIG.  62. 


SHIELD  FOR  THE  OBJECTIVE  IN  INTERMITTENT  PROJECTION, 
WHEN  SLOW-LIGHTING  RADIANTS  ARE  USED. 


S1     Shield  up  to  allow  the  light  to  pass  from  the  objective  to  the  screen. 

5  Shield  down  to  cut  the  light  off  from  the  screen.  This  shield  is  especially 
desirable  when  slides  are  to  be  shown  at  intervals,  as  in  a  demonstration  lecture 
with  the  lime  light,  a  Nernst  light,  a  kerosene  light,  or  an  alcohol  light  (§  169). 

Sometimes  also  to  avoid  using  so  much  gas  and  burning  out  the 
lime  too  quickly  there  are  regulating  valves,  by  which  only  a  small 
amount  of  the  two  gases  is  allowed  to  pass,  without  changing  the 
relative  proportions.  When  these  valves  are  opened  again  the  full 
amount  needed  and  in  the  original  proportions  is  allowed  to  flow 
again.  Even  in  this  case  there  should  be  a  shield  before  the  objec- 
tive to  avoid  lighting  the  screen. 

§  170.  Proper  lighting  for  the  screen. — The  light  on  the  screen 
should  be  uniformly  brilliant.  This  can  be  attained  by  following 
the  directions  for  centering  and  getting  the  proper  distance  of  the 
lamp  from  the  condenser  exactly  as  with  the  direct  current  arc 
lamp  (§  51-57). 

If  there  are  shadows  on  the  screen  make  the  proper  change  in  the 
position  of  the  lamp,  etc.,  as  indicated  in  fig.  27-30,  §  83-93. 


112  LIME  LIGHT  WITH  OXYGEN  AND  GAS  [Cn.  IV 

If  everything  has  been  put  in  perfect  order  before  the  exhibition 
the  changes  required  during  the  exhibition  should  be  very  slight. 

§  171.  Arrangement  of  lantern  slides,  their  insertion  and 
focusing. — Follow  the  directions  in  Ch.  I,  §  21-23;  SS^1- 

§  172.  Lighting  the  room. — As  the  lime  light  gives  only  about 
Vs  to  V&  as  much  light  as  the  arc  lamp  the  room  must  be  darker 
if  the  same  brilliant  contrast  is  desired.  One  can  determine  by  a 
little  experiment  with  the  set  of  slides  to  be  exhibited  at  any  time 
how  dark  to  have  the  room.  The  more  transparent  the  lantern 
slides,  the  lighter  can  the  room  be.  Many  lantern  slides  are 
altogether  too  opaque,  and  require  a  dark  room,  no  matter  what 
light  is  used  in  the  lantern. 

§  173.    Avoidance  of  intervals  of  total  darkness  in  the  room. — 

This  can  be  accomplished  by  leaving  the  lantern  on  all  the  time  and 
by  using  the  objective  shield  (fig.  62).  If  that  device  is  not  used, 
then  the  operator  should  not  turn  out  the  lime  light  until  the  room 
lights  are  turned  on.  And  whenever  the  lantern  is  to  be  used,  the 
lecturer  must  give  two  or  three  minutes  warning  to  the  operator 
before  turning  off  the  room  lights. 

THE  LIME  LIGHT  WITH  OXYGEN  AND  ILLUMINATING  GAS 

§  174.  Frequently  the  lime  light  is  produced  with  illuminating 
gas  drawn  from  the  house  supply,  and  with  oxygen  gas  in  a  steel 
cylinder  (§  154). 

If  illuminating  gas  is  used  instead  of  hydrogen  it  is  to  be  remem- 
bered that  the  pressure  as  drawn  from  the  house  supply  is  very 
slight,  i.  e.,  about  equal  to  a  column  of  water  from  5  to  12  cm. 
high  (2  to  5  in.)  or  only  about  Vs  the  pressure  of  the  hydrogen 
and  oxygen  when  these  gases  are  drawn  from  steel  cylinders  (§  154). 

The  oxygen  is  used  at  a  much  higher  pressure  than  the  house  gas, 
and  many  operators  use  for  this  combination  the  "blow-through 
jet"  (fig.  58).  Mixed  jets  are  also  constructed  for  this  combina- 
tion, but  the  "blow- through"  is  considered  safer.  The  user  of  this 
form  of  apparatus  would  do  well  to  get  the  combination  found  best 
by  the  manufacturers  of  his  apparatus. 


CH.  IV]    OXYGEN  GENERATOR  AND  ETHER  SATURATOR          113 

§  175.  For  lighting  the  lamp. — Whatever  form  of  burner  is  used 
turn  on  the  illuminating  gas  first  and  light  it;  then  turn  on  the 
oxygen  until  the  flame  is  made  much  smaller,  as  with  hydrogen. 
For  warming  and  arranging  the  lime  and  its  distance  from  the 
nozzle  of  the  jet  see  §  158,  162-164. 

§  176.  Putting  out  the  lamp. — Turn  off  the  oxygen  first,  then 
the  illuminating  gas. 

Remember  that  oxygen  is  always  on  last,  and  off  first. 

LIME  LIGHT  WITH  OXYGEN  GENERATOR  AND  ETHER  SATURATOR 

§  177.     Oxygen  generator. — There  has  recently  been  perfected 
a  method  of  preparing  sodium  peroxide  so  that  it  gives  off  oxygen 
gas  when  water  is  added,  somewhat  as 
calcium  carbide  gives  off  acetylene  gas 
when  put  in  water.     This  substance  gives 
about  300  times  its  volume  of  oxygen, 
and  serves  very  well  for  an  oxygen  sup- 
ply when  used  in  a  proper  generator. 

§    178.     Hydrogen     substitute. — The 

substitute  for  hydrogen  with  this  outfit 
is  sulfuric  ether  or  gasoline.  But  ether 
and  gasoline  should  never  be  mixed. 

§  179.     Use  of  the  apparatus.— There 

must  be  a  burner  and  lime  holder  as  for 
the  oxy-hydrogen  lime  light.  The  sodi- 
um peroxide  (Oxone,  oxodium,  oxy lithe 
are  trade  names)  is  put  into  the  generator 
and  the  oxygen  gas  conducted  over  to  the 
ether  saturator.  In  the  saturator,  the 
stream  of  oxygen  from  the  generator  is 
divided,  one  stream  of  the  oxygen  going 
directly  to  the  burner  through  one  tube,  FlG  63  PORTABLE  OXYGEN 

and  another  part  going  through  the  ether      GENERATOR  AND  ETHER 
i        i          r  , ,  .  SATURATOR. 

chamber  of  the  saturator  and  becoming 

1       J    j        -j.1        j_i  /-rv-i  •  (Cut  loaned  by  the  Edison  Manu- 

loaded  with  ether  vapor.     This  oxygen-   *  /<«/««,*«  Co.). 


114  TROUBLES  WITH  THE  LIME  LIGHT  [Cn.  IV 

ether  vapor  is  inflammable  and  takes  the  place  of  hydrogen  or 
coal  gas.  The  pure  oxygen  mixed  with  it  just  before  it  emerges 
from  the  burner  gives  the  necessary  intensity  to  the  flame. 

In  using  this  outfit  it  is  necessary  to  follow  very  precisely  the 
directions  of  the  manufacturers  to  avoid  accidents.  In  particular, 
one  must  be  sure  to  turn  on  the  oxygen-ether  first  and  light  it; 
then  turn  on  the  pure  oxygen  until  the  light  is  best.  In  turning 
the  light  out:  Turn  off  the  oxygen  first,  then  after  a  moment, 
turn  off  the  oxygen-ether  supply. 

The  oxygen  produced  from  one  charge  of  3^  pounds  of  the 
sodium  peroxide  (oxone)  gives  about  6.6  cubic  feet  of  oxygen  gas, 
enough  to  last  from  two  to  three  hours  for  the  magic  lantern.  One 
filling  of  the  ether  saturator  requires  about  one  pound  of  sul- 
furic  ether  and  will  supply  the  ether  vapor  for  the  charge  of 
oxone.  It  is  said  by  the  manufacturers  that  if  used  economic- 
ally the  single  charge  of  oxone  and  ether  will  supply  a  double  lan- 
tern for  an  entertainment  lasting  an  hour  or  an  hour  and  a  half. 

TROUBLES  WITH  THE  LIME  LIGHT 

§  180.  Snapping  out  of  the  light. — This  is  usually  due  to  an 
excess  of  oxygen.  The  oxygen  should  always  be  less  than  the 
hydrogen  or  any  of  its  substitutes,  i.  e.,  illuminating  gas,  ether  or 
gasoline  vapor,  acetylene  gas.  To  invert  the  statement,  the 
hydrogen  or  its  substitutes,  i.  e.,  the  inflammable  gas  or  vapor 
should  be  in  excess  of  the  actual  combining  proportions.  If  the 
lime  is  too  close  to  the  burner  tip  the  light  will  snap  out. 

In  case  the  light  snaps  out,  at  once  turn  off  the  oxygen.  Light 
the  hydrogen  and  slowly  turn  on  the  oxygen  again  until  a  satis- 
factory flame  is  obtained.  Be  sure  the  lime  is  not  too  close  to  the 
burner  tip. 

§  181.  Going  out  of  the  light. — This  may  be  due  (i)  to  a  lack  of 
one  or  of  both  the  gases  used,  that  is,  the  supply  may  be  exhausted. 
Look  at  the  capacity  meter. 

(2)  Some  of  the  valves  may  be  clogged. 

(3)  A  rubber  tube  may  have  split  or  come  off  at  the  connection. 


CH.  IV]  TROUBLES  WITH  THE  LIME  LIGHT  115 

(4)  A  lime  may  have  broken  so  that  there  is  nothing  for  the 
hot  flame  to  make  incandescent. 

Remedy.— Turn  off  the  gases  the  first  thing;  oxygen  first  then 
the  hydrogen  or  other  gas.  One  can  then  investigate  each  of  the 
possible  causes  for  the  going  out  of  the  lamp.  The  broken  lime 
and  the  split  or  separated  rubber  tube  can  be  most  easily  detected 
and  corrected,  and  consequently  should  be  looked  for  first. 

§  182.  Irregular  light  or  shadows  on  the  screen. — The  fault 
may  lie  in  any  of  the  following,  to  name  which  is  to  suggest  a 
remedy : 

(1)  The  lime  may  be  too  deeply  pitted  where  the  flame  strikes 
it.     Change  the  position  of  the  lime  (§  164). 

(2)  The  lime  may  be  in  bad  position,  too  high  or  too  low,  too 
far  from  or  too  close  to  the  burner  tip. 

(3)  The  incandescent  spot  may  not  be  centered  on  the  axis,  i.  e., 
be  too  high  or  too  low;   too  far  to  the  right  or  to  the  left  with  the 
resulting  shadows  as  with  the  crater  of  the  arc  lamp  (fig.  27-30). 

(4)  The  light  may  be  too  close  to  or  too  far  from  the  condenser. 

(5)  The  nozzle  of  the  burner  may  be  in  the  way  and  cast  a 
shadow.     If  so,  it  must  be  lowered  or  the  distance  from  the  lime 
or  the  angle  changed  (see  also  §  82-91). 

§  183 .  Roaring  or  hissing  of  the  burner. — A  slight  hissing  sound 
is  usually  heard  when  the  right  amount  of  oxygen  is  being  used. 
But  when  the  roaring  becomes  annoying  its  cause  must  be  found 
and  remedied.  It  may  be  due  to:  (i)  The  inside  of  the  nozzle 
tube  may  be  rough. 

(2)  The  lime  may  not  be  the  right  distance  from  the  tip  of  the 
nozzle. 

(3)  The  pitting  of  the  lime  may  be  too  great. 

(4)  There  may  be  too  great  a  supply  of  the  gases  for  the  bore 
of  the  nozzle. 

§  184.  Cracking  of  the  lime. — This  is  usually  due  to  a  sudden 
heating  of  the  lime.  If  it  is  warmed  gradually  by  rotating  it  at 
first  at  some  distance  and  then  closer  to  the  flame  the  breaking  is 
usually  avoided.  If  broken,  the  lime  should  be  removed  from  the 


n6  TROUBLES  WITH  THE  LIME  LIGHT  [Cn.  IV 

holder  and  a  new  one  put  in  place.     This  should  then  be  grad- 
ually warmed  (§  162). 


SPECIAL  PRECAUTIONS  IN  USING  THE  LIME  LIGHT 

§  185.  Remember  that  hydrogen  and  all  the  substitutes  used 
for  it,  illuminating  gas,  ether  and  gasoline,  are  very  inflammable. 

Oxygen  with  hydrogen  and  also  with  the  other  substances  forms 
an  explosive  compound.  Hence,  the  greatest  care  must  be  taken 
to  avoid  mixing  these  gases  except  in  the  mixer  of  the  burner 
(fig.  59).  Hence  also  in  filling  any  part  of  the  apparatus  and  in 
working  about  it  there  should  be  no  open  flames  or  glowing  parts 
to  ignite  any  accidentally  escaping  hydrogen,  gasoline,  ether,  etc. 
Fill  the  apparatus  by  daylight,  or  use  an  electric  light  or  an  elec- 
tric flash-light  if  the  work  must  be  done  in  a  dark  place.  In  this 
way  no  chance  for  igniting  the  gases  will  occur.  Naturally  one 
should  not  smoke  when  filling  the  apparatus. 

It  is  economical  to  buy  the  best  apparatus  throughout.  The 
makers  adapt  the  burners  and  all  other  parts  to  give  the  best 
results  in  the  safest  manner,  therefore,  unless  one  is  an  expert  in 
such  matters  it  is  safer  to  take  the  outfit  assembled  and  recom- 
mended by  some  reliable  manufacturer. 

The  makers  send  out  with  their  apparatus  very  precise  directions 
for  using  it  with  safety,  and  it  is  the  height  of  wisdom  to  follow 
their  directions  faithfully. 


CH.  IV]          DO  AND  DO  NOT  WITH  THE  LIME  LIGHT 


117 


§  186.     Summary  of  Chapter  IV: 


Do 

1.  Use   gas   cylinders   which 
are     plainly    marked     Oxygen 
and  Hydrogen,  and  have  right- 
handed  screws  for  the  oxygen 
and  left-handed  screws  for  the 
hydrogen  (§  154).     Be  sure  that 
there  is  plenty  of  gas  in  each 

(§  156). 

2 .  Connect  the  cylinders  with 
the  burner  by  means  of  rubber 
or  metallic  tubing,  colored    to 
correspond  with  the  cylinders 
(OorH)  (§  154,  159,  isga). 

3.  In    starting    the    burner, 
turn   on   the   hydrogen   or   its 
substitute    first    and    light    it, 
then  turn  on  the  oxygen  slowly 
(§  160). 

4.  Heat  up  the  lime  slowly  by 
having  it  at  some  distance  from 
the  flame  (§  162). 

5.  Turn  the  lime  occasionally 
so  that  the  pit  will  not  get  too 
deep  (§  164). 

6.  In  putting  out  the  lamp, 
turn  off  the  oxygen  first,  then 
the  hydrogen  after  a  moment. 

7.  If  the  light  snaps  out,  turn 
off  the  oxygen  then  the  hydro- 
gen.    Turn   on   the   hydrogen, 
light  it  and  then  turn  on  the 
oxygen  slowly  as  in  (3). 


Do  NOT 

i.  Do  not  use  gas  cylinders 
which  are  not  plainly  marked. 
Do  not  start  an  exhibition 
unless  there  is  plenty  of  gas. 


2.  Do  not  be  careless  in  con- 
necting the  cylinders  with  the 
gas  burner. 


3 .  Do  not  turn  on  the  oxygen 
first.     Oxygen  is  last  on,  first  off. 


4.  Do  not  turn  the  full  heat 
of  the  O-H  flame  against  a  cold 
lime  which  is  close  up  to  it. 

5.  Do  not  let  the  lime  stay 
too  long  in  one  position.     Ro- 
tate it  occasionally. 

6.  Do  not  turn  off  the  hydro- 
gen   first,    but    turn    off    the 
oxygen  first.    Oxygen  is  on  last, 
off  first. 

7.  Do   not   leave    the   gases 
turned  on  if  the  light  snaps  out. 
Oxygen  off  first,  then  Hydrogen. 


n8 


DO  AND  DO  NOT  WITH  THE  LIME  LIGHT          [Cn.  IV 


8.  After  the  exhibition  is  over 
remove  the  lime  or  it  will 
slake  in  the  holder. 


8.  Do  not  leave  the  lime  in 
the  holder  to  slake  after  the 
lecture. 


9.  Conduct  the  exhibition 
exactly  as  with  an  electric 
lantern  (Ch.  I,  §  21-40). 


9.  Do  not  spare  any  pains  in 
conducting  an  exhibition  with 
the  lime-light  magic  lantern. 
More  care  and  skill  are  neces- 
sary than  with  the  electric  light 
lantern. 


10.  As  the  hydrogen  or  its 
substitute  is  inflammable,  and 
the  oxygen  is  a  perfect  supporter 
of  combustion,  follow  the  direc- 
tions given  by  the  manufac- 
turers of  a  special  apparatus 
intelligently  and  exactly. 


10.  Do  not  take  any  chances 
when  dealing  with  the  oxy- 
hydrogen  lantern.  Do  things 
in  the  right  order,  and  do  not 
neglect  the  directions  of  the 
manufacturers. 


CHAPTER  V 

MAGIC  LANTERN  WITH  PETROLEUM   LAMP;  VERTICAL 

AND  REFLEX  MANTLE  GAS  LAMPS;   ACETYLENE 

LAMP;  ALCOHOL  LAMP  WITH  MANTLE 

§  190.    Apparatus  and  Material  for  Chapter  V: 

Suitable  projection  room  with  screen ;  Magic  lantern  with  lamp 
and  chimney  for  petroleum  (fig.  65-67) ;  High  grade  petroleum  for 
burning  in  the  lamp ;  Gas  burners  for  vertical  and  reflex  mantles 
(fig.  68-69);  Illuminating  gas  supply;  Acetylene  burner  and 
reflector,  (fig.  70) ;  Acetylene  gas  supply  (house  supply,  prestolite 
tank  of  compressed  acetylene  in  acetone  or  an  acetylene  generator) ; 
Special  alcohol  lamp  with  mantle  (fig.  72-73):  Strong  alcohol 
(95%)  ethyl,  methyl  or  denatured.  The  magic  lantern  for  all  but 
the  oil  lamp  must  have  a  lamp-house  into  which  the  burner  can  be 
placed.  There  must  be  lantern  slides,  screw  drivers,  pliers  and 
matches  or  safety  lighters  (§  160),  for  all  of  them. 

§  191.     Historical  development  and  references  to  literature.— 

For  the  history  see  the  Appendix,  and  for  general  works  of  reference 
see  the  list  of  books  in  the  first  chapter  (§2). 

The  directions  sent  out  by  the  manufacturers  of  these  light 
sources  should  be  studied  carefully  and  followed  exactly  unless  one 
has  technical  knowledge  on  the  subject. 

OIL  AND  GAS  LAMPS 

§  192.  Early  sources  of  light. — For  a  long  time  after  the  inven- 
tion of  projection  apparatus  there  were  but  two  sources  of  light 
known : 

(1)  The  sun,  which  has  ever  remained  the  most  brilliant  source 
of  light  available,  and 

(2)  Some  form  of  torch,  candle,  or  oil  lamp. 

The  first  oil  lamps  burned  animal  or  vegetable  oil  and  had  no 
lamp  chimney. 

After  the  discovery  and  proper  refinement  of  petroleum,  that 
became  and  has  remained  the  oil  most  used  for  illumination. 

If  one  reads  the  early  works  on  projection  it  seems  astonishing 
that  the  workers  of  those  times  were  able  to  produce  screen  images 

119 


120 


MAGIC  LANTERN  WITH  OIL  AND  GAS  LAMPS       .[Cn.  V 


which  showed  general  form  and  details  with  anything  like  satisfac- 
tion to  large  audiences.  But  screens  as  large  as  four  meters  square 
(12  ft.  sq.)  were  used  with  the  petroleum  light. 

When  the  feeble  lights  discussed  in  this  chapter  are  compared 
with  the  powerful  electric  arc  light  giving  from  1,000  to  5,000  candle- 
power  it  would  seem  that  the  results  of  earlier  times  must  have  been 
very  unsatisfactory. 

But  the  older  lanternists  gave  very  successful  exhibitions.  They 
did  this  by  observing  with  scrupulous  care  the  requirements  for 
projection  with  their  appliances. 


Condenser 


FIG.  64.     MAGIC  LANTERN  WITH  LARGE  LIGHT  SOURCE. 

Lamp     Illuminating  gas  lamp"  with  Welsbach  mantle. 
Condenser    Triple-lens  condenser  without  water-cell. 
vS     Lantern  slide. 

Objective     Projection  objective  with  inverted  image  of  the  luminous  mantle 
between  the  lenses. 

Screen  Image     The  image  of  the  lantern  slide  on  the  white  screen. 

§  193.    Requirements  for  projection  with  a  feeble  light: 

(A)  The  lantern  slides  must  be  very  transparent ;   and  the  old, 
hand-painted  slides  were  very  transparent. 

(B)  The  room  must  be  very  dark.     There  must  be  no  stray 
light  from  the  windows  or  from  the  apparatus ;  the  only  light  must 
be  that  issuing  from  the  lantern  objective  and  reflected  from  the 
screen. 

(C)  The  management  of  the  lantern  must  be  the  best  possible, 
so  that  all  the  available  light  may  be  utilized  for  producing  the 
screen  image. 

(D)  The  projection  objective  must  be  of  large  aperture  so  that 
as  much  as  possible  of  the  light  issuing  from  the  large  source  (lamp 


CH.  V]         MAGIC  LANTERN  WITH  PETROLEUM  LAMP  121 

flame  or  incandescent  mantle),  may  be  utilized  in  making  the  screen 
image.  This  is  of  fundamental  importance  (fig.  64,  90). 

(E)  Use  of  twilight  vision. — It  is  astonishing  how  dim  a  picture 
can  be  clearly  seen  after  one's  twilight  vision  has  become  fully 
established.  According  to  careful  investigations  the  sensitiveness 
of  the  eye  may  be  increased  from  35  to  2500  times  by  the  adapta- 
tion to  dim  light  (§  281). 

The  old  lanternists  used  to  advise  that  the  exhibition  should  not 
begin  until  the  audience  had  been  in  the  darkened  room  for  half  an 
hour  "to  get,"  as  they  said,  "the  sunlight  out  of  their  eyes."  We 
would  say  to  "get  the  twilight  vision  well  established." 

§  194.  Time  required  for  lighting  up. — The  gas  light  and  the 
acetylene  light  are  quickly  established,  but  the  petroleum  and 
the  alcohol  lights  require  several  minutes  to  get  up  the  best  illumi- 
nation. These  two  should  then  burn  during  the  entire  time  of  an 
exhibition.  If  the  lecturer  cannot  arrange  to  have  all  the  slides 
continuously,  but  must  have  them  at  intervals  during  the  lecture, 
the  operator  should  make  use  of  an  objective  shield  (fig.  14,  62), 
and  leave  the  lights  on  all  the  time. 

§  195.  Rehearsals. — As  these  lights  are  more  difficult  to  man- 
age and  the  results  are  less  satisfactory  than  with  the  more  power- 
ful radiants,  so  much  the  more  should  the  operator  rehearse  before 
the  lecture  and  make  sure  that  everything  is  in  as  nearly  perfect 
order  as  human  skill  can  make  it. 

THE  MAGIC  LANTERN  WITH  A  PETROLEUM  LAMP 

§  196.  The  petroleum  lamps  now  used  as  radiants  for  projec- 
tion have  two,  three  or  four  wicks.  The  wicks  are  wide  (about  five 
cm.,  two  in.)  and  are  placed  edgewise  to  the  condenser.  If  more 
than  two  wicks  are  used  the  two  outer  ones  are  inclined  inward 
(fig.  66). 

Sometimes  instead  of  being  ranked  side  by  side,  the  different 
wicks  are  arranged  like  the  lines  forming  the  letter  W,  but  there  is 
no  advantage  in  this. 


122 


MAGIC  LANTERN  WITH  PETROLEUM  LAMP          [Cn.  V 


FIG.  65.     MARCY'S  MAGIC  LANTERN  OR  "SCIOPTICON"  WITH    A 
MULTIPLE-WICK,  PETROLEUM  LAMP. 

(From  Dolbear's  Art  of  Projecting). 

a-b,  c-d     The  lenses  of  the  projection  objective. 

p-q     The  condenser  lenses. 

Z  61     The  oil  reservoir  of  the  lamp. 

E     The  flames  of  the  lamp  with  their  edges  toward  the  condenser. 

G-G  Two  glass  plates  at  opposite  ends  of  the  lamp-house  to  allow  the  light 
to  pass  to  the  condenser,  and  so  that  the  reflector  H  can  return  the  backward 
extending  light. 

C  I  J    The  chimney  and  ventilator  of  the  lamp-house. 

W  W    At  the  right,  the  milled  heads  for  turning  the  lamp-wicks  up  or  down. 

There  is  a  common  reservoir  and  a  common  chimney,  but  each 
wick  has  a  separate  burner  and  a  separate  mechanism  for  raising 
and  lowering  the  wick. 


§  197.  Chimney  and  reflector. — There  is  a  common  chimney. 
This  is  usually  of  metal  with  a  window  on  opposite  sides,  and  with 
either  a  telescoping  extension  or  a  segment  which  can  be  put  on  top 


CH.  V]          MAGIC  LANTERN  WITH  PETROLEUM  LAMP 


123 


for  getting  the  best  draught  when  the  lamp  is  turned  up  full 
height. 

The  reflector  is  a  concave  mirror  placed  with  its  center  of  curva- 
ture coinciding  with  the  flame.  This  serves  to  reflect  the  backward 
extending  light  to  a  focus  on  the  flame  again,  and  from  thence  it 
passes  onward  to  the  condenser  with  the  rays  passing  directly  from 
the  flame  to  the  condenser. 


FIG.  66.     MULTIPLE- WICK,  PETROLEUM  LAMP  FOR  THE  MAGIC  LANTERN. 

(From  the  Catalogue  of  the  Mclntosh  Battery  and  Optical  Company,  1889). 

This  figure  shows  that  there  is  a  single  oil  reservoir  but  four  separate  wicks, 
each  with  a  mechanism  for  turning  the  wick  up  or  down.  It  also  shows 
clearly  the  inclination  toward  each  other  of  the  separate  wick  holders,  and 
finally  that  the  lamp  has  a  single  chimney. 

The  openings  in  the  metal  chimney  for  the  reflector  and  the  con- 
denser must  be  covered  with  glass  or  with  clear  mica  or  the  lamp 
will  smoke. 

§  198.  Management  of  the  lamp. — Before  an  exhibition  the 
reservoir  is  filled  nearly  full  with  good  petroleum  (kerosene  oil). 
The  wicks  must  be  carefully  trimmed  until  the  flame  burns  without 
tails.  One  must  be  careful  in  preparing  the  lamp  not  to  get  any 
oil  on  the  outside,  for  when  the  lamp  gets  hot  this  oil  is  sure  to  smell 
badly. 


124 


MAGIC  LANTERN  WITH  PETROLEUM  LAMP          [Cn.  V 


Light  the  wicks  and  turn  them  up  moderately  and  allow  them  to 
burn  for  five  or  ten  minutes  before  the  exhibition.  This  is  to  get 
the  apparatus  warmed  up.  One  cannot  get  the  best  light  from  a 
petroleum  lamp  instantly,  but  only  after  it  has  become  warm. 
Finally  turn  up  each  wick  as  high  as  possible  without  having  it 
smoke.  The  central  wicks  can  usually  be  turned  higher  than  the 
marginal  ones.  When  the  wicks  are  at  their  full  height  the  chim- 
ney, if  adjustable,  must  also  be  at  its  full  height  to  give  the  best 
draught. 

After  the  exhibition  is  over  the  lamp-wicks  are  turned  down,  the 
small  flames  blown  out,  and  then  the  unused  oil  poured  into  a  con- 
tainer, the  wicks  taken  out  and  carefully  dried  between  blotting 


FIG.  67.     NEWTON'S  FOUR- WICKED,  PETROLEUM  LAMP  FOR  THE  MAGIC 

LANTERN. 
(From  Catalogue  No.  4  of  Newton  &  Co.). 

The  chimney  is  in  two  segments.     For  the  maximum  light  after  the  lamp  is 
warmed  up,  the  top  segment  is  added. 


CH.  V]          MAGIC  LANTERN  WITH  MANTLE  GAS  LAMP  125 

papers.  If  the  lamp  is  kept  perfectly  clean,  and  no  oil  is  allowed  to 
remain  on  the  outside,  the  disagreeable  smell  of  partly  oxidized  oil 
will  be  avoided. 

§  199.  Amount  of  oil  used. — It  takes  about  half  a  liter  (one 
pint)  of  kerosene  per  hour  for  the  best  lamps. 

§  200.  Candle-power  and  size  of  screen. — The  candle-power  of 
the  best  petroleum  lamps  is  not  much  above  100.  While  the  older 
lanternists  used  large  screens  (4  meters,  12  ft.  square)  it  is  better 
to  use,  with  this  light,  screens  of  small  size,  2  to  3  meters  square 
(6-9  ft.),  and  to  keep  in  mind  the  requirements  for  good  images 
with  these  feeble  lights  (§  193). 

§  201.  Relative  position  of  lamp  and  condenser. — In  general, 
the  middle  of  the  flame  should  be  in  the  axis  of  the  condenser  and 
it  should  be  at  about  the  principal  focal  distance  of  the  first  ele- 
ment of  the  condenser  from  it  (fig.  64).  One  must  get  the  best 
possible  position  at  any  one  time  by  experiment,  i.  e.,  by  moving 
the  light  a  little  closer  or  farther  away  than  the  focus  of  the  con- 
denser. For  the  two-lens  condenser  the  lamp  must  be  closer  than 
for  the  three-lens  condenser  (§  17,  55). 

§  202.  The  management  of  an  exhibition  is  as  described  in 
Chapter  I,  §  21-41,  and  above,  §  193-194. 

MAGIC  LANTERN  WITH  A  MANTLE  GAS  LAMP 

§  203.  Gas  and  gas  lamps. — The  illuminating  gas  may  be 
drawn  from  the  house  lighting  supply. 

The  lamps  are  of  two  kinds,  the  vertical  and  the  inverted  or 
reflex  form  (fig.  68-69).  The  burner  is  of  the  Bunsen  type.  It 
heats  the  mantle  to  incandescence.  While  there  is  a  very  brilliant 
light  and  a  great  deal  of  it,  the  source  is  very  large,  and  cannot  be 
utilized  so  completely  as  the  small  source  of  the  electric  arc  lamp 
(see  fig.  i,  64). 

§  204.  Position  of  the  incandescent  mantle. — As  this  is  the 
source  of  illumination,  the  middle  of  the  face  next  the  condenser 
should  be  on  the  horizontal  axis  (fig.  64). 


126 


MAGIC  LANTERN  WITH  MANTLE  GAS  LAMP        [Cn.  V 


FIG.  68.     UPRIGHT  GAS  BURNER  WITH  WELSBACH  MANTLE  AND 
CONCAVE  REFLECTOR  FOR  THE  MAGIC  LANTERN. 

(From  Max  Kohl,  A.  G.,  Price  List  No.  50,  Vol.  /).     • 

The  distance  from  the  condenser  giving  the  best  light  must  be 
determined  by  experiment,  as  with 
other  extended  sources.  But,  in  gen- 
eral, it  will  be  found  to  be  at  about 
the  principal  focal  distance  from  the 
first  element  of  the  condenser,  as 
with  the  arc  lamp,  but  closer  for  the 
two-lens  than  for  the  three-lens  con- 
denser §  (55). 

§  205.  Reflector.  —  As  with  the  pe- 
troleum light,  a  concave  reflector  is 
sometimes  used  behind  the  mantle  to 
reflect  back  to  the  mantle  and  thence 
to  the  condenser  the  light  which  passes 
backward  from  the  mantle.  This  is 
not  always  used,  but  it  would  increase 
the  light  somewhat  (§  210). 

-.  FIG.  69.     INVERTED  GAS  BUR- 

§  206.      Connecting  the  gas  supply      NER  WITH  WELSBACH  MAN- 


with  the  lamp.—  Use  for  this  a  perfect 

11        ^   1  £  ^      n     -1  1 

rubber  tube  or  one  of  the  flexible  me- 

tallic   tubes  (fig.  60),  and  secure  the 


TLE  FOR  THE  MAGIC 

LANTERN. 
(From  Schmidt  und  HaenMs  Cata_ 

ggg.**  Iv'  P"***** 


CH.  V]  MAGIC  LANTERN  WITH  ACETYLENE  LAMP  127 

ends  to  their  connections  by  tying  a  string  tightly  around 
them,  if  rubber  tubes  are  used.  If  the  supply  is  at  a  con- 
siderable distance  there  should  be  a  stop-cock  at  the  lamp  to 
regulate  the  amount  of  gas,  and  to  turn  it  off  completely  if  desired. 
At  the  end  of  the  exhibition  the  gas  must  be  turned  off  at  the  source 
of  supply. 

§  207.  The  management  of  the  exhibition  is  simple,  and  should 
follow  the  general  lines  laid  down  in  Chapter  I  (§  21-41).  It  is 
not  wise  to  try  to  use  a  screen  more  than  two  to  three  meters  square 
(6-9  ft.),  and  one  must  keep  in  mind  the  requirements  for  feeble 
lights  (§  193). 

THE  MAGIC  LANTERN  WITH  AN  ACETYLENE  LAMP 

§  208.  Source  of  acetylene. — This  may  be  from  a  house  supply, 
a  special  generator,  or  from  a  tank  or  cylinder  of  acetylene  dis- 
solved in  acetone  under  pressure  (prestolite  tank). 

§  209.  Acetylene  lamp. — The  burners  now  used  are  in  pairs. 
Two  jets  set  at  an  angle  give  a  fused,  flat  flame.  For  the  magic 
lantern  the  lamp  has  from  one  to  four  of  these  twin  burners  in 
a  line.  Behind  the  burner  is  a  concave  reflector  returning  the 
backward  reflected  light  to  the  burner  and  from  thence  on  to  the 
condenser,  so  that  as  much  of  the  light  as  possible  is  utilized  for  the 
screen  image  (fig.  70). 

§  210.  Position  of  the  concave  mir- 
ror.— If  a  concave  mirror  is  used  to  save 
the  light  extending  away  from  the  screen, 
its  center  of  curvature  should  coincide 
with  the  flame  of  a  single  burner,  or  its 
center  should  be  at  the  middle  flame,  if 
there  are  several  burners  in  a  row. 
FIG.  70.  DOUBLE-JET  The  acetylene  flame  is  very  transpar- 

REEFLEEcNTEoR  TOR  T™  ent'  so  that  *  m{rrQr  behind  the  burner 
MAGIC  LANTERN.  will  increase  the  light  nearly  the  theo- 

*     retical  amount  (75%),  while  with  nearly 


128 


MAGIC  LANTERN  WITH  ACETYLENE  LAMP 


[CH.  V 


opaque  sources,  such  as  the  incandescent  mantle  light  or  the 
petroleum  flame,  a  mirror  placed  behind  the  light  does  not  in- 
crease the  brilliancy  so  much. 

§  211.  Position  of  the  acetylene  lamp. — This  should  be  so  that 
the  middle  point  of  the  flame  is  on  the  axis  (fig.  64)  and  it  should 
be  at  a  distance  from  the  condenser  of  about  the  principal  focal 
length  of  the  first  element  of  the  condenser  and  the  middle  flame 
of  the  burner.  For  the  best  position  in  practice  one  must  experi- 
ment while  looking  at  the  screen  image  or  disc  of  light,  and  arrange 
the  lamp  to  give  the  best  effect  (§  17,  55). 


B 


FIG.  71.    UPPER  AND  LOWER  ENDS  OF  A  PRESTOLITE  TANK  USED  WITH 

THE  MAGIC  LANTERN. 
FIG.  7 1 A.     UPPER  END  OF  THE  PRESTOLITE  TANK. 

V    Outlet  valve.     It  is  opened  and  closed  by  a  special  wrench. 

Connector  The  metal  connector  for  joining  the  gas  supply  and  the  acetylene 
burner. 

Rt  Rubber  or  flexible  metal  tube  extending  from  the  connector  to  the 
burner. 

N  Nut  for  holding  the  conical  part  of  the  connector  in  gas-tight  union  with 
the  hollow  cone  of  the  tank-valve.  This  valve  must  be  set  gas-tight  before 
opening  the  outlet  valve  (V}. 

FIG.  7iB.     LOWER  END  OF  THE  PRESTOLITE  TANK  SHOWING  THE 

PRESSURE  GAUGE. 

P  G  Pressure  gauge  indicating  the  pressure  of  the  gas  within  the  tank. 
The  pressure  is  given  in  atmospheres  or  in  pounds  per  square  inch  or  in  both. 


CH.  V]          MAGIC  LANTERN  WITH  ACETYLENE  LAMP  129 

§  212.  Connecting  the  burner  to  the  gas  supply. — For  this  a 
heavy  and  perfect  rubber  tube  or  a  flexible  metallic  tube  (fig.  60) 
should  be  used  and  the  connections  with  the  supply  and  with  the 
burner  should  be  tied  unless  special  fittings  are  present. 

As  with  illuminating  gas,  the  best  light  is  obtained  when  the 
correct  amount  of  gas  is  delivered  at  the  tip  of  the  burner.  If  too 
much  gas  is  flowing  the  jets  will  blow,  and  if  too  little,  there  will 
not  be  light  enough. 

If  a  tank  of  compressed  acetylene  in  acetone  is  used  (fig.  71  A), 
the  adjustments  must  be  made  at  the  valve  on  the  cylinder.  If  one 
turned  this  on  full  head  and  tried  to  regulate  by  the  stop-cock  at 
the  burner  the  pressure  accumulating  in  the  rubber  tube  would 
probably  blow  the  tube  from  its  connections  or  burst  it  (§  2 1 2a) . 


§  212a.     Prestolite  tanks  supplying  acetylene  for  the  Magic  Lantern. — A 

steel  cylinder  is  packed  with  asbestos  and  this  is  saturated  with  acetone. 
Acetylene  gas  is  then  pumped  into  the  cylinder  and  is  dissolved  by  the  acetone. 

The  tanks  are  charged  under  a  pressure  of  approximately  15  atmospheres  at 
183^3  degrees  centigrade  (65°  F.)  this  is  15.82  kilos  per  square  centimeter  or  225 
Ibs.  to  the  square  inch. 

The  tanks  are  of  various  sizes,  and  their  holding  capacities,  under  15  atmos- 
pheres pressure,  are  as  follows: 

"A"  contains  70  cubic  feet  of  gas,  (1982  liters),  cost $25.00 

"B"  contains  40  cubic  feet  of  gas,  (1132.6  liters),  cost $18.00 

"E"  contains  30  cubic  feet  of  gas,  (849.5  liters),  cost $15.00 

Motor-cycle  tank  contains  10  cubic  feet  of  gas,  (283  liters),  cost $  8.00 

The  burner  for  a  magic  lantern  requires  from  one  to  two  cubic  feet  of  acety- 
lene gas  per  hour.  The  motor-cycle  tank  full  of  gas  will  then  supply  light,  for 
from  five  to  ten  hours.  It  costs  less  than  $1.00  to  have  the  tank  recharged, 
hence,  the  cost  of  gas  per  hour  is  from  ip  to  20  cents. 

It  is  of  importance  to  know  at  any  given  time  whether  there  is  gas  enough 
to  last  for  an  exhibition  or  for  a  number  of  exhibitions.  As  shown  with  the 
lime  light  the  cylinders  are  supplied  with  a  gauge  showing  the  pressure  of  the 
gas  within  the  cylinder,  and  from  Boyle's  law  that  the  amount  of  a  gas  in  a 
given  space  depends  on  the  pressure,  it  is  easy  to  determine  at  any  time  the 
amount  of  gas  available.  It  is  only  necessary  to  know  the  capacity  of  the 
cylinder  under  ordinary  atmospheric  pressure  and  to  multiply  that  volume  by 
the  number  of  atmospheres  indicated  on  the  pressure  gauge  (see  also  §  156). 

For  example,  the  gauge  of  a  motor-cycle  tank  (fig.  71  B),  shows  that  the 
pressure  is  12  atmospheres,  how  many  cubic  feet  of  acetylene  gas  are  avail- 
able? 

As  the  tank  under  15  atmospheres  holds  10  cubic  feet  of  gas  its  capacity  at 
atmospheric  pressure  must  be  10-^-15  =  %  of  a  cubic  foot.  If  it  holds  %  of 
a  cubic  foot  under  one  atmosphere,  under  12  atmospheres  pressure  it  will  hold 
Yz  multiplied  by  12  =8  cubic  feet. 

The  tank  will  then  supply  gas  for  four  or  for  eight  hours  of  continuous  light 
depending  upon  the  capacity  of  the  burner. 


130  LANTERN  WITH  ALCOHOL  LAMP  AND  MANTLE        [Cn.  V 

§  213.  The  management  of  an  exhibition  is  as  for  the  direct 
current  arc  lamp,  keeping  in  mind  the  general  statements  in  this 
chapter  (Ch.  I,  §  21-40;  §  193). 

THE  MAGIC  LANTERN  WITH  ALCOHOL  LAMP  AND  MANTLE 

(  §  214.  An  alcohol  flame  burning  in  the  air,  is  very  hot.  This 
has  been  taken  advantage  of  to  heat  a  mantle  to  incandescence  in 
the  same  way  that  illuminating  gas  with  a  Bunsen  burner  heats  a 
mantle  to  incandescence. 


FIG.  72.     MAGIC  LANTERN  WITH  THE  ALCO-RADIANT. 

(Cut  loaned  by  Williams,  Brown  &  Earle). 
For  the  details  see  fig.  32  and  73. 

For  the  best  results  the  alcohol  is  vaporized,  and  the  vapor  burn- 
ing in  a  special  burner  gives  the  Bunsen  flame  necessary  to  heat  the 
mantle. 

The  light  is  as  intense  or  more  intense  than  gas  light  with  a 
mantle. 

§  215.  Alcohol  supply  and  burner. — There  must  be  a  reservoir 
for  alcohol  (95%  ethyl,  methyl,  or  denatured).  This  is  connected 
with  the  burner  by  means  of  a  metal  tube  with  a  stop-cock.  In 
use  the  reservoir  is  filled  over  half  full,  but  must  always  have  an 
air  space  above.  Connected  with  this  air  space  is  a  force-pump 
by  which  the  alcohol  is  put  under  pressure. 


CH.  V]       LANTERN  WITH  ALCOHOL  LAMP  AND  MANTLE  131 

§  216.  Lighting  the  lamp. — (i)  Place  the  lamp  in  a  metal  tray; 
put  a  mantle  in  position  over  the  burner,  and  burn  it  off  as  for  a  new 
gas  mantle. 

(2)  Place  the  heater  or  torch  in  position  under  the  burner 
(fig.  73  L) .  Wet  the  torch  well  with  strong  alcohol,  using  a  pipette. 
Sometimes  the  torch  is  saturated  with  alcohol  by  pouring  the 
alcohol  upon  it  from  a  bottle  before  it  is  put  in  place  under  the 
burner.  This  is  usually  wasteful,  as  some  alcohol  is  almost  sure 
to  be  spilled. 


FIG.  73.     ALCO-RADIANT,  SHOWING  THE  PARTS. 

(Cut  loaned  by  Williams,  Brown  &  Earle). 

BM    The  mantle. 

BS    The  gas  burner  for  the  volatilized  alcohol. 

L  H    The  heater  to  start  the  volatilization. 

S    The  handle  for  opening  and  closing  the  air  valve  of  the  burner. 

R     Valve  for  turning  on  and  off  the  alcohol  supply. 

W    The  tank  holding  the  alcohol  supply. 

T    Connection  for  the  pressure  tube. 

Z  Rubber  bulb  for  forcing  air  into  the  alcohol  reservoir.  The  round 
object  in  the  course  of  the  rubber  tube  is  an  air  reservoir  to  make  the  pressure 
steady. 


132          LANTERN  WITH  ALCOHOL  LAMP  AND  MANTLE      [Cn.  V 

(3)  When  the  torch  is  in  place  and  wet  with  alcohol,  open  the 
stop-cock  from  the  supply  tank  (fig.  73   R),  and  then  light  the 
torch.     The  alcohol  flame  will  heat  the  burner  and  stand-pipe,  and 
the  alcohol  in  the  stand-pipe  will  be  vaporized  and  pass  over 
through  the  small  pipe  to  the  burner  where  it  will  catch  fire  and 
burn.     Open  the  air  intake  partly.     In  using  the  lamp  this  air 
intake  must  be  regulated  as  for  a  Bunsen  burner,  the  more  pressure 
the  more  the  valve  must  be  opened. 

Soon  the  mantle  should  begin  to  glow  brightly  from  the  burning 
vapor  in  the  burner.  When  this  occurs  commence  to  put  pressure 
on  the  alcohol  tank  (fig.  73  W).  This  is  done  by  connecting  the 
pressure  apparatus  by  means  of  the  rubber  tube  to  the  alcohol 
tank,  at  T,  (fig.  73),  and  squeezing  the  bulb. 

In  case  the  first  burning  off  of  the  torch  does  not  start  the  lamp 
one  must  burn  it  off  again,  but  do  not  add  the  alcohol  until  the 
torch  or  heater  is  out,  and  then  use  a  pipette.  Relight  the  heater 
and  it  will  almost  surely  start  the  lamp. 

Do  not  connect  the  pressure  apparatus  until  the  mantle  com- 
mences to  glow.  If  pressure  were  on  the  alcohol  tank  at  first  the 
liquid  alcohol  would  be  forced  over  from  the  stand-pipe  into  the 
burner  and  would  run  down  on  the  torch  and  upon  the  table. 
Remember  that  alcohol  is  very  inflammable  and  also  very  unman- 
ageable when  it  is  on  fire,  so  be  exceedingly  careful. 

(4)  As  soon  as  the  mantle  begins  to  glow  brilliantly  consider- 
able pressure  can  be  put  on  the  alcohol  tank.     The  greater  the 
pressure  the  wider  must  the  air-intake  at  the  burner  be  opened  and 
the  more  brilliant  will  be  the  light ;   but  as  the  pressure  increases 
the  lamp  roars  more  loudly  until,  when  the  pressure  is  considerable, 
it  roars  like  a  young  blast  furnace.     By  watching  the  results  one 
can  avoid  the  excessive  noise,  and  still  get  a  brilliant  light. 

§  217.  Management  of  the  exhibition. — This  is  in  general  like 
any  other  magic  lantern,  but  as  the  light  depends  largely  on  the 
pressure  regulation,  one  must  be  careful  to  keep  up  the  proper 
amount  of  pressure  during  the  entire  time.  Do  not  expect  too 
much  of  this  light.  It  gives  fairly  good  lantern-slide  images  for  a 
screen  from  two  to  three  meters  (six  to  nine  ft.)  square.  As  the 


CH.  V]  TROUBLES  IN  CHAPTER  V  133 

source  is  large,  one  needs  a  good  projection  objective  of  large  aper- 
ture (see  §85 5). 

§  218.  Putting  out  the  lamp. — As  this  lamp  is  difficult  to  light 
it  should  be  kept  burning  during  the  entire  exhibition.  One  can 
shut  the  light  from  the  screen  by  the  objective  shield  (fig.  62). 

At  the  close  of  the  exhibition,  take  the  lamp  from  the  lamp-house, 
remove  the  rubber  tube  from  the  pressure  apparatus  to  the  tank 
to  relieve  all  pressure  on  the  alcohol.  Close  the  supply  valve  so 
that  no  more  alcohol  can  pass  over  to  the  stand-pipe.  Close  the 
air-intake  of  the  burner.  Use  a  sponge  well  wet  with  water  and 
apply  it  to  the  burner  as  near  the  mantle  as  possible  without  touch- 
ing the  mantle.  The  sponge  will  naturally  rest  against  the  small 
conducting  pipe  and  the  stand-pipe  in  this  operation.  This  cools 
the  burner  and  the  stand-pipe  and  stops  the  vaporization  of  the 
alcohol.  The  flame  then  goes  out  as  with  any  gas  burner  when  the 
supply  of  gas  is  cut  off. 

§  219.  Precautions. — Remember  that  alcohol  is  very  inflam- 
mable, therefore  special  care  should  be  exercised  that  none  of  it 
overflows  from  the  reservoir  or  leaks  from  poor  joints.  It  is  per- 
fectly safe  in  burning  through  the  burner,  but  any  alcohol  outside 
the  lamp  is  dangerous,  for  if  it  catches  fire  it  cannot  be  extinguished 
unless  one  has  plenty  of  sand  or  non-inflammable  dust  to  throw  on 
it  and  smother  the  flame,  or  one  of  the  modern  chemical  fire  extin- 
guishers. 

TROUBLES  IN  CHAPTER  V 

§  220.  The  prime  difficulty  with  these  relatively  weak  lights 
is  the  dim  screen  pictures.  That  is,  they  will  be  dim  in  comparison 
with  the  bright  pictures  obtainable  with  the  direct  current  arc 
light. 

Remember  the  conditions  requisite  for  screen  images  with  weak 
lights  (§  193). 

§  221.  Smoking  of  the  petroleum  lamp  or  of  the  acetylene 
burner. — This  shows  that  the  wicks  are  not  properly  trimmed  or 
that  they  are  turned  up  too  high  for  the  height  of  the  chimney. 


134  TROUBLES  IN  CHAPTER  V  iCH.  V 

With  the  acetylene  flame  if  too  much  gas  is  turned  on  the  flame 
will  smoke  and  roar. 

§  222.  The  image  of  the  lamp  flame  may  show  on  the  screen. 
This  is  because  the  objective  is  too  far  from  the  condenser  or  the 
lamp  flame  is  not  in  the  proper  position  with  reference  to  the  con- 
denser. Try  removing  the  lamp  farther  from  the  condenser  or 
bringing  it  nearer.  When  it  is  in  the  correct  position  its  image 
will  not  appear  on  the  screen. 

§  223.  Roaring  of  the  alco-radiant  lamp.  If  the  roaring  is 
excessive  it  shows  that  the  pressure  on  the  alcohol  reservoir  is  too 
great.  This  can  be  remedied  by  ceasing  to  pump  the  air  in 
till  the  noise  is  within  reasonable  bounds. 


CH.  V] 


DO  AND  DO  NOT  IN  CHAPTER  V 


135 


§  224.     Summary  of  Chapter  V: 
Do 

i.  For  these  relatively  weak 
sources  of  light  use  a  good 
screen,  and  make  the  room  dark 

(§  193). 


2.  Use 
slides. 


transparent    lantern 


Do  NOT 

i.  Do  not  try  to  give  an  ex- 
hibition with  these  weak  lights 
in  a  room  with  much  stray  light, 
and  do  not  use  a  dirty  screen. 


2.  Do  not  try  to  use  opaque 
lantern  slides. 


3.  The  objective  to  select  is 
one  of  large  aperture  for  these 
large  sources  (§  217,  855). 

4.  Have    perfect    containers 
for  liquids  and  gases   so  that 
none  can  escape  into  the  room. 


5.  For  the  petroleum  light 
and  the  alco-radiant  use  the 
objective  shield  (fig.  62)  as  it 
takes  so  long  to  get  a  good  light. 


3.  Do  not  use  an  objective  of 
small  aperture  with  these  large 
sources. 

4.  Do    not    use    leaky    con- 
tainers for  the  gases  or  liquids 
used  in  this  chapter.     They  are 
all  dangerous  when  out  of  their 
proper  containers. 

5.  Do  not  turn  off  the  alco- 
radiant  or  the  petroleum   light 
during  the  exhibition;    it  takes 
too  long  to  start  them. 


6.  Follow  carefully  the  direc- 
tions sent  with  the  apparatus 
by  the  manufacturers. 


6.  Do  not  fail  to  read  care- 
fully and  follow  strictly  the 
directions  sent  out  by  the  manu- 
facturers. 


7.  Do  your  part  with  great 
care  and  even  these  weak  lights 
will  give  good  projection  within 
their  range  of  possibility,  i.  e., 
for  a  screen  two  to  three  meters 
(six  to  nine  feet)  square. 


7.  Do  not  expect  too  much  of 
these  weak  sources,  but  give 
them  a  chance  to  do  their  best. 


136 


DO  AND  DO  NOT  IN  CHAPTER  V 


[CH.  V 


Do 

1.  Use    a    good    quality    of 
petroleum  (kerosene). 

2.  Keep  the  lamp  clean,  and 
the  wicks  properly  trimmed. 


3.  Use    a    chimney    of    the 
proper  height  for  the  flame. 

4.  Turn  the  flame  up  as  high 
as  possible  without  having  it 
smoke. 

5.  The   edge    of   the   flames 
should  face  the  condenser,  the 
middle  flame  being  in  the  axis. 


Do  NOT 

1 .  Do  not  use  poor  oil,  it  will 
not  give  a  good  light,  and  may 
explode. 

2.  Do  not  let  the  lamp  get 
dirty  or  the  wicks  burn  with 
tails.     Clean  and  trim. 

3 .  Do  not  use  a  low  chimney 
for  a  large,  high  flame. 

4.  Do  not  turn  the  wicks  up 
till     they     smoke.     Stop     just 
before  that. 

5.  Do  not  have  the  face  of  the 
flame,  but  the  edge  toward  the 
condenser. 


Do 

1.  For  gas  use  the  best  kind 
of  mantles. 

2 .  Make  the  connections  with 
rubber  tubing  of  good  thickness 
and  quality  or  flexible  metallic 
tubing  (fig.  60). 


Do  NOT 

1 .  Do  not  use  mantles  of  poor 
quality,  or  that  are  broken. 

2.  Do  not  make  connections 
with   thin   or  used   up   rubber 
tubing. 


Do 

1 .  For  acetylene  use  a  proper 
burner   and  reflector,   that  is, 
one  which  is  made  by  a  reliable 
house  that  has  proved  its  safety 
and  excellence. 

2.  Use    a    safe    gas    supply, 
such  as  a  house  supply   or   a 
prestolite  tank  is  best. 


Do  NOT 

1.  Do    not    use    an    untried 
lamp  and  general  outfit  for  the 
acetylene  light.     Acetylene  is  a 
good  servant  but  a  cruel  master. 

2.  Do  not  try  to  use  a  make- 
shift gas  generator.     The  smell 
will   be    disagreeable    and    the 
escaping  gas  possibly  dangerous 


CH.  V] 


DO  AND  DO  NOT  IN  CHAPTER  V 


137 


3.  Use  thick  and  good  quality 
rubber  tubing  or  flexible  metal- 
lic tubing  (fig.  60)  to  make  the 
connections. 

4.  Study  carefully  the  direc- 
tions for  the  use  of  the  acetylene 
outfit   with  the  magic  lantern 
sent  out  by  the  manufacturers. 

5.  Use  perfect  burners  with 
the  gas  turned  on  sufficiently, 
but  not  enough  to  blow. 

6.  Keep  all  naked  lights  away 
from  an  acetylene  supply.     Use 
an  electric  torch  light  if  a  light 
must  be  used. 


3.  Do  not  use  poor  rubber 
tubing  for  connections. 


4.  Do  not  neglect  the  careful 
study  of  the  directions  for  using 
the    acetylene    outfit  with  the 
magic  lantern. 

5.  Do  not  try  to  use  broken 
burners,  and  do  not  turn  the  gas 
on  until  it  blows. 

6.  Never  let  any  naked  lights 
come  near  an  acetylene  gas  sup- 
ply. 


Do 

i .  For  the  alcohol  light ,  follow 
with  care  the  directions  accom- 
panying your  alco-radiant  lamp. 
Alcohol  is  dangerous  stuff  and 
should  not  be  trifled  with. 


Do  NOT 

i.  Do  not  fail  to  follow  with 
scrupulous  care  the  directions  of 
the  manufacturers  of  the  lamp 
you  use. 


CHAPTER  VI 
THE  MAGIC   LANTERN  WITH   SUNLIGHT:    HELIOSTATS 

§  230.     Apparatus  and  material  for  Chapter  VI : 

Suitable  room  for  projection,  preferably  one  with  southern 
exposure ;  Screen  of  proper  size ;  Porte-Lumiere  or  hand-regulated 
heliostat;  Heliostat  with  clock-work  for  regulation;  Condenser 
for  bringing  the  parallel  rays  of  sunlight  to  a  focus  (plano-convex 
or  achromatic  combination) ;  Slide-carrier  and  projection  objective. 

See  also  Ch.  I,  §  i. 

§  231.     Historical. 

For  the  history  of  the  magic  lantern  and  all  other  projection 
apparatus  with  sunlight,  see  the  Appendix. 

For  Foucault's  clock-driven  heliostat  see  his:  Recueil  des 
Travaux  Scientifiques,  1878,  pp.  427-433. 

For  the  Heliostat  of  Mayer,  using  a  lens  and  prisms,  see  Amer. 
Journal  of  Science,  IV  Ser.  Vol.  IV,  (1897),  pp.  306-308. 

For  the  Heliostats  of  Fuess,  see  C.  Leiss,  Die  Optischen  Instru- 
mente  der  Firma  R.  Fuess,  1899,  pp.  284-305.  For  Heliostats 
like  fig.  82,  see  Ambronn's  Handbuch  Astron.  Instr.  p.  649,  fig.  637. 

Dolbear. — Art  of  Projecting. 

LIGHT  FROM  THE  SUN 

§  232.  The  limitless  supply  of  light  from  the  sun  would  be  used 
in  preference  to  any  artificial  source  if  it  were  only  always  avail- 
able. In  many  regions  it  is  available  during  most  of  the  year,  and 
will  no  doubt  be  much  more  utilized  as  time  goes  on.  Its  use  is 
strongly  recommended  in  sunny  regions. 

The  sun  is  the  brightest  known  source  of  light.  Its  intrinsic 
brilliancy  is,  in  round  numbers,  421,000  candle-power  per  square 
centimeter  (2,720,000  candle-power  per  sq.  inch).  (See  §  232a). 

Sunlight  also  serves  as  the  standard  for  color  values. 

§  232a.  The  intrinsic  brilliancy  of  the  sun. — The  intrinsic  brilliancy  of  a 
source  can  be  determined  if  its  area  and  its  candle-power  are  known.  With  the 
sun  it  is  inconvenient  to  make  the  reckoning  in  these  terms  as  both  the  candle- 
power  and  distance  are  so  enormous.  The  light  from  the  sun  near  the  zenith 
in  clear  weather  amounts  to  288,000  meter  candles,  that  is,  the  sunlight  is  as 
powerful  as  the  illumination  due  to  288,000  standard  candles  at  a  distance  of 
one  meter.  (A.  Arrhenius — Lehrbuch  der  kosmischen  Physik). 

138 


CH.  VI] 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


139 


§  233.     Heliostat. — From  the  rotation  of  the  earth  on  its  axis 
from  west  to  east  the  sun  seems  to  move  over  the  face  of  the  sky 


N 


FIG.  74.     MAGIC  LANTERN  WITH  SUNLIGHT. 

5     Sunlight. 

Mirror  The  plane  mirror  serving  to  direct  the  sunlight  horizontally  into 
the  condenser. 

Condenser  The  single  plano-convex  lens  serving  to  converge  the  parallel 
beam  of  sunlight.  (Compare  the  second  element  of  the  condenser  in  fig.  2). 

Ls     Lantern  slide. 

Objective  The  projection  objective  for  projecting  an  image  upon  the  white 
screen.  The  projection  objective  and  the  condenser  should  be  of  approxi- 
mately the  same  focus. 

c  Center  of  the  projection  objective  where  the  rays  from  the  condenser 
should  cross. 

Axis  Axis  The  principal  optic  axis  of  the  condenser  and  of  the  projection 
objective. 

Screen  Image     The  image  of  the  lantern  slide  upcn  the  white  screen. 


The  apparent  diameter  of  the  sun's  disc  is  32 '36"  in  midwinter  and  31'  32"  in 
midsummer,  or  it  averages  32'  04"  (Abbot,  The  Sun,  p.  3;  Ball's  Astronomy, 
P-  127). 

The  apparent  area  of  the  sun's  disc  at  a  distance  of  one  meter  is  determined 
as  follows:  Its  diameter  is  32'o4"  or  .5343°.  One  centimeter  at  a  distance  of 
one  meter  subtends  an  angle  of  .573°,  hence  at  one  meter  the  sun's  disc  would 

appear  to  have  a  diameter  of =  .933  centimeters.     The  area  of  such  a 

•573 


circle  is 


•9332 


x  TT  =  .684  square  centimeters. 


The  intrinsic  brilliancy  is  then, 


Candle-power         288,000 


=  in  round  num- 


Area  .684 

bers,  421,000  candle-power  per  square  centimeter  or  2,720,000  candle-power  per 
square  inch. 


140  HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 

from  east  to  west.  In  order  that  the  sun's  rays  may  shine  in  one 
place  continuously  it  is  necessary  to  counterbalance  in  some  way 
the  apparent  motion  of  the  sun. 

If  one  holds  a  plane  mirror  in  the  hands,  it  is  possible  to  keep  a 
spot  of  sunlight  on  one  place  indefinitely  by  making  slight  changes 
in  the  position  of  the  mirror  to  correspond  with  the  changes  in 
apparent  position  of  the  sun.  This  is  possible  from  the  law  of 
reflection:  "The  angle  of  incidence  and  the  angle  of  reflection  are 
equal."  (See  fig.  80  and  Chap.  XIV,  §  794). 

A  heliostat  is  then  simply  a  mechanism  for  holding  the  mirror 
so  that  the  sun's  rays  may  be  reflected  in  a  constant  direction.  As 
this  seems  to  make  the  sun  stand  still,  the  name  is  appropriate.  It 
was  given  by  the  original  inventor,  s'Gravesande,  1742  (fig.  77). 

There  are  three  principal  forms  of  heliostats : 

(1)  The  hand-regulated  heliostat  or  porte-lumiere  with  one 
mirror  and  a  double  movement  up  and  down,  and  on  the  axis,  so 
that  it  may  be  made  to  follow  accurately  the  sun's  apparent  motion 

(%.  75). 

(2)  A  heliostat  with  one  mirror,  in  which  the  movements  of  the 
mirror  are  brought  about  by  clock-work  (fig.  77-79). 

(3)  A  Heliostat  with  two  mirrors.     One  mirror  is  attached  to 
the  end  or  side  of  the  clock-shaft.     The  other  mirror  is  not  con- 
nected with  the  clock-work.     The  second  mirror  serves  to  reflect 
the  beam  from  the  movable  mirror  in  the  desired  direction,  and 
is  set  by  hand,  once  for  all,  at  the  beginning  of  the  experiment. 

The  clock-shaft  rotates  once  in  24  hours  with  the  single  mirror 
heliostats  (fig.  77-79),  and  also  with  the  two  mirror  heliostat 
with  the  mirror  at  the  end  of  the  clock-shaft  (fig.  81). 

When  the  mirror  is  attached  parallel  to  the  clock-shaft  (fig.  82), 
the  clock-shaft  rotates  once  in  48  hours  (fig.  82,  A,  B,  C). 

INSTALLATION  AND  USE  OF  A  HAND-REGULATED  HELIOSTAT  OR 
PORTE-LUMIERE 

§  234.  The  hand-regulated  heliostat  or  porte-lumiere  consists 
of  a  plane  mirror  so  mounted  that  it  can  move  on  two  axes.  The 
mirror  should  be  about  15  x  30  cm.  (6x12  in.)  in  size  and  sup- 


CH.  VI] 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


141 


ported  by  a  framework.  This  frame  should  be  hinged  so  that  it 
can  be  moved  from  the  horizontal  up  to  the  vertical  position.  It 
must  also  be  so  mounted  that  it  can  be  rotated  around  at  right 
angles  to  the  hinge  motion. 


FlG.  75.     PORTE-LUMIERE  WITH  PLANO-CONVEX  LENS  AND  PROJECTION 

OBJECTIVE. 
(Cut  loaned  by  the  C.  H.  Stoelting  Co.). 

The  mirror  for  the  reflection  of  the  sunlight  into  the  condenser  is  moved  as 
necessary  by  the  milled  head  just  above  the  condenser. 

The  two  movements  are  made  by  hand  as  often  as  needed  while 
one  is  using  the  apparatus.  In  the  original  forms  of  Cuff  and 
Adams  (1744-1746,  fig.  75a),  and  in  some  modern  forms,  there  are 
two  handles  or  milled  heads  extending  into  the  projection  room; 
with  one  of  them  the  operator  can  raise  or  lower  the  mirror  on  its 
hinges,  and  by  the  other  he  can  rotate  it  on  the  other  axis.  In  the 
form  here  shown  there  is  but  one  handle.  This  serves  as  a  crank 


142 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


[CH.  VI 


to  turn  the  mirror  around  in  a  circle  on  its  axis,  and  as  a  screw  by 
means  of  which  it  is  raised  or  lowered  on  its  hinges  (fig.  75). 

§  235.  Setting  up  the  hand-regulated  heliostat. — The  appara- 
tus must  be  so  placed  that  it  receives  the  full  sunshine  on  the 
mirror. 

In  the  forenoon  an  eastern  exposure  can  be  used,  and  in  the 
afternoon  a  western  one;  or  a  southern  one  nearly  all  day.  In 
practice  a  person  will  naturally  use  the  window  best  adapted  to  his 
particular  needs  if  he  has  a  choice. 


FIG.  75a.     SOLAR,  PROJECTION  MICROSCOPE  OF  ADAMS,  WITH 

PORTE-LUMI^RE. 

(From  Adams'  Essays,  1771,  PL  VI). 

Fig.  4  shows  the  movable  mirror  (K-L)  placed  outside  the  shutter  in  the  sun, 
0-P,  screws  in  the  square  plate  to  fasten  the  instrument  in  the  shutter;  M-N; 
thumb  screws  by  which  the  mirror  is  turned  to  hold  the  sun's  rays  in  the  right 
direction.  The  large  tube,  A-C-D,  contains  the  condenser  and  receives  the 
shorter  tube,  fig.  5.  Fig.  5  shows  the  tube  into  which  the  objectives  are  fixed. 
If  for  large  objects  the  lens  (fig.  6)  is  screwed  into  the  end,  g,  for  smaller  objects, 
the  objectives  are  arranged  in  a  piece  (fig.  8)  sliding  into  the  opening  at  q. 
Notches  along  the  objective  slider  indicate  when  the  lens  is  centered.  The 
specimen  to  be  examined  is  inserted  at  In.  For  high  powers  the  substage  con- 
denser shown  at  fig.  7  is  put  in  the  tube  between  d-h.  At  b  is  a  rack  and  pinion 
for  focusing  the  object. 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN  143 

§  236.  Darkening  the  room. — The  room  is  darkened  in  the 
usual  way  with  curtains  or  shutters.  The  window  where  the 
apparatus  is  to  be  placed  must  be  darkened  by  a  shutter  or  a  cur- 
tain with  a  hole  in  it,  through  which  the  instrument  may  be  ex- 
tended out  into  the  sunshine  and  through  which  the  sunshine 
can  be  reflected  into  the  room. 

The  window  frame  must  either  be  raised  entire  or  one  of  the 
panes  must  be  hinged  so  that  it  can  be  opened  when  desired.  One 
can  use  the  heliostat  within  the  room  utilizing  the  sunlight  passing 
through  the  window  glass,  but  this  is  far  less  satisfactory  than  hav- 
ing the  heliostat  out  in  the  free  air  where  the  sun  shines  directly 
upon  it. 

Finally  it  must  be  possible  to  close  the  openings  completely  so 
that  the  room  may  be  made  as  dark  as  desired. 

§  237.  Operation  of  the  apparatus. — In  starting  work  at  any 
time  the  mirror  is  inclined  on  its  hinges  and  rotated  until  the  sun 
shines  upon  it,  and  then  until  the  light  is  reflected  into  the  con- 
denser. Finally  some  further  slight  changes  may  be  necessary  to 
get  the  light  accurately  centered  so  that  it  will  pass  from  the 
condenser  along  the  common  axis  to  the  objective  and  thence  to 
the  screen  (fig.  74).  By  changing  the  position  of  the  mirror 
slightly  every  three  to  five  minutes  to  compensate  for  the  apparent 
motion  of  the  sun,  the  light  will  continue  to  pass  through  the  magic 
lantern  to  the  screen. 

§  238.    Adjustments  necessary  for  the  different  windows.— 

(A)  For  a  southern  exposure — For  this  exposure  it  is  desirable 
to  have  the  entire  outfit  in  a  north  and  south  direction  with  the 
objective  pointing  toward  the  north.  In  the  morning  the  mirror 
is  turned  on  its  hinges  to  about  45°  and  then  rotated  toward  the 
east  until  it  receives  the  light  of  the  sun  (fig.  76).  It  must  then 
be  turned  slightly  by  one  or  both  of  its  possible  movements  until 
the  light  is  reflected  in  the  desired  direction.  As  the  sun  continues 
to  rise  in  the  sky  the  mirror  must  be  rotated  on  the  axis  from  the 
east  toward  the  west  to  follow  the  apparent  movement  of  the  sun. 
As  the  sun  gets  higher  and  higher  the  mirror  must  be  turned  on  its 


144 

r 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


[Cn.  VI 


hinges  more  and  more  until  at  noon  it  will  be  nearly  horizontal 
(fig.  76).  In  the  afternoon,  as  the  sun  moves  toward  the  west,  the 
mirror  must  be  rotated  to  follow  it.  At  the  same  time  it  must  be 
turned  more  and  more  on  its  hinges  until  late  in  the  afternoon,  it 
will  be  at  the  same  angle  as  in  the  morning,  and  rotated  as  far 
toward  the  west  as  it  was  toward  the  east  in  the  earlier  part  of  the 
day  (fig.  76). 

(B)  For  an  eastern  exposure — In  this  position,  the  axis  of  the 
entire  instrument  is  preferably  east  and  west  with  the  objective 
pointing  westward.  The  earlier  the  time  the  more  nearly^hori- 


f 


12 


28° 


M 


North 

FIG.  76.     DIAGRAM  SHOWING  THE  POSITION  OF  THE  MIRROR  NECESSARY 
TO  RELFECT  THE  SUNLIGHT  DIRECTLY  NORTH  AT  THREE  DIFFERENT 
PERIODS  OF  THE  DAY — (6  A.  M.;   12  M.;  6  p.  M.)- 

The  diagram  is  for  the  latitude  of  Ithaca  and  at  the  season  of  the  equinox 
when  the  sun  seems  to  rise  directly  in  the  east  and  set  directly  in  the  west.  In 
the  morning  the  mirror  is  turned  toward  the  east  at  an  angle  of  45°  and  inclined 
about  10°  toward  the  south.  In  the  evening  it  is  turned  similarly  toward  the 
west  and  south. 

At  noon  the  mirror  is  raised  on  its  hinges  about  28°  above  the  horizontal.  At 
all  intermediate  points  the  mirror  must  be  set  accordingly:  that  is,  so  that  it 
will  reflect  the  sun  directly  north. 

The  diagram  also  shows  the  apparent  course  of  the  sun  from  sunrise  to 
sunset. 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN  145 

zontal  must  the  mirror  be.  As  the  sun  gets  higher  and  higher  the 
mirror  must  be  raised  more  and  more  on  its  hinges ;  and  as  the  sun 
seems  to  move  toward  the  south  as  well  as  upward,  the  mirror  must 
be  rotated  on  its  axis  toward  the  south. 

(C)  For  a  western  exposure — If  a  western  exposure  is  used, 
the  entire  instrument  should  be  placed  pointing  east  and  west  if 
possible.  The  mirror  will  be  raised  on  its  hinges  and  turned  south- 
ward early  in  the  afternoon.  As  the  sun  sinks  toward  the  west  the 
mirror  will  be  made  more  and  more  nearly  horizontal,  and  as  the 
sun  seems  to  move  toward  the  north  as  well  as  toward  the  west, 
the  mirror  will  finally  be  nearly  horizontal  on  its  hinges  and  rotated 
somewhat  northward. 

These  movements  of  the  mirror  become  intelligible  if  one 
observes  the  position  of  the  sun  in  the  different  periods  of  the  day. 
By  consulting  fig  86,  87,  it  is  also  clear  that  the  mirror  must  have 
different  positions  owing  to  the  declination  or  position  of  the  sun 
with  reference  to  the  horizon  at  different  times  of  the  year. 

HELIOSTATS  DRIVEN  BY  CLOCK-WORK 

§  239.  Types  of  clock-driven  heliostats. — A  fundamental 
character  of  all  heliostats  is  that  the  clock-work  rotates  a  shaft 
corresponding  with  the  post  carrying  the  hour  hand  of  an  ordinary 
clock,  and  that  it  is  this  shaft  which  directly  or  indirectly  gives 
motion  to  the  mirror. 

This  shaft  must  be  made  parallel  with  the  earth's  axis  wherever 
the  instrument  is  used. 

(A)  Single-mirror  type. — This  is  so  constructed  that  the  clock- 
work gives  a  double  motion  to  the  mirror  something  as  one  can 
give  a  double  motion  to  a  mirror  held  in  the  hands,  i.  e.,  an  up  and 
down  motion  and  a  motion  of  rotation  on  the  axis  (fig.   77-79). 

(B)  Double-mirror  type. — In  this  type  one  mirror  is  fixed  at 
the  end,  or  the  side  of  the  clock-shaft.     The  second  mirror  is  not 
moved  by  the  clock-work,  but  is  set  by  hand  at  the  beginning  of 
each  experiment  (fig.  81-84). 

As  one  might  conclude,  the  second  or  two-mirror  type  is  of 
simpler  construction  and  therefore  correspondingly  inexpensive. 


146 


HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 


FIG.  77.     ONE-MIRROR  HELIOSTAT  OF  S'GRAVESANDE. 
(From  his:     Elementa  Mathematica  Phy sices,  Tomus  II,  Tabula  LXXXIII). 

This  picture  is  a  facsimile  except  that  the  clock-shaft  has  been  extended 
above  and  below. 

B  B     Base  of  the  mirror  support  with  leveling  screws. 

P     Hollow  cylinder  in  which  the  mirror  support  C  can  rotate. 

C    The  pillar  with  mirror  fork  at  the  upper  end. 

5     Plane  mirror. 

D  E  Shaft  at  right  angles  to  the  mirror  and  extending  to  the  fork  at  the 
end  of  the  clock-hand  (N  0). 

L  L  M    Foot  of  the  support  for  the  clock-work. 

Ill    The  leveling  screws  of  the  support. 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN  147 

G  F    Column  supporting  the  clock-work. 

N  P  L°  The  clock-shaft.  It  is  parallel  with  the  earth's  axis  and  hence 
points  toward  the  celestial  north  pole.  The  angle  L°  at  the  lower  end  is  equal 
to  the  latitude  of  the  place  where  the  heliostat  is  used. 

T  R  Fork  where  the  movement  of  the  clock-hand  (N  O)  is  transferred  to 
the  shaft  actuating  the  mirror  (D  E  S). 

f  g  Plate  bearing  the  clock-work.  It  must  be  elevated  sufficiently  to  make 
the  angle  of  the  clock-shaft  equal  to  the  latitude  of  the  place  (see  fig.  85). 

It  answers  very  well  for  all  the  work  required  by  the  photographer 
and  the  projectionist. 

§  240.  How  to  make  the  clock-shaft  parallel  with  the  earth's 
axis  at  any  given  place. — For  this  it  is  necessary  to  know  two 
things : 

(1)  One  must  know  the  north  and  south  direction. 

(2)  One  must  know  the  latitude  of  the  place. 

The  first  information  can  be  gained  by  referring  to  the  pole  star. 
Buildings  are  often  set  due  north  and  south,  and  thus  serve  as 
guides ;  or  one  might  use  a  compass.  If  a  magnetic  needle  is  used 
it  must  not  be  forgotten  that  there  is  a  certain  variation  from  the 
true  north  and  south  line  assumed  by  the  compass  needle,  and  for 
accurate  observations  it  is  necessary  to  know  the  magnetic  varia- 
tion at  any  given  place  and  to  correct  for  it. 

For  the  latitude,  a  good  map  like  that  issued  by  the  U.  S.  geologi- 
cal survey  will  give  the  information.  The  geological  survey  maps 
also  give  the  magnetic  variation. 

Making  the  clock-shaft  parallel  with  the  earth's  axis  is  easily 
accomplished  if  one  knows  the  latitude  and  the  north  and  south 
direction.  As  a  general  statement  all  that  is  necessary  is  to  make 
the  clock-shaft  point  toward  the  north  star  or  more  accurately, 
toward  the  celestial  north  pole. 

By  referring  to  fig.  85  it  is  evident  that  this  is  brought  about  by 
putting  the  instrument  due  north  and  south  and  then  elevating  the 
clock-shaft  above  the  level  or  horizontal  line  an  amount  equal  to 
the  latitude  of  the  place. 

For  example,  if  an  experiment  with  the  heliostat  is  to  be  made 
in  one  of  the  buildings  of  Cornell  University  at  Ithaca  with,  in 
round  numbers,  a  latitude  of  42.5  degrees,  the  instrument  is  set  on 
a  level  place  and  due  north  and  south,  then  the  free  end  of  the 


148 


HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 


\ 


FIG.  78.    ONE-MIRROR  HELIOSTAT  OF  FOUCAULT. 
(From  his  Recueil  des  Travaux  Scientifigues) . 

Modified  by  extending  the  clock-shaft  above  and  below. 
B     Clock-work. 
T    The  clock-shaft. 
C    The  upper  end  of  the  clock-shaft. 

N  P     Continuation  of  the  clock-shaft  above  the  mirror  (M). 
L°     The  angle  above  the  horizontal  to  make  the  clock-shaft  parallel  with  the 
earth's  axis.     It  equals  the  latitude  of  the  place  where  the  heliostat  is  used  (see 

fig.  85). 

D     Divided  semicircle  to  enable  one  to  set  the  instrument  according  to  the 
sun's  declination. 

N1  N,  N  C1     Connections  between  the  clock-work  and  the  mirror. 


clock-shaft  is  raised  above  the  level  42.5  degrees.  If  now  one  were 
to  sight  along  the  clock-shaft  it  would  be  found  pointing  directly 
toward  the  north  star. 

As  seen  from  the  diagram,  fig.  85,  it  will  then  also  be  parallel 
with  the  earth's  axis. 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN  149 

Sometimes  the  co-latitude  and  the  vertical  line  are  used  instead 
of  the  horizontal  line  and  the  latitude.  This  brings  about  the  same 
result,  for  if  the  clock-shaft  is  vertical  to  start  with,  it  must  be 
tipped  over  toward  the  north  from  the  vertical  an  amount  equal  to 
the  co-latitude.  That  is,  in  Ithaca,  it  must  be  inclined  47.5  degrees 
from  the  vertical. 

In  general  the  clock-shaft  must  be  inclined  upward  from  the 
horizontal,  a  number  of  degrees  corresponding  with  the  latitude  at 
the  place  of  observation  or  it  must  be  inclined  downward  toward 
the  north  from  the  vertical  position  a  number  of  degrees  correspond- 
ing with  the  co-latitude.  The  sum  of  the  latitude  and  the  co-lati- 
tude in  every  case  equals  90  degrees.  (See  fig.  85  and  its  explana- 
tion) . 

INSTALLATION  OF  A  SINGLE-MIRROR  HELIOSTAT 
§  241.     Setting  up  the  heliostat.— 

1.  In  the  first   place  the  instrument  must   be  leveled  and 

arranged  accurately  in  a  north  and  south  direction. 

2 .  The  clock-shaft  must  next  be  elevated  to  an  angle  correspond- 

ing with  the  latitude  of  the  place  where  it  is  to  be  used 
(§  240)  so  that  it  will  point  toward  the  north  star.  It  will 
then  be  parallel  with  the  earth's  axis  (fig.  85). 

3.  To  give  the  proper  angle  to  the  mirror,  depending  on  the 

declination  of  the  sun,  and  to  get  also  the  correct  local 
time,  loosen  the  clamp  holding  the  clock-arm  (fig.  79  c) 
and  turn  the  clock-arm  toward  the  sun  until  the  light 
shines  through  both  sights  along  the  line  q-p.  Then 
clamp  the  set  screw  at  c. 

4.  To  get  the  spot  of  light  in  the  desired  place,  loosen  the  clamp- 

screws  in  the  position  arm  F-B  and  below  H  in  the  rotating 
collar  and  then  raise  or  lower  the  shaft  o,  fig.  79  and  rotate 
the  position  arm  around  the  column  A  till  the  light  is 
reflected  where  it  is  wanted,  then  tighten  the  clamping 
screws  and  the  clock-work  should  cause  the  mirror  to  move 
so  that  it  will  reflect  the  beam  of  light  in  the  same  place  so 
long  as  the  sun  shines  on  the  mirror. 


150 


HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 


FIG.  79.     UNIVERSAL,  ONE-MIRROR  HELIOSTAT. 
(From  the  Catalogue  of  R.  Fuess). 

Modified  by  extending  the  clock-shaft  above  and  below  and  by  adding  the 
abbreviations  in  and  rf  for  the  incident  and  reflected  ray  on  the  mirror  (M). 

This  heliostat  is  called  universal  for  it  is  adjustable  so  that  it  can  be  used  in 
any  latitude  and  at  any  season  of  the  year.  See  fig.  80  and  §  241  for  further 
explanation. 

The  dials  showing  the  time  and  declination  may  be  used  for 
setting  the  heliostat,  but  one  can  get  the  apparatus  set  accurately 
by  trial  as  just  described.  If  the  time  and  declination  scales  are 
to  be  used  one  must  consult  a  nautical  almanac  for  the  sun's 
declination  for  the  given  date,  and  an  accurate  clock  for  the  time 
of  day. 

§  242.  For  centering  the  magic  lantern  when  a  heliostat  is  used 
the  same  general  principles  must  be  followed  as  with  the  arc  light 
magic  lantern  (Ch.  I,  §  51-57,  fig.  i,  74). 

To  center  the  light  one  must  be  able  to  adjust  the  mirror  by  hand 
after  it  has  been  set  to  follow  the  sun.  This  is  provided  for  in  all 


CH.  VI]          HELIOSTATS  FOR  THE  MAGIC  LANTERN  151 

forms  of  single-mirror  heliostats.  In  fig.  79,  for  example,  the 
position  arm  B-F  can  be  raised  or  lowered  and  the  entire  arm  can 
be  rotated  around  the  column  A.  When  the  light  is  accurately 
directed,  all  the  clamps  can  be  tightened  and  the  clock-work  should 
cause  the  mirror  to  hold  the  light  constantly  in  position.  It  will 
be  found  much  easier  to  center  the  light  on  one  axis  if  the  heliostat 
is  at  about  the  same  level  as  the  condenser  and  objective.  This 
position  can  be  secured  by  raising  the  heliostat  or  the  lantern, 
whichever  is  more  convenient,  provided  the  two  are  not  on  the 
same  level  to  start  with. 


o 

FIG.  80.   PRINCIPLE  OF  THE  UNIVERSAL  HELIOSTAT  SHOWN  IN  FIG.  79. 

O  A     The  clock  arm  pointing  directly  towards  the  sun. 

0  B  The  position  arm,  pointing  in  the  direction  in  which  it  is  desired  to 
reflect  the  light. 

in     The  incident  light  parallel  to  O  A. 

rf    The  reflected  light. 

.4  B  The  mirror.  The  mirror  is  perpendicular  to  the  plane  passing  through 
A,  0  and  B. 

O  N    Perpendicular  to  the  mirror  A  B. 

In  order  to  prove  that  incident  light  parallel  to  A  O  will  be  reflected  from 
the  mirror  parallel  to  O  B  it  is  necessary  to  prove  that  A  O,  O  B  and  O  N  are 
in  the  same  plane  and  that  O  N  bisects  the  angle  A  O  B.  The  mirror  being 
perpendicular  to  the  plane  containing  A ,  O  and  B  and  the  line  O  N  perpendicu- 
lar to  A  B  must  also  be  in  this  same  plane.  The  triangle  A  O  B  is  isosceles  by 
construction,  as  A  O  and  0  B  are  made  equal,  hence  the  perpendicular  to  the 
base  must  bisect  the  vertex  angle. 


15-2  HELIOSTATS  FOR  THE  MAGIC  LANTERN  ICn.  VI 

INSTALLATION  AND  USE  OF  TWO-MIRROR  HELIOSTATS 

§  243.    Heliostat  with  the  mirror  at  the  end  of  the  clock-shaft.— 

Place  the  heliostat  in  a  position  either  inside  a  room  or  outside  a 
window  where  the  full  light  of  the  sun  can  fall  upon  the  movable 
mirror.  The  stand  supporting  the  clock-work,  etc.,  must  be  made 
level,  and  set  in  a  north  and  south  direction  (fig.  81). 

Elevate  the  clock-shaft  above  the  level  to  an  angle  equal  to  the 
latitude  of  the  place  where  it  is  to  be  used.  One  can  use  a  good 
protractor  for  this.  The  clock-shaft  will  then  point  toward  the 
north  star,  and  be  parallel  with  the  earth's  axis  (fig.  85). 

This  form  of  heliostat  often  has  the  clock-shaft  in  a  fixed  position 
for  cheapness  of  construction  (fig.  81).  If  such  a  heliostat  is 
purchased,  the  manufacturer  must  know  the  latitude  of  the  place 
where  it  is  to  be  used,  then  he  will  give  the  proper  inclination  to  the 
clock-shaft  so  that  when  the  instrument  is  arranged  in  a  north  and 
south  line  the  shaft  will  point  toward  the  north  star. 

§  244.  Arranging  the  movable  mirror. — The  mirror  is  fixed  to 
the  end  of  the  shaft  by  a  collar  which  permits  it  to  rotate  around 
the  shaft.  It  is  also  held  in  a  kind  of  fork,  which  permits  the 
mirror  to  be  raised  and  lowered  in  a  way  similar  to  the  hinge 
movement  of  the  porte-lumiere  (fig.  75). 

For  setting  this  mirror  so  that  the  clock-work  will  cause  it  to 
throw  a  beam  of  light  in  one  direction  continuously,  it  is  necessary 
first  of  all  to  set  the  mirror  for  the  local  time.  This  is  done  by  the 
use  of  a  perforated  screen  admitting  a  narrow  pencil  of  light  from 
the  sun.  This  screen  is  so  placed  that  the  pencil  of  light  falls  upon 
the  mirror.  The  mirror  is  then  turned  by  loosening  the  clamp 
(fig.  8 1  c)  and  rotating  it  on  the  shaft,  and  by  tipping  it  in  the  fork 
until  the  pencil  of  light  is  reflected  back  along  its  path  through  the 
hole  again. 

Then  the  clamp  is  tightened  and  the  screen  removed.  The 
mirror  is  now  tipped  in  the  fork  until  the  light  is  reflected  from  it 
directly  in  line  with  the  clock-shaft,  i.  e.,  directly  toward  the  north 
star  (fig.  81  N.  P.).  The  easiest  way  to  do  this  is  to  take  a  piece 
of  white  cardboard  with  parallel  black  lines  on  it  and  place  it 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN 


153 


FIG.  81.     TWO-MIRROR  HELIOSTAT  WITH  THE  MOVABLE  MIRROR  AT  THE 

END  OF  THE  CLOCK-SHAFT. 
(From  the  Catalogue  of  Max  Kohl). 

The  figure  has  been  modified  by  extending  the  clock-shaft  and  by  adding  the 
second  mirror  and  the  light  rays.  This  heliostat  is  usually  fixed  for  a  given 
latitude,  hence  in  ordering  one,  the  latitude  of  the  place  should  be  given.  It 
could  be  made  adjustable  for  latitude,  but  that  would  naturally  increase  the 
cost. 

N-P,  L°  The  clock-shaft  pointing  to  the  celestial  north  pole  above,  and 
indicating  the  angle  corresponding  with  the  latitude  below. 

c     Clamp  to  hold  the  mirror  (M]  to  the  revolving  clock-shaft. 

M  Movable  mirror.  It  is  adjusted  in  the  fork  and  on  the  clock-shaft  until 
the  reflected  rays  proceed  parallel  with  the  earth's  axis,  hence  also  parallel  with 
the  clock-shaft. 

M2    The  fixed  mirror  to  be  set  by  hand  in  the  beginning. 

5    and  the  Arrows.     The  sun's  rays. 


154  HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 

parallel  with  the  clock-shaft.  When  the  beam  of  light  from  the 
mirror  extends  out  parallel  with  these  lines,  as  indicated  by  the 
streak  of  light,  the  mirror  will  be  in  the  correct  position.  ' 

§  245.  Arranging  the  second  mirror. — For  getting  the  light  in  a 
desired  direction,  a  second  mirror  is  used  in  the  path  of  the  beam 
extending  directly  northward,  from  the  first  mirror,  and  so  arranged 
that  the  light  is  reflected  as  desired  (fig.  81  M2). 

§  246.    Heliostat  with  the  mirror  parallel  with  the  clock-shaft. — 

With  the  other  heliostats  described  in  this  chapter,  the  clock-work 
rotates  the  shaft  once  in  24  hours,  but  with  this  form,  the  rotation 
is  once  in  48  hours,  i.  e.,  half  the  rate  of  rotation  of  the  earth.  The 
clock-shaft  is  somewhat  extended  and  the  mirror  is  fixed  directly  to 
the  shaft  and  parallel  with  it.  The  mirror  is  therefore  in  a  plane 
which  if  extended  would  cut  the  celestial  north  pole  (fig.  82). 

Light  reflected  from  this  mirror  may  be  made  to  take  any  direc- 
tion in  a  circle. 

§  247.  Setting  up  the  heliostat  with  the  mirror  parallel  with  the 
clock-shaft. — The  heliostat  is  placed  in  a  proper  position  for 
receiving  the  sunlight.  The  support  is  made  level,  and  the  instru- 
ment set  north  and  south.  The  clock-shaft  is  then  elevated  from 
the  horizontal  until  it  is  at  an  angle  equal  to  the  latitude  of  the 
place  where  it  is  to  be  used.  As  the  mirror  in  this  form  may  be  set 
to  reflect  the  light  anywhere  in  a  circle,  it  is  best  to  loosen  the  clamp 
of  the  clock-shaft  and  rotate  the  mirror  until  it  receives  the  full 
light  of  the  sun  and  reflects  it  in  a  convenient  direction.  Then 
clamp  the  shaft  to  the  clock-work  and  the  mirror  will  follow  the 
sun. 

§  248.  Arranging  the  second  mirror. — The  second  mirror  is  now 
placed  so  that  it  will  receive  the  beam  from  the  movable  mirror, 
and  then  it  is  turned,  raised,  or  lowered  on  its  stand,  until  the  light 
extends  in  the  desired  direction.  It  should  continue  to  hold  the 
light  in  one  place  so  long  as  the  sun  shines  on  the  movable  mirror 
(fig.  82).  One  must  make  sure  that  the  position  of  the  second 
mirror  is  such  that  it  will  not  shade  the  heliostat  mirror  as  the 
sun  moves  toward  the  west. 


CH.  VI] 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


155 


FIG.  82. 


TWO-MIRROR  HELIOSTAT  WITH  THE  MOVABLE  MIRROR  ATTACHED 
PARALLEL  TO  THE  CLOCK-SHAFT. 


This  heliostat  is  adjustable  for  latitude  and  can  be  used  anywhere  in  the 
northern  hemisphere,  and  by  reversing  the  motion,  in  the  southern  hemis- 
phere (§  253). 

C    Clock-work  mounted  on  a  hinged  plate. 

Ml  Rotating  mirror  attached  to  the  side  of  the  clock-shaft.  From  this 
arrangement  its  plane  would  pass  through  the  celestial  north  pole  if  extended. 


*To  get  this  picture,  the  heliostat  was  set  in  the  west  window  of  Stimson  Hall 
at  2:30  P.  M.,  May  20,  1912,  and  the  mirrors  arranged  to  receive  and  reflect  a 
small  beam  of  sunlight  as  indicated.  A  black  cord  was  extended  from  the 
small  hole  in  the  shutter  to  the  point  on  the  first  mirror  receiving  the  sunbeam, 
and  from  thence  to  the  second  mirror  along  the  path  of  the  sunbeam ;  and  from 
the  second  mirror  to  a  point  on  the  screen  receiving  the  sunbeam.  The 
apparatus  was  then  photographed.  To  make  the  course  of  the  sunbeam  very 
sharp  for  this  cut  its  course  was  traced  on  the  photograph  by  a  right  line  pen. 
The  clock-shaft  was  also  extended  above  and  below  and  an  arc  of  a  circle  added 
between  the  clock-shaft  and  the  horizon  to  indicate  the  angle  of  elevation  of 
the  clock-shaft,  corresponding  with  the  latitude  of  Ithaca  (42.5°  North  Lati- 
tude). 


156 


HELIOSTATS  FOR  THE  MAGIC  LANTERN 


[CH.  VI 


M2  The  fixed  mirror.  This  is  adjusted  at  the  beginning  of  the  experiment 
to  reflect  the  light  in  the  desired  place  and  usually  needs  no  attention  during 
the  experiment. 

N  P     Continuation  of  the  clock-shaft  pointing  toward  the  north  pole. 

L  42.5°  The  angle  made  by  the  clock-shaft,  and  the  horizon  at  Ithaca, 
New  York,  U.  S.  A.  It  indicates  the  latitude  of  that  place,  and  the  elevation 
of  the  clock-shaft  to  make  it  point  toward  the  celestial  north  pole. 

S  B  Sunbeam  admitted  through  a  hole  in  the  shutter.  It  strikes  the  first 
mirror  and  is  reflected  to  the  second  mirror,  and  from  it  in  any  desired  direction. 


12  M 


B 


FIG.  82  A,  B,  C.  DIAGRAMS  SHOWING  THE  POSITION  OF  THE  FIRST 
MIRROR  OF  THE  HELIOSTAT  (Fig.  82)  AT  DIFFERENT  TIMES  OF  THE  DAY 
TO  REFLECT  THE  SUNLIGHT  CONSTANTLY  IN  THE  SAME  DIRECTION. 

The  eye  is  supposed  to  be  looking  along  the  axis  of  the  clock-shaft.  It  is  to 
be  noted  that  between  6  A.  M.  and  6  P.  M.  (12  hours)  the  mirror  has  turned 
through  an  angle  of  90°,  and  at  this  rate  it  takes  48  hours  for  the  mirror  to 
make  a  complete  revolution  of  360°. 

The  arrows  indicate  the  direction  of  the  light  before  and  after  reflection  from 
the  mirror. 

A     Position  of  the  mirror  at  6  A.  M. 

B     Position  of  the  mirror  at  12  M. 

C    Position  of  the  mirror  at  6  p..  M. 

At  intermediate  periods  the  mirror  will  be  in  correspondingly  intermediate 
positions  to  reflect  the  sun  constantly  in  the  same  direction,  that  is,  the  mirror 
must  follow  the  sun. 

,  This  is  one  of  the  easiest  heliostats  to  manage,  as  one  needs  to 
know  only  the  latitude  and  the  north  and  south  direction.  The 
arrangement  of  the  two  mirrors  can  be  easily  made  at  any  time 
and  in  any  place  by  trial. 


HELIOSTATS  IN  THE  SOUTHERN  HEMISPHERE 

§  249.  Up  to  the  present,  the  discussion  has  been  with  reference 
to  heliostats  in  the  northern  hemisphere.  For  those  to  be  used  in 
the  southern  hemisphere  certain  modifications  are  necessary  as  seen 
from  the  following  considerations : 


CH.  VI]          HELIOSTATS  FOR  THE  MAGIC  LANTERN 


157 


FIG.  83.     LENS  AND  PRISM  HELIOSTAT  OF  ALFRED  M.  MAYER. 
(From  the  American  Journal  of  Science,    Vol.  154,  1897}. 

This  heliostat  is  in  principle  like  the  two-mirror  heliostat  with  the  movable 
mirror  attached  to  the  end  of  the  clock-shaft  (fig.  81). 

/  Biconvex  lens  about  10  cm.  (4  in.)  in  diameter  to  receive  the  sun's  rays 
and  render  them  convergent. 

K     Concave  lens  to  render  the  converging  beam  parallel. 

g  Rack  and  pinion  movement  to  change  the  position  of  the  concave  lens 
and  thus  increase  or  diminish  the  size  of  the  beam. 

/  Right-angled  prism  receiving  the  parallel  bundle  from  K  and  reflecting  it 
to  a  fixed  prism  (L)  or  to  a  mirror,  by  which  it  is  reflected  in  any  desired 
direction. 

The  two  lenses  /  K  and  the  prism  /,  are  all  on  one  common  axis  and  are 
rotated  by  the  clock-shaft  G,  and  thus  made  to  follow  the  sun  like  the  mirror 
on  the  end  of  the  clock-shaft  in  figure  81.  The  clock-shaft  G  must  be  at  an 
elevation  corresponding  to  the  latitude  of  the  place  (see  also  fig.  84). 


158  HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 

§  250.  If  one  were  looking  at  the  north  pole  of  the  earth  from 
a  position  along  the  earth's  axis,  the  direction  of  the  earth's  rota- 
tion would  appear  in  a  direction  opposite  to  the  hands  of  a  clock  or 
watch.  To  compensate  for  this,  a  mirror  to  hold  a  spot  of  sunlight 
in  one  position  would  need  to  be  rotated  around  an  axis  parallel 
with  that  of  the  earth,  but  in  an  opposite  direction  to  the  earth's 
rotation,  that  is  in  the  clockwise  direction. 

§  251.  At  the  equator,  the  clock-shaft  must  be  horizontal  in 
order  to  be  parallel  with  the  earth's  axis.  The  clock-shaft  must  be 
turned  from  east  to  west.  This  can  be  accomplished  either  by  a 
clock-work  located  at  the  southern  end  of  the  shaft  turning  in  the 
clockwise  direction  as  in  fig.  77-79,  or  by  a  clock-work  located  at 
the  northern  end  of  the  shaft  turning  in  a  counter-clockwise  direc- 
tion. 

§  252.  At  the  north  pole  of  the  earth,  the  axis  of  rotation  of  the 
shaft  would  be  vertical  and  the  direction  of  rotation  as  seen  from 
above,  would  be  clockwise. 

At  the  south  pole  the  axis  would  also  be  vertical  and  the  direction 
of  rotation  would  be  clockwise  as  seen  from  below — i.  e.,  from  the 
north — or  counter-clockwise  as  seen  from  above. 

§  253.  A  heliostat  constructed  for  the  southern  hemisphere 
would  be  exactly  similar  to  one  for  the  northern  hemisphere  except 
that  the  clock-shaft  must  rotate  in  the  counter-clockwise  direction, 
that  is,  from  right  to  left. 

§  254.     Setting  up  a  heliostat  in  the  southern  hemisphere. — If 

a  heliostat  is  properly  constructed  for  the  southern  hemisphere  it 
is  set  up  at  any  given  south  latitude  by  arranging  the  instrument 
due  north  and  south  with  the  free  end  of  the  clock-shaft  pointing 
south.  Then  the  clock-shaft  would  be  elevated  above  the  horizon 
a  number  of  degrees  corresponding  with  the  south  latitude.  This 
would  make  the  clock-shaft  parallel  with  the  earth's  axis  and  it 
would  point  toward  the  celestial  south  pole  (fig.  85).  Indeed,  the 
entire  procedure  for  getting  the  light  in  the  desired  direction,  the 
use  of  the  condenser  and  projection  objective,  etc.,  is  exactly 
as  for  the  northern  hemisphere. 


CH.  VI]  HELIOSTATS  FOR  THE  MAGIC  LANTERN  159 


FIG.  84.     LENS  AND  PRISM  HELIOSTAT  OF  ALFRED  M.  MAYER. 

(From  the  Catalogue  of  Optical  Instruments  by  R.  Fuess). 

The  figure  has  been  modified  by  extending  the  clock-shaft  above  and  below. 
As  here  shown  the  instrument  is  suitable  for  any  latitude.  It  uses  a  mirror 
instead  of  a  second  prism  as  in  the  original  of  Mayer  (fig.  83). 

U    Clock-work. 

N  P,  L°  The  clock-shaft  extended  to  indicate  the  direction  of  the  celestial 
north  pole  above,  and  below  the  angle  of  elevation  corresponding  to  the  lati- 
tude of  the  place  where  the  instrument  is  used. 

P  D  Z  Three  divided  scales ;  P  for  the  latitude,  D  for  the  Sun's  declination, 
Z  for  the  time  of  day. 

S,  k  and  Pr.  The  convex  and  the  concave  lens,  and  the  prism  as  shown  in 
fig.  83. 

Sp    Mirror  to  take  the  place  of  the  prism  (L)  in  fig.  83. 


i6o 


HELIOSTATS  FOR  THE  MAGIC  LANTERN  [Cn.  VI 


FIG.  85.     DIAGRAM  SHOWING  THAT  THE  ELEVATION  OF  THE  CLOCK-SHAFT 
AT  AN  ANGLE  EQUAL  TO  THE  LATITUDE  OF  A  PLACE  WILL  MAKE  THE 
CLOCK-SHAFT  PARALLEL  WITH  THE  EARTH'S  Axis. 

EQ    Equator  of  the  earth. 

Axis  Axis  The  earth's  axis  with  the  north  pole  of  the  earth  above  and  the 
south  pole  below. 

N  P     The  earth's  north  pole. 
S  P    The  earth's  south  pole. 

f.5°     Latitude  of  Ithaca,  New  York,  U.  S.  A. 
h     Horizontal  lines,  that  is,  tangents  to  the  earth's  surface  at  the  two 
latitudes  shown  (42.5°  north,  30°  south). 

Z     Zenith. 

A  A  Clock-shaft  elevated  from  the  horizon  an  amount  equal  to  the  latitude. 
If  continued  toward  the  equator  the  clock-shaft  would  meet  the  plane  of  the 
equator  at  right  angles,  hence  it  is  parallel  with  the  earth's  axis  and  points 
toward  the  celestial  poles. 

A  h     Latitude  (42.5°  north  and  30°  south). 

A  Z     Co-latitude  (47.5°  north,  60°  south). 

§  255.  Finally,  a  heliostat  constructed  for  the  northern  hemi- 
sphere would  work  equally  well  for  the  southern  hemisphere  if  it 
were  attached  to  the  ceiling  (i.e.  wrong  side  up)  instead  of  being  on 
a  table  or  window-sill  right  side  up,  for  this  change  in  position  would 
make  the  clock-shaft  rotate  in  the  counter-clockwise  direction,  as 
seen  from  above. 


CH.  VI]  CONDENSER  FOR  SUNLIGHT  161 

CONDENSER  FOR  SUNLIGHT 

§  256.  As  sunlight  is  composed  of  practically  parallel  rays,  the 
condenser  consists  of  a  single  plano-convex  lens  with  the  convexity 
receiving  the  light  (fig.  74) ;  or  one  may  use  an  achromatic  com- 
bination (fig.324). 

The  condition  is  practically  like  the  ordinary  condenser  after  the 
light  has  been  rendered  parallel  by  the  first  element  of  the  condenser 
(fig.  3).  Having  parallel  rays  to  start  with,  only  the  second  ele- 
ment of  the  condenser  is  needed. 

§  257.  Increasing  the  illumination. — The  greatest  difference 
between  the  use  of  sunlight  and  the  arc  light  for  projection  appears 
when  one  wishes  to  increase  the  illumination.  With  the  arc  lamp 
one  simply  uses  more  current,  and  this  increases  the  candle-power 
and  makes  the  screen  image  more  brilliant.  With  the  same  size 
condenser  and  picture  the  illumination  of  the  screen  with  the  arc 
light  is  directly  proportional  to  the  illumination  of  the  condenser 
face. 

With  sunlight,  the  illumination  of  the  condenser  face  is  a  con- 
stant quantity  except  for  haze,  etc.  As  all  the  light  which  strikes 
the  screen  must  pass  through  the  condenser,  the  screen  illumina- 
tion can  be  increased  with  sunlight  only  by  using  a  condenser  of 
larger  diameter  and  correspondingly  greater  focal  length.  And  for 
this  one  must  have  heliostat  mirrors  of  sufficient  size  to  fill  the 
condenser  with  light. 

§  258.  The  water-cell  with  sunlight. — This  light  is  accom- 
panied by  so  much  radiant  heat  that  it  is  desirable  to  use  a  water- 
cell  with  the  apparatus,  and  thus  reduce  the  liability  of  over-heating 
lantern  slides  or  other  specimens  used  for  projection  (see  §  848  for 
the  discussion  of  the  need  of  a  water-cell) . 

CONDUCT  OF  AN  EXHIBITION  WITH  SUNLIGHT 

§  259.  The  general  principles  given  in  Ch.  I,  §  21-41  are 
applicable. 

§  260.  Lighting  of  the  room. — Sunlight  is  sufficiently  powerful 
so  that  the  room  used  need  not  be  very  dark  for  showing  lantern 


162  USE  OF  SUNLIGHT  FOR  PROJECTION  lCn.  VI 

slides.  Care  must  be  taken  to  have  no  direct  light  fall  on  the  screen 
except  that  from  the  lantern,  but  the  room  can  have  sufficient 
diffused  light  to  take  notes  comfortably  (see  also  Ch.  XII, 
§  605-608). 

§  261.  Size  of  the  room  and  the  screen. — By  using  a  condenser 
of  proper  size  and  of  a  focal  length  adapted  to  the  projection 
objective,  there  is  no  practical  limit  to  the  possibilities  of  projection 
with  sunlight. 

§  262.  Turning  on  and  off  the  light. — For  shutting  out  the  sun- 
light one  can  use  a  metal  shield  between  the  mirror  and  the  con- 
denser or  one  can  use  the  objective  shield  (fig.  14  and  62).  The 
first  method  is  preferable,  for  there  will  be  less  heating  of  the 
apparatus. 

TROUBLES 
§  263.    The  troubles  with  sunlight  are: 

1 .  The  difficulty  of  keeping  the  beam  of  sunlight  in  a  constant 

direction.  With  the  porte-lumiere  one  must  be  con- 
stantly on  the  alert  to  make  the  slight  adjustments  of  the 
mirror  necessary. 

2.  The  clock-driven  heliostats,  if  well  made  and  regulated 

accurately,  should  give  no  trouble  when  they  are  prop- 
erly set  up. 

If  a  person  is  fortunate  enough  to  live  near  an  astronomical 
observatory  and  can  get  the  help  of  the  astronomer  in  charge  he  can 
learn  to  overcome  difficulties  that  seem  to  be  insurmountable  when 
working  alone.  The  apparatus  of  an  observatory  is  also  of  first 
rate  quality,  and  it  helps  any  worker  to  know  what  good  apparatus 
looks  like. 

§  264.  Lack  of  sunlight. — This  is  the  one  great  trouble.  Of 
course  it  is  not  available  at  night  anywhere.  And  in  the  most 
thickly  populated  regions  where  projection  apparatus  is  used  there 
is  liable  to  be  so  much  cloudy  weather  that  sunlight  is  not  available 
even  in  the  daytime  during  much  of  the  year.  Smoke  also  obscures 
the  sun  when  clouds  are  absent. 


CH.  VI] 


TROUBLES  WITH  SUNLIGHT 


163 


Fortunately,  in  many  parts  of  America  the  sun  can  be  counted 
on  in  the  daytime;  and  for  those  parts  the  use  of  sunlight  for 
projection  of  all  kinds  is  strongly  recommended. 


North  Pole 


South  Pole 


FIG.  86.     DIAGRAM  OF  THE  CELESTIAL  SPHERE  WITH  THE  PLANES  OF  THE 

CELESTIAL  EQUATOR  AND  OF  THE  ECLIPTIC;  AND  WITH  THE  APPARENT 

POSITION  OF  THE  SUN  AT  DIFFERENT  SEASONS. 

Earth     This  is  shown  as  a  small  black  sphere  at  the  center. 

North  Pole,  South  Pole  The  two  poles  of  the  celestial  sphere.  They  are  at 
an  infinite  distance  from  the  earth. 

West,  East  East  and  west  points  of  the  celestial  sphere.  The  plane  of  the 
celestial  equator  touches  these  points. 

Equator  The  plane  of  the  celestial  equator  (shaded  in  lines)  dividing  the 
celestial  sphere  into  a  northern  and  a  southern  hemisphere.  A  plane  at  right 
angles  to  this  traversing  the  north  and  south  poles  would  divide  it  into  an 
eastern  and  western  hemisphere. 

Ecliptic  The  plane  (shaded  in  dots)  around  the  outer  edge  of  which  the 
sun  seems  to  move  during  the  year.  It  is  inclined  to  the  equator  at  an  angle 
of  23°  27.' 

Equinox  When  the  sun  appears  at  the  equator  the  days  and  nights  are  of 
equal  length  (March  21,  Vernal  or  Spring  Equinox,  and  Sept.  23,  Autumnal  or 
Fall  Equinox). 

Solstice  The  point  on  the  Ecliptic  the  farthest  north  or  south  of  the  Equa- 
tor. (Summer  Solstice,  when  north  of  the  equator,  June  22 ;  Winter  Solstice, 
when  south  of  the  equator,  Dec.  22). 

(See  also  fig.  87). 


164 
§  265. 


DO  AND  DO  NOT  WITH  SUNLIGHT 


[CH.  VI 


Summary  of  Chapter  VI: 

Do  Do  NOT 

i .  Utilize  sunlight  when  that         i .  Do  not  use  artificial  light 
is  available,  for  it  is  the  bright-     in  a  region  where  bright  sun- 
est  light  to  be  had  on  our  planet     light  is  constantly  available. 
(§  232). 


2.  For  sunlight  some  sort  of 
a    heliostat    is    necessary    to 
counterbalance  the  rotation  of 
the  earth,  and  make  the  sun 
shine  in  one  place  continuously 

(§  233)- 

3 .  Two  motions  to  the  mirror 
are  necessary,  an  up  and  down 
motion  and  a  rotary  motion  at 
right  angles  to  this  (§  233). 

4.  If  a  clock-driven  heliostat 
is  used,  the  instrument  must  be 
set  up  so  that  the  shaft  of  the 
clock   shall   point   toward   the 
celestial    pole     and     thus     be 
parallel  with   the   earth's   axis 
(§  239-241). 

5.  To  make  the  shaft  parallel 
to  the  earth's  axis  raise  it  from 
the  horizontal  an  amount  equal 
to   the   latitude    of   the   place 
where  it  is  to  be  used  (§  240). 

6.  The   two-mirror   heliostat 
is  simplest  and  least  expensive 

(§  239). 


2.  If  a  porte-lumiere  is  used 
to  keep  the  sun  shining  in  one 
place,  do  not  forget  to  adjust 
the  mirror  frequently.  Remem- 
ber that  the  earth  never  stops 
rotating. 


3-4.  For  a  clock-driven  helio- 
stat do  not  forget  that  the  shaft 
moving  the  mirror  must  point 
toward  the  north  pole  (or 
south  pole,  if  south  of  the 
equator). 


5.  Do  not  forget  to  elevate 
the  clock-shaft  an  amount  equal 
to  the  latitude  of  the  place. 


6-7.  Do  not  put  the  second 
mirror  of  the  heliostat  so  that 
the  sun  cannot  shine  on  the 
first  mirror. 


CH.  VI] 


DO  AND  DO  NOT  WITH  SUNLIGHT 


165 


7.  One  mirror  is  attached  to 
the  shaft  and  is  driven  by  the 
clock-work.     The  other  mirror 
is  set  by  hand  at  the  beginning 
of  the  experiment  (§  239,  248). 

8.  As  the  rays  of  sunlight  are 
practically    parallel,    only    one 
element    of    the    condenser    is 
needed,  viz.,  the  one  next  the 
lantern  slide  (fig.  74,  §  256). 

9.  To  increase  the  illumina- 
tion  use    a  larger  mirror  and 
condenser  (§257). 

10.  To  turn  the  light  on  and 
off,  use  a  metal  shield  (§262). 


n.  Use  a  heliostat  designed 
for  the  hemisphere  where  you 
are  to  work  (§249-255). 


8.  Do  not  use  a  condenser 
with  two  or  three  lenses  for  sun- 
light as  for  a  near  light  source, 
use  only  one  lens  or  an  achroma- 
tic   combination    designed    for 
parallel  light  (fig.  74). 

9.  One   cannot   increase   the 
illumination  without  increasing 
the  size  of  the  mirror  and  con- 
denser. 

10.  Do  not  use  inflammable 
shields  to  block  the  light.     Use 
a    metal   shield    between    the 
mirror  and  condenser. 

1 1 .  Do  not  try  to  use  a  helio- 
stat in  the  southern  hemisphere 
which  was  constructed  for  use 
in  the  northern  hemisphere. 


CHAPTER  VII 

PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS 
§  270.    Apparatus  and  material  for  Chapter  VII : 

Suitable  projection  room  with  screen  (§  286);  Lantern  with 
projection  objective  of  large  aperture  and  with  suitable  radiant 
and  condenser  (§  275,  277,  279,  294-296);  Suitable  objects  for 
projection  (§  285). 


FIG.  88.     CAMERA  FOR  DRAWING  LANDSCAPES. 
(From  the  Catalogue  of  Queen  &  Co.,  1880). 

The  dark  room  is  made  of  opaque  cloth  over  a  tripod.  The  45°  mirror  at 
the  top  rotates  to  take  in  any  desired  part  of  the  surrounding  country  and  an 
objective  projects  the  image  down  upon  the  horizontal  drawing  shelf. 

If  there  is  to  be  combined  projection,  a  tinted  glass  to  make  the 
lantern-slide  image  as  dim  as  the  opaque  image  (§  282). 
See  also  the  outfit  given  in  §  i,  Ch.  i. 

1 66 


CH.  VII]      PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS          167 

§  271.    Historical  development.    See  appendix. 

References  to  literature :  See  the  books  referred  to  in  Ch.  i,  §  2, 
also  the  special  catalogues  and  directions  furnished  by  the  manu- 
facturers of  Opaque  Lanterns  and  combined  projection  apparatus. 


PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS 
§  272.    All  of  the  images  seen  on  a  white  screen  within  a  dark 
room  were  originally   of   opaque   objects.     These   objects  were 
brilliantly  illuminated  by  the  sun,  and  the  light  reflected  from  them 


FIG.  89.     CAMERA  FOR  EXHIBITING  SURROUNDING  LANDSCAPES. 
(From  the  Catalogue  of  McAllister}. 

In  a  kind  of  cupola  at  the  top  is  situated  a  plane  mirror  and  beneath  that  a 
projection  objective.  The  cupola  rotates,  thus  enabling  the  operator  to  bring 
any  desired  scene  upon  the  horizontal  screen  within  the  room.  Such  cameras 
were  once  common  at  fairs  and  in  parks. 


168         PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [CH.  VII 


\ 


FIG.   90.    DIAGRAM   SHOWING   OPAQUE   PROJECTION. 

In  both  these  diagrams  (fig.  90-91)  the  same  amount  of  light  illuminates  the 
object,  and  the  objects  are  of  the  same  size,  and  the  objectives  have  the 
same  aperture. 

Fig.  90.  Opaque  projection  In.  L.  Incident  light  of  parallel  rays  imping- 
ing upon  a  picture  in  white  and  black. 

Object    The  opaque  object  in  black  and  white  the  size  of  a  lantern  slide. 

1-75  The  beams  of  light  illuminating  the  object.  The  light  must  of 
course  fall  upon  the  surface  facing  the  projection  objective. 

R  L  Reflected  light.  From  each  point  on  the  surface  of  the  opaque  object 
the  light  falling  upon  it  is  reflected  nearly  equally  throughout  the  entire  hemi- 
sphere. 

Ax    Axial  beam  on  the  principal  optic  axis  of  the  objective. 

Objective  The  projection  objective.  Its  aperture  is  such  that  it  receives 
and  transmits  about  20°  of  the  180°  reflected  from  each  point. 

From  the  formula  given  in  §  8573  such  an  objective  transmits  to  form  the 
screen  image  approximately  3%  of  the  light  reflected  from  the  opaque  object. 


CH.  VII]     PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS          169 


15 


FIG.  91.     TRANSPARENCY  PROJECTION. 


In  L  Incident  light.  This  is  supposed  to  be  exactly  the  same  as  that 
striking  the  face  of  the  opaque  object.  In  this  case  it  traverses  the  condenser 
lens,  passes  through  the  transparency,  and  the  objective,  and  passes  on  to 
the  screen  with  very  little  loss. 

7-15     Parallel  beams  of  light  reaching  the  condenser  and  passing  onward. 

Condenser    A  plano-convex  lens  to  render  parallel  rays  converging. 

L  S    Transparent  lantern  slide. 

Ax    The  principal  optic  axis. 

Objective  The  projection  objective.  Its  aperture  is  the  same  as  in  fig.  90, 
but  is  much  larger  than  necessary  for  the  transparency. 


1 70        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [Cn.  VII 

passing  through  a  hole,  or  later  a  lens,  in  the  wall  of  a  dark  room 
sufficed  to  produce  the  picture  on  the  white  wall  or  screen. 

Later  it  was  found  that  it  was  possible  to  illuminate  objects 
sufficiently  with  artificial  light  to  get  screen  pictures;  and  still 
later  transparencies  were  used  (§  272a). 

Every  one  who  looks  at  the  picture  of  a  landscape,  etc.,  depicted 
on  the  ground  glass  of  a  photographic  camera  sees  inverted  images 
like  those  originally  observed  in  darkened  rooms  on  translucent 
screens. 


CONDITIONS  FOR  OPAQUE  PROJECTION:     COMPARISON  OF  PROJEC- 
TION WITH  OPAQUE  AND  TRANSPARENT  OBJECTS 

§  273.  In  order  to  deal  intelligently  and  successfully  with 
opaque  projection  it  is  necessary  to  comprehend  in  the  very  begin- 
ning the  difference  in  the  conditions  for  obtaining  a  screen  image 
of  an  opaque  object,  and  for  a  screen  image  of  a  transparency 
(lantern  slide,  moving  picture  film  or  microscopic  specimen). 

With  a  transparent  or  semi-transparent  object,  the  light  comes 
from  behind  and  traverses  the  object,  and  goes  on  with  practically 
no  variation  in  direction  to  the  projection  objective.  As  the  light 
reaching  the  lantern  slide  or  transparency  is  directed  by  the  con- 
denser (fig.  91),  the  light  which  illuminates  the  transparency  passes 
on  and  enters  the  projection  objective  and  therefore  serves  for  the 
production  of  the  screen  image  (fig.  1-2). 

With  the  opaque  object,  on  the  other  hand,  all  the  light  which 
produces  the  screen  image  must  be  reflected  from  the  surface  of  the 
object,  and  the  light  which  illuminates  the  object  must  strike  its 


§  272a.  In  the  early  days  of  opaque  projection  with  artificial  light  the  whole 
face  of  a  man  was  sometimes  shown;  this,  of  course,  required  very  large  lenses. 

This  is  what  Hepworth  says  concerning  these  exhibitions:  "At  one  time  a 
large  instrument  of  this  type  was  made  for  casting  the  image  of  a  human  face 
on  the  screen,  the  lenses  being  of  immense  size1.  .  .  It  was,  of  course,  fitted 
with  a  reversing  (erecting)  lens  (fig.  208),  so  that  the  face  should  appear  right 
way  up.  The  owner  of  this  face,  by  the  way,  suffered  tortures  during  the  short 
time  of  exhibition,  for  the  powerful  lime  lights  close  to  and  on  each  side  of  his 
head,  were  so  hot  that  they  blistered  his  skin.  He  was  made  to  smile  at  the 
audience,  and  then  to  drink  their  good  health  in  a  glass  of  wine,  a  refreshment 
which  the  poor  man  really  needed  after  his  grilling."  (P.  246). 


CH.  VII]     PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS  171 

face  instead  of  traversing  it, — that  is,  it  must  extend  in  the  opposite 
direction  from  that  used  with  the  transparency. 

The  light  falling  upon  the  face  of  the  opaque  object  must  then 
be  reflected  from  each  point.  But  unlike  the  transparent  object, 
in  which  practically  all  of  the  light  illuminating  each  point  of  the 
object  goes  directly  to  the  projection  objective  (fig.  91),  with  the 
opaque  object,  each  point  reflects  the  light  irregularly  and  in  all 
directions  within  the  entire  hemisphere  (i  80  degrees,  fig.  90) .  This 
being  the  case,  only  a  part  of  the  light  reflected  from  each  point  can 
get  into  the  projection  objective,  all  the  rest  falling  outside  the 
objective.  Of  course,  the  larger  and  closer  the  objective,  the  more 
of  the  light  will  be  received;  hence,  in  selecting  an  objective  for 
opaque  projection,  keep  in  mind  that  the  greater  the  diameter  of 
the  lenses  the  more  light  from  each  point  can  be  received,  and  con- 
sequently the  more  brilliant  will  be  the  screen  picture. 

It  is  assumed  in  this  discussion,  and  in  the  accompanying  dia- 
grams (fig.  90-91),  that  the  opaque  object  is  black  and  white  and 
that  it  and  the  transparent  lantern  slide  are  of  the  same  size; 
that  both  are  lighted  by  a  similar  beam  of  parallel  light  rays,  and 
that  none  of  the  light  is  lost  by  absorption. 

§  274.  Relative  amount  of  light  for  the  images  with  trans- 
parencies and  opaque  objects. — If,  for  example,  as  in  the  diagram, 
the  projection  objective  can  receive  but  20  degrees  of  the  hem- 
isphere of  light  from  each  point,  then  160  degrees  will  fall  outside  the 
objective  and  not  aid  at  all  in  the  formation  of  the  screen  image. 
If  the  objective  could  take  in  all  of  the  light  from  each  point,  the 
opaque  object  would  give  as  brilliant  a  screen  image  as  the  lantern 
slide,  but  the  actual  proportion  of  light  represented  by  the  angle  of 
twenty  degrees  is  only  three  per  cent,  of  that  represented  by  180 
degrees.  As  only  three  per  cent,  of  the  light  from  each  point  helps 
in  the  formation  of  the  screen  image  of  the  opaque  object,  the 
opaque  object  can  give  a  screen  picture  only  three  per  cent,  as 
bright  as  the  transparency  where  practically  all  of  the  light  helps 
to  form  the  screen  image  (fig.  90—91). 

In  practice,  how  great  a  proportion  of  light  serves  for  the  screen 
image  and  how  much  is  absorbed  or  lost  depends  upon  the  opacity 


172         PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      [Cn.  VII 

of  the  lantern  slide  and  the  reflecting  qualities  of  the  opaque  object 
(see  §  274a). 

§  275.  Aperture  of  the  projection  objective  for  transparencies 
and  for  opaque  objects. — By  comparing  figures  90-91  it  will  be 
seen  that  for  a  transparency,  relatively  small  aperture  for  the 
projection  objective  is  sufficient.  This  also  shows  that  if  one  were 
to  use  the  same  objective  for  both  transparencies  and  for  opaque 
objects,  that  the  difference  in  brightness  would  be  enormously 
exaggerated,  if  one  used  only  the  necessary  aperture  for  the  trans- 
parencies. If  one  used  the  proper  objective  for  the  opaque  object, 
it  would  answer  well  for  the  transparency,  but  only  a  part  of  the 
aperture  would  be  utilized.  As  the  large  aperture  makes  the 
objective  very  expensive,  one  wastes  money  by  having  the  large 
aperture  for  transparencies  In  the  best  practice,  an  objective  of 
moderate  aperture  is  used  for  transparencies,  and  one  of  relatively 
very  large  aperture  for  opaque  projection. 

§  276.  As  will  beshownlater  (Ch.  XIV,  §  857a),with  a  given 
object  and  a  given  illumination,  the  brilliancy  of  the  screen  image 
depends  upon  the  aperture  of  the  objective  and  its  distance  from 
the  screen.  The  larger  the  diameter  of  the  lenses  of  an  objective 


§  274a.     Light  flux  getting  through  the  objective  with  opaque  projection.  — 

It  will  be  shown  in  §  857a  that  the  light  received  from  a  perfectly  white,  per- 
fectly diffusing  surface  is 


Sin  26  d28    _    _    ^B 

=   ~       (i-cos  26) 


=   2QOOO  (i  —  cos  28}  lumens  per  square  centimeter  of  the  white  reflecting 

surface,  where  I  is  the  intensity  of  illumination  of  the  surface  measured  in 
meter  candles,  and  6  is  the  half  angle  of  light  subtended  by  the  objective,  or  20 
is  the  angle  of  light  subtended  by  the  objective.  The  light  received  by  the 
surface  is  I/  10,000  lumens  and  the  proportion  of  light  received  by  the  surface 

which  strikes  the  objective  is  then  — 

In  this  problem  the  angle  of  light  subtended  by  the  objective  is  20°,  i.  e. 
26  =  20°.  The  proportion  of  light  received  by  the  objective  is  then  (i  —  cos 
20°)  /2  =  (i  —  .9397)  /2  =  .0603/2  =  .0302  or  about  3%. 


CH.  VII]     PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS          173 

of  given  focus,  the  greater  will  be  the  brightness.  With  the  same 
objective,  the  greater  the  distance  of  the  objective  from  the  screen, 
the  less  will  be  the  brightness 


FIG.    92.       CHADBURN'S    OPAQUE 
LANTERN  WITH  ONE  SOURCE. 

OF  LIGHT. 
(From  Chad-wick,  Hepworth  and 

Wright}. 

L  Source  of  light  shining  directly 
upon  the  opaque  object. 

M  Beam  of  light  from  the  opaque 
object  to  the  objective  and  to  the 
screen. 


FIG.  93.     CHADBURN'S  OPAQUE   LAN- 
TERN WITH  Two  SOURCES  OF  LIGHT. 
(From  Chadwick,  Hepworth  and  Wright). 

This  form  requires  two  sources  of 
light  and  two  condensers.  The  light 
is  projected  directly  upon  the  object 
and  from  the  object  it  extends  out 
through  the  objective  to  the  screen. 
This  method  is  still  often  employed. 

The  same  lantern,  connected  in  the 
usual  way,   was  employed  for  trans- 
parency projection  (fig.  i). 
L-L     Source  of  light  and  condenser  arranged  to  send  the  light  directly  to 
the  opaque  object. 

D-D  Hinged  door  for  the  support  of  the  book,  picture  or  other  object. 
When  the  door  is  closed,  the  light  from  both  sources  shines  directly  upon  the 
opaque  object. 

B     Beam  of  light  from  the  object  to  the  objective. 

A  Objective  of  large  aperture  for  projecting  the  image  of  the  opaque  object 
upon  the  screen. 


174        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      [Cn.  VII 

§  277.  Brilliant  screen  images  of  opaque  objects. — It  is  intel- 
ligible from  the  above  discussion  and  the  diagrams  that  to  produce 
a  brilliant  screen  image  of  an  opaque  object  five  things  are  neces- 
sary: 

1.  The  light  for  illuminating  the  opaque  object  must  be  very 

brilliant,  like  sunlight  or  the  electric  arc  light. 

2.  The  opaque  object  must  be  capable  of  reflecting  most  of  the 

light  illuminating  it,  or  must  be  on  a  white  background. 

3.  The  projection  objective  must  have  lenses  of  large  diameter. 

4.  The  distance  of  the  objective  from  the  screen  must  not  be 

too  great. 

5.  Besides  the  above,  the  projection  room  must  be  dark  or  the 

screen  image  will  not  have  sufficient  contrast. 


FIG.  94.    DOLBEAR'S  OPAQUE  PROJECTOR  WITH  SUNLIGHT. 
(From  Dolbear's  Art  of  Projecting}. 

H  Heliostat,  porte-lumiere  or  simply  a  plane  mirror  to  direct  the  sunlight 
through  the  bi-convex  condenser. 

r  Movable  mirror  to  reflect  the  sunlight  upon  the  opaque  object  at  d. 
The  handle  for  changing  the  inclination  of  the  mirror  is  seen  at  the  right. 

d     Opaque  object  with  the  light  from  the  mirror  (r}  illuminating  it. 

o     Projection  objective. 

S    Screen  for  the  image. 

§  278.  Position  of  the  radiant. — The  radiant  or  source  of  light 
for  illuminating  opaque  objects  for  projection  may  have  either  of 
two  positions: 


CH.  VII]      PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         175 

1.  It  may  be  in  front  of  the  object  so  that  the  light  emitted 

shines  directly  on  it.  This  is  the  original  device  and  gives 
the  greatest  amount  of  light  (fig.  92-93);  or  the  radiant 
may  be  tilted  (fig.  105,  in). 

2.  The  second  method  is  to  have  the  light  not  in  front,  but  a 

mirror  reflects  the  light  from  the  radiant  upon  the  opaque 
object  (fig.  94,  95).  This  is  usually  a  more  convenient 
arrangement  than  the  above,  but  a  certain  amount  of  the 
light  (between  10%  and  25%)  is  lost  when  reflected 
from  a  mirror. 

§  279.  Use  of  a  condenser  or  concave  reflector  with  opaque 
projection. — This  is  frequently  employed  for  the  object  is  often  at  a 
considerable  distance  from  the  radiant,  and  too  small  a  part  of  the 
light  from  the  radiant  would  be  available  but  for  the  help  of  the 
condenser. 

In  most  cases  only  the  first  element  of  the  condenser  is  used. 
This  projects  upon  the  object  or  the  mirror  a  cylinder  of  parallel 
rays  (fig.  90,  103) .  Sometimes  also  a  converging  lens  of  long  focus 
is  put  in  the  path  of  the  parallel  cylinder  to  concentrate  it  more  or 
less,  depending  upon  the  size  of  the  object  to  be  shown.  Instead 
of  a  condenser,  there  is  sometimes  used  a  reflector  (fig.  95,  96) 
behind  the  radiant. 

§  280.  Darkness  of  the  projection  room. — Owing  to  the  diffi- 
culty of  obtaining  a  sufficiently  brilliant  screen  image  it  is  necessary 
to  have  the  projection  room  very  dark. 

COMBINATION  LANTERN  SLIDE  AND  OPAQUE  PROJECTION 
§  281.  Daylight  and  twilight  vision. — Nearly  all  modern 
apparatus  giving  opaque  projection  also  gives  transparency  pro- 
jection with  a  slight  change.  These  two  kinds  of  projection  are 
mutually  antagonistic  for  the  adjustments  of  the  eyes  of  the  specta- 
tors. For  transparency  projection  the  image  is  so  brilliant  that 
the  eyes  are  adjusted  for  daylight  vision  in  large  part,  while  for 
the  opaque  projection  the  image  is  so  dim  that  the  eyes  should  be 
adjusted  for  twilight  or  night  vision. 


176         PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      [Cn.  VII 


FIG.  95. 


ZEISS  EPIDIASCOPE  FOR  OPAQUE  AND  FOR  TRANSPARENT  OBJECTS 
IN  A  HORIZONTAL  POSITION. 


(Zeiss'  Special  Catalogue). 

As  shown  in  this  figure  the  apparatus  is  set  up  for  opaque  objects.  For 
transparent  objects  M2  (mirror  2)  is  removed  when  the  light  striking  M3  is 
reflected  to  M4  and  thence  up  through  the  object  to  M1  and  to  the  screen. 

Commencing  at  the  right:  R  Parabolic  reflector,  which  projects  the  light 
from  the  crater  through  (W)  the  water-cell  to  M1  the  mirror  which  is  at  the 
proper  angle  for  reflecting  the  light  down  upon  the  opaque  object.  From  the 
opaque  object  the  light  is  irregularly  reflected  through  the  objective  to  M1. 
M1  serves  to  reflect  the  rays  from  the  objective  to  the  screen. 

V  Ventilator.  M3  and  M4  are  mirrors  for  use  in  reflecting  the  light  through 
horizontal  transparent  objects. 


CH.  VII]       PROJECTION  OF  IMAGES  v)F  OPAQUE  OBJECTS        177 

This  apparatus  is  designed  to  project  opaque  objects  as  large  as  22  centi- 
meters in  diameter,  at  a  magnification  of  five  to  ten  with  a  30  ampere  current. 
For  a  smaller  object  one  may  magnify  as  high  as  25  diameters.  With  a  50 
ampere  current  and  a  larger  reflector  the  magnification  may  be  from  14  up  to 
37  diameters. 

In  this  instrument  the  carbons  are  horizontal  and  in  the  optic  axis.  The 
parabolic  reflector  (R)  serves  to  direct  the  light  in  a  parallel  beam  along  the 
line  of  the  optic  axis. 

It  takes  considerable  time  for  the  eyes  to  adjust  themselves, 
hence,  if  one  passes  quickly  from  opaque  projection  to  lantern 
slides  the  screen  images  are  dazzling.  On  the  other  hand  in  passing 
from  lantern-slide  images  to  opaque  images,  the  eyes  being  adjusted 
for  daylight  vision,  the  screen  images  seem  exceedingly  dim  at 
first,  although  the  screen  image  may  be  as  brilliant  as  it  is  possible 
to  obtain  with  the  best  apparatus.  After  the  eyes  gain  their 
twilight  vision  the  images  on  the  screen  appear  much  brighter,  as 
if  the  light  had  been  greatly  increased.  As  old  observers  put  it: 
"It  is  necessary  to  get  the  brilliant  sunshine  out  of  the  eyes  before 
the  relatively  dim  screen  images  are  satisfactory." 

§  282.  Dim  and  brilliant  light  in  combined  projection. — This 
difficulty  can  be  avoided  in  two  ways : 

1.  In  showing  lantern  slides,  the  current  may  be  lessened  until 

the  light  forming  the  image  of  the  transparency  is  of  about 
the  same  intensity  as  is  that  of  the  opaque  object  with  the 
full  current. 

2.  A  neutral  tinted  glass  of  the  proper  shade  can  be  put  in  the 

path  of  the  beam  going  to  the  lantern  slide,  to  tone  down 
the  brilliancy  (§  2820). 


§  282a.  In  1908-1909  this  difficulty  was  in  part  overcome  by  Mr.  A.  O. 
Potter  by  putting  a  tinted  glass  of  the  proper  light  reducing  power  in  the  path 
of  the  beam  going  to  the  lantern  slide.  This  reduces  the  image  of  the  trans- 
parency to  the  same  dimness  as  the  opaque  object,  hence  one  can  pass  from 
one  to  the  other  without  any  adjustment  of  the  eyes. 

If  only  lantern  slides  are  to  be  shown,  the  tinted  glass  can  be  removed  and 
the  full  light  employed. 

Some  combined  lanterns,  as  those  of  the  Bausch  &  Lomb  Optical  Co.,  and 
perhaps  others,  are  now  regularly  supplied  with  the  light  reducing  glass  for  the 
magic  lantern  part. 


1 78          PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      ICn.  VII 


FIG.  96.     UNIVERSAL  PROJECTION  APPARATUS  WITH  THE  PROJECTION 
MICROSCOPE  IN  POSITION. 
(Cut  loaned  by  E.   Leitz). 


CH.  VII]      PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         179 

This  apparatus  is  designed  for  all  kinds  of  projection,  and  with  the  objects 
either  in  a  vertical  or  in  a  horizontal  position.  When  the  object  is  in  a  vertical 
position  the  illuminating  device  (arc  lamp  with  parabolic  reflector)  sends  the 
light  horizontally  through  the  specimen,  apparatus  and  to  the  screen  as  would 
be  the  case  in  the  figure  here  shown. 

If  the  object  is  in  a  horizontal  position  the  lamp  and  reflector  remain  in  a 
horizontal  position  and  the  light  is  reflected  by  a  mirror  upon  the  opaque 
object;  or  for  vertical  opaque  objects  the  radiant  is  turned  sidewise. 

For  transparencies  in  a  horizontal  position  the  lamp  and  reflector  are  lowered 
to  the  level  of  one  of  the  mirrors  below,  and  this  mirror  reflects  the  horizontal 
beam  up  through  the  transparent  object  whence  it  passes  to  the  projector  and 
the  screen. 

The  entire  apparatus  is  covered  by  a  dark  curtain    (compare  fig.  95). 


USE  OF  OPAQUE  PROJECTION  FOR  EXHIBITIONS  AND  FOR 
DEMONSTRATIONS 

§  283.  Testing  the  lantern. — The  directions  given  in  Chapter  I, 
§  26  are  applicable  here. 

§  284.  Size  of  objects  for  opaque  projection. — The  size  of 
object  which  can  be  shown  with  an  opaque  projector  varies  greatly. 
The  smallest  size  is  usually  larger  than  a  lantern  slide.  The  lan- 
tern-slide opening  is  rarely  greater  than  6.5  x  7.5  cm.  (2.6  x  3  in.), 
while  the  smallest  picture  usually  shown  in  the  opaque  lantern  is 
rarely  less  than  postal  card  size  (8  x  12.5  cm.,  3x5  in.).  From 
this  minimum  the  size  ranges  all  the  way  up  to  50  cm.  (20  in.) 
square. 

Of  course  the  radiant  and  condenser  must  vary  accordingly 
(see  fig.  107). 

§  285.  Objects  for  opaque  projection. — The  best  of  all  are  dull 
white  objects,  like  marble  figures,  or  black  print  on  white  paper, 
pictures  in  black  and  white.  Colored  pictures  in  which  the  bright 
colors  of  the  spectrum  like  red,  yellow  and  green,  are  predominant, 
give  good  images.  Metallic  objects  with  polished  surfaces  give 
good  images.  Among  these  the  works  of  a  watch  or  small  clock 
show  well;  also  coins  and  medals.  Bright  metallic  objects  show 
best  on  a  dark  ground. 

Objects  and  pictures  which  are  very  light-absorbing  naturally 
will  not  give  good  screen  images,  no  matter  how  brilliant  the  light 
or  good  the  apparatus.  If  the  outlines  of  such  objects  are  what  is 


i8o       PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS        [Cn.  VII 


FIG.  97.     THOMPSON'S  REFLECTOSCOPE,  MODEL  G-2,  1913. 
(Cut  loaned  by  A.  T.  Thompson  &  Co.). 

As  here  shown  the  instrument  is  ready  for  opaque  and  for  transparency 
projection. 

There  are  additional  attachments  by  which  microscopic  projection  can  be 
done  with  either  a  horizontal  or  a  vertical  microscope.  There  is  also  an 
arrangement  for  placing  the  magic  lantern  objective  in  a  vertical  position, 
and  thus  projecting  horizontal  objects. 

Commencing  at  the  right :  The  lamp-house  with  arc  lamp  and  condenser. 
This  is  at  an  angle  so  that  opaque  objects  in  a  vertical  position  are  lighted 
directly  as  in  Chadburn's  opaque  lantern  (fig.  92).  In  this  case  the  screen 
picture  has  the  rights  and  lefts  reversed. 

Above  is  the  magic  lantern  objective  for  transparencies. 

Below  is  the  large  aperture,  long  focus  projection  objective  for  opaque  ob- 
jects. The  objective  is  inserted  in  the  dark  chamber  containing  mirrors  for 
reflecting  the  light  upward  for  transparency  projection,  or  downward  for  the 
opaque  objects  in  a  horizontal  position. 

Above  is  shown  a  lantern  slide  in  the  carrier  and  below  a  book  in  a  horizontal 
and  a  picture  in  a  vertical  position. 

With  the  opaque  object  in  a  horizontal  position  the  light  is  reflected  from  a 
mirror  down  upon  the  object,  the  light  from  the  opaque  object  is  then  reflected, 
in  part,  back  to  the  same  mirror  and  from  the  mirror  out  through  the  projec- 
tion objective  to  the  screen.  The  screen  image  in  this  case  will  be  erect  in 
every  way  if  properly  placed  on  the  holder. 


CH.  VII]      PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         181 

wanted  very  good  results  can  be  obtained  by  using  a  white  back- 
ground. They  will  appear  like  silhouettes,  but  almost  no  details 
will  show. 

§  286.  Screens  for  opaque  projection. — On  the  whole  no  screen 
is  so  satisfactory  as  a  white  one  of  the  best  quality  (see  §  621). 

If  the  room  is  narrow,  so  that  all  the  spectators  are  included 
in  about  30  degrees,  the  metallic  screen  answers  fairly  well.  If  the 
room  is  wide,  those  on  the  sides  near  the  screen  will  get  only  a  very 
dim  screen  image  from  the  metallic  screen.  With  the  white  screen 
it  is  practically  as  good  in  one  place  as  in  another,  for  the  reflection 
is  about  equal  throughout  the  entire  180  degrees  (§  622,  630). 

For  darkening  the  room  see  §  280  and  §  608. 

§  287.     Magnification  of  the  picture  and  size  of  screen  image. — 

For  lantern  slides  the  magnification  can  be  30  to  60,  with  resulting 
brilliant  pictures;  but  with  opaque  projection  one  can  rarely 
magnify  more  than  six  to  ten  times  and  get  good  results. 

If  the  area  to  be  shown  is  relatively  small  and  the  illuminating 
beam  is  made  converging  and  a  powerful  radiant  (50  amperes)  is 
used,  the  magnification  may  be  carried  up  to  25  or  37  diameters 
(Zeiss,  p.  6)  or  perhaps  more. 

The  screen  image  should  not  exceed  2  x  2,  or  3  x  3  meters  (8  x  10 
feet),  (Zeiss,  p.  6). 

§  288.  Screen  distance. — In  opaque  projection,  the  screen 
images  are  usually  not  magnified  so  much  as  lantern-slide  images 
and  the  screen  distance  is  usually  from  three  to  ten  meters.  The 
correct  magnification  (six  to  ten)  is  obtained  by  using  an  objective 
of  the  proper  focal  length,  i.  e.,  for  a  magnification  of  six  and  a 
screen  distance  of  three  meters  there  should  be  an  objective  of  50 
cm.  or  20  in.  If  the  magnification  is  to  be  10  and  the  screen  dis- 
tance three  meters  then  the  objective  should  have  a  focus  of  30  cm. 
or  12  inches.  For  the  discussion  relating  to  magnification,  screen 
distance,  and  focus  of  the  objective  see  §  392a. 

Sometimes  it  is  necessary  to  project  at  a  screen  distance  of  15  to 
20  meters  (50  to  70  feet) .  As  the  magnification  of  the  screen  image 
must  not  usually  exceed  six  to  ten,  a  very  long  focus  projection 


182         PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      [Cn.  VII 


FIG.  98.     THE  NEW  REFLECTING  LANTERN  OF  WILLIAMS  BROWN  &  EARLE 

(No.   3   BR   15). 
(Cut  loaned  by  Williams  Brown  &  Earle). 

This  is  a  combination  projector  for  lantern  slides  and  for  opaque  objects. 

Commencing  at  the  right: 

N  Arc  lamp  in  the  lamp-house  with  the  feeding  screws  extending  beyond 
the  lamp-house. 

M    Lamp-house  of  metal  with  the  ventilator  at  the  top. 

C    First  element  of  the  condenser  for  giving  approximately  parallel  rays. 

D  The  opaque  object  in  position.  The  light  from  the  lamp  shines  directly 
upon  it  and  is  reflected  outward  toward  the  projection  objective  (£). 

E     Projection  objective  for  opaque  objects. 

F  Mirror  for  reflecting  the  image  of  the  opaque  object  to  the  screen  and  for 
correcting  the  right  to  left  inversion. 

B  Water-cell  and  second  element  of  the  condenser  for  transparency  pro- 
jection. 

A     Opening  for  the  lantern-slide  carrier. 

L     Projection  objective  for  lantern  slides. 

For  lantern-slide  projection  a  mirror  at  C  is  brought  into  position  to  reflect 
the  light  out  along  the  optic  axis  of  B  and  L. 

objective  must  be  used  for  such  a  screen  distance.  (For  a  magnifi- 
cation of  six  and  a  15  meter  screen  distance,  an  objective  of  250  cm. 
(100  inches)  is  necessary). 

§  289.  Arc  lamp  and  amount  of  current. — If  one  wishes  to  use 
more  than  25  amperes,  the  arc  lamp  should  be  hand-feed.  Up  to 
25  amperes,  the  right-angled  carbons  work  well.  Beyond  that 
amount  the  inclined  or  vertical  carbons  are  more  satisfactory  for 


CH.  VII]        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         183 

the  right-angled  arc  goes  out  easily  from  the  magnetic  blow  when 
the  current  is  above  25  to "30  amperes.  tOf.  course,  in  opaque  pro- 
jection,  where  the  most  powerful  light  available  is  demanded, 
alternating  current  is  far  less  satisfactory  than  direct  current ;  still 
with  skillful  application  of  the  light  available  even  alternating 


FIG.  99.     THE  INDEPENDENCE  POST-CARD  PROJECTOR. 
(Cut  loaned  by  Williams  Brown  &  Earle). 

This  is  in  principle  exactly  like  Chadburn's  opaque  lantern  with  two  lamps 
(fig-  93)-  In  this  projector  the  lamps  are  usually  of  the  incandescent  form,  and 
connection  is  made  with  the  house-electric  lighting  system. 

current  radiants  give  fairly  good  opaque  projection  (see  Ch.  XIII, 
§  753a  for  size  of  carbons  with  different  currents,  etc.). 

For  favorable  objects  and  good  conditions  one  must  use  not  less 
than  20  to  25  amperes  of  direct  current  for  successful  screen  pic- 
tures of  opaque  objects.  Those  with  most  experience  in  the  work 
use  40  to  50  amperes. 

For  alternating  current  satisfactory  results  can  hardly  be 
obtained  with  less  than  40  amperes,  and  60  to  80  are  better. 


1 84        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [Cn.  VII 


FIG.  'loo.     HOME  BALOPTICON  FOR  OPAQUE  OBJECTS. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

In  this  instrument  there  is  used  a  small  arc  light  for  attachment  to  the  house 
lighting  system.  The  rheostat  is  shown  at  the  left. 

The  object  is  horizontal  and  the  lamp  shines  in  part  directly  upon  the  object 
and  in  part  the  light  is  reflected  upon  the  object  by  a  mirror.  From  the  object 
light  is  reflected  to  a  mirror  above  the  arc  light,  and  from  the  mirror  directed 
out  through  the  objective  to  the  screen.  The  projected  mirror  image  appears 
erect  on  the  screen. 


FIG.  10 1.     HOME  BALOPTICON  FOR  LANTERN  SLIDES  AND 
OPAQUE*  OBJECTS. 

•  (Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

jThe  opaque  projection  is  precisely  as  in  fig.  100.  For  lantern-slide  projec- 
tion thejiiirror  in  front  of  the  arc  lamp  is  turned  up  out  of  the  way  and  the 
light  passes  on  to  the  condenser,  lantern  slide  and  objective  as  in  ordinary 
lantern-s;lide  projection  (fig.  i). ...  , :  .  ,>  .  .  ^ 


CH.  VII]     PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS          185 


FIG.  1 02.    UNIVERSAL  PROJECTOSCOPE. 
(Cut  loaned  by  C.  H.  Stoelting  Company}. 


This  instrument  as  shown  in  the  picture  is  designed  to  project : 

(1)  Lantern  slides  and  other  transparencies  in  the  usual  vertical  position 
or  in  a  horizontal  position. 

(2)  Opaque  objects. 

(3)  Microscopic  objects.     For  this  the  lantern-slide   objective  is  turned 
back  and  the  microscope  turned  up  in  place. 


1 86        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [CH.  VII 

§  290.  Precaution  for  heavy  currents. — The  lamps  for  heavy 
currents  are  mostly  of  the  hand-feed  type  and  burn  large  carbons. 
When  starting  the  lamp  it  is  much  safer  to  make  sure  that  the  car- 
bons are  separated  before  closing  the  knife  switch.  Then  one  can 
use  the  feeding  screws  and  bring  the  carbons  together  to  strike  the 
arc,  and  separate  them  a  short  distance  immediately.  If  the 


FIG.  103.    DIAGRAM  OF  THE  PARTS  AND  COURSE  OF  THE  RAYS  IN  THE 

UNIVERSAL    PROJECTOSCOPE    FOR    OPAQUE    AND    LANTERN-SLIDE 

PROJECTION. 

(Cut  loaned  by  the  C.  H.  Stoelting  Company}. 

The  instrument  is  here  arranged  for  the  projection  of  opaque  objects.  The 
mirror,  Mlt  reflects  the  parallel  beam  from  the  first  element  of  the  condenser 
(C),  down  on  the  horizontally  placed  object.  The  large  aperture  projection 
objective  directly  above,  and  the  45°  mirror  beyond,  project  the  image  upon 
the  screen. 

Ordinary  lantern-slide  projection  is  shown  by  the  broken  lines,  (for  a  de- 
tailed description  of  all  the  parts  see  fig.  16). 


CH.  VII]       PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         187 

carbons  are  in  contact  after  striking  the  arc,  so  much  current  flows 
that  there  is  danger  of  blowing  the  fuses  or  burning  out  some  con- 
nection. Be  sure  that  the  fuses  and  wiring  are  adapted  to  the 
current  (fig.  3,  §  691). 

§  291 .  Illuminating  the  entire  opaque  object. — For  illuminating 
opaque  objects,  Zeiss  uses  the  principle  of  the  search-light.  That 
is,  the  two  carbons  are  horizontal,  the  positive  one  has  its  crater 
facing  the  concave  mirror  (fig.  95,  96).  This  mirror  then  reflects 
the  light  toward  the  object.  Depending  upon  its  position,  it  can 


FIG.   104.     NEW  MODEL  CONVERTIBLE  BALOPTICON  IN  POSITION  FOR 

OPAQUE  PROJECTION. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

In  the  new  (1913)  models  of  projectors  by  the  Bausch  &  Lomb  Optical  Com- 
pany provision  is  made  in  each  case  to  place  the  object  in  a  horizontal  position 
and  then  to  illuminate  it  either  by  a  mirror  (fig.  iO5a)  or  preferably  by  tilting 
the  radiant  and  first  element  of  the  condenser  (fig.  105),  so  that  the  light  from 
the  lamp  is  projected  directly  upon  the  object.  From  the  object  a  part  of  the 
light  extends  out  through  the  vertically  placed  projection  objective  to  the 
mirror  and  from  the  mirror  to  the  screen.  The  mirror  gives  correct  images  on 
the  screen. 


1 88         PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [Cn.  VII 

direct  a  parallel  beam,  a  converging  or  a  diverging  beam  (see  also 
Ch.  XIII-XIV  on  radiants  and  lighting). 

If  a  condenser  is  used,  its  size  must  be  adapted  to  the  size  of  the 
object,  that  is,  the  diameter  of  the  cylinder  of  light  must  be  some- 


FIG.  105.     DIAGRAM  SHOWING  THE  OPTICAL  PARTS  AND  THE  COURSE  OF 
THE  RAYS  IN  THE  CONVERTIBLE  BALOPTICON  IN  OPAQUE  PROJECTION. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

The  lamp-house,  radiant  and  first  element  of  the  condenser  are  so  inclined 
upward  that  the  light  from  the  condenser  falls  directly  upon  the  opaque  object. 

A     Upper  carbon  of  the  arc  lamp  furnishing  the  light. 

B  First  element  of  the  condenser  to  render  the  diverging  light  parallel. 
The  lens  beyond  the  meniscus  is  double-convex  instead  of  plano-convex  as  in 
fig-  3- 

D  Position  of  the  opaque  object.  Objects  as  large  as  20  x  20  cm.  (8x8 
inches)  can  be  illuminated  and  projected. 

E     Large  aperture  projection  objective  in  a  vertical  position. 

F  Mirror  beyond  the  objective  to  reflect  the  image  to  the  screen  and  correct 
the  inversion. 

C  Mirror.  It  serves  to  increase  the  illumination  of  the  opaque  object  by 
reflecting  back  upon  it  some  of  the  scattered  light. 

G     Second  element  of  the  condenser  for  lantern-slide  projection  (fig.  3). 

H    Projection  objective  for  lantern  slides. 

O     Bellows. 

M    Lathe  bed  on  which  slide  the  objective,  etc. 


CH.  VII]        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS        189 


a  /  /0\ 

^     /  /  A-^  \ 


FIG.  losa.     DIAGRAM  SHOWING  THE  COURSE  OF  THE  LIGHT  RAYS  FOR 
TRANSPARENCY  AND  OPAQUE  PROJECTION  WITH  THE  RADIANT  HORIZONTAL. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

A     Upper  carbon  of  the  arc  lamp. 
B     The  first  element  of  the  condenser  (fig.  3). 

C  C    Mirror  horizontal  when  using  lantern  slides  and  inclined  for  opaque 
projection. 

D     Horizontal  surface  for  opaque  objects  (20  x  20  cm.,  8x8  in.). 

E     Projection  objective  for  opaque  objects. 

F    Mirror  for  reflecting  the  light  to  the  screen  and  correcting  the  inversion. 

G     Second  element  of  the  condenser  for  lantern  slides. 

H    Projection  objective  for  lantern  slides. 

N    Support  for  condenser  and  bellows. 

O    Bellows. 

M   Lathe  bed  on  which  move  the  various  supports. 

what  greater  than  the  diagonal  measuring  the  size  of  the  picture,  as 
for  lantern  slides  (see  §  314,  fig.  114).  A  diverging  beam  could  be 
used  by  pushing  the  radiant  within  the  focal  distance,  and  a  con- 
verging by  separating  farther  than  the  focal  distance.  Sometimes 
there  is  no  condenser  but  the  radiant  shines  directly  upon  the 
object  (fig.  99,  100,  107). 


i  go        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [Ce.  VII 

§  292.  Avoidance  of  shadows. — With  solid  objects  there  will  be 
very  heavy  shadows  unless  the  light  is  evenly  distributed.  With  a 
single  lamp  this  is  not  easily  accomplished,  and  if  no  mirror  is  used 
practically  impossible.  It  is  better  to  use  two  lamps,  one  on  each 
side,  as  in  the  original  apparatus  of  Chadburn  (fig.  93).  The  two 
lamps  have  the  further  advantage  of  doubling  the  light.  Two  arc 
lamps  are  used  in  the  large  opaque  lantern  of  the  Bausch  &  Lomb 
Opt.  Co.  (fig.  107). 

In  the  Spencer  Lens  Co.'s  opaque  lantern,  plane  mirrors  line  a 
part  of  the  projection  chamber  where  the  object  is  placed,  and  much 
of  the  light  lost  by  absorption  without  this  arrangement  is  reflected 
back  upon  the  object.  This  also  helps  to  obviate  the  shadows 
when  one  lamp  is  used  (fig.  in). 

ERECT  IMAGES  WITH  OPAQUE  OBJECTS 

§  293.  Inversion  of  the  image  with  an  opaque  object.— Besides 
being  upside  down  the  image  of  an  opaque  object  on  an  ordinary 
white  screen  has  the  rights  and  lefts  reversed. 

§  294.  How  to  get  an  erect  image  with  the  object  in  a  vertical 
position. — Put  the  opaque  object  in  the  vertical  position  upside 
down.  Point  the  objective  at  right  angles  to  the  screen,  use  a 
mirror  at  45  degrees,  or  use  a  45  degree  prism  to  direct  the  image- 
forming  rays  upon  the  vertical  opaque  screen  (fig.  95.  in).  If 
the  inversion  of  the  rights  and  lefts  is  unimportant,  put  the  object 
upside  down  in  the  vertical  holder  and  point  the  objective  directly 
toward  the  screen  (fig.  97,  109). 

If  a  translucent  screen  like  ground  glass  is  used  the  image  will  be 
erect  in  every  way  if  it  is  put  upside  down  in  the  holder  and  the 
objective  pointed  directly  toward  the  screen. 

§  295.  How  to  get  an  erect  image  of  an  opaque  object  in  a 
horizontal  position. — Place  the  opaque  object  with  its  upper  edge 
away  from  the  screen.  The  objective  is  usually  in  a  vertical 
position  so  that  the  image  would  appear  on  the  ceiling  above  the 
instrument.  The  mirror  or  prism  used  to  direct  the  image  forming 
rays  upon  the  vertical  screen  corrects  also  the  mirror  image,  and 


CH.  VII]     PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS  191 


FIG.    1 06.     NEW   MODEL    UNIVERSAL   BALOPTICON   IN   POSITION   FOR 

OPAQUE  PROJECTION. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

Opaque  objects  are  placed  in  a  horizontal  position  and  the  lamp-house,  lamp 
and  first  element  of  the  condenser  are  inclined  as  in  fig.  105.  The  light  from 
the  opaque  object  is  reflected  upward  to  the  right  face  of  an  inclined  mirror 
and  from  the  mirror  reflected  out  through  the  projection  objective,  giving  an 
erect  screen  image. 

When  used  for  lantern  slides  the  lamp-house  is  horizontal  and  the  horizontal 
light  is  reflected  upward  by  the  left  face  of  the  mirror  to  the  mirror  at  the  left 
of  the  lantern-slide  attachment.  This  second  mirror  reflects  the  light  hori- 
zontally through  the  lantern  slide. 

the  object  will  be  erect  in  every  way  (fig.  95-111).  (See  also  the 
discussion  of  the  reflecting  lantern  of  Thompson  in  which  a  mirror 
image  is  projected,  and  hence  appears  erect  on  the  screen  (fig.  97, 
100).  If  a  translucent  screen  is  used  with  the  object  in  a  hori- 


1 92          PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS      lCn.  VII 


FIG.  107.     BALOPTICON  FOR  THE  PROJECTION  OF  LARGE  OPAQUE  OBJECTS. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

This  opaque  projector  is  especially  designed  to  show  large  objects  and  large 
surfaces  (20  inches,  50  cm.  square).  To  avoid  shadows  in  projecting  machines 
and  other  solid  objects,  and  to  supply  the  needed  illumination  there  are  two 
25  ampere  lamps  tilted  to  throw  their  light  directly  upon  the  two  opposite 
sides  of  the  object.  Each  lamp  has  its  own  rheostat  and  table  switch. 


CH.  VII]       PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS         193 

The  projection  objective  is  of  the  Tessar  Ic  series  of  very  large  aperture 
(114  mm.,  4^  in.,  in  diameter  and  50  cm.  19^  in.  equivalent  focus).  The 
focusing  is  accomplished  by  a  screw  which  raises  or  lowers  the  table  supporting 
the  object. 

This  instrument  enables  one  to  demonstrate  to  an  audience  the  workings  of 
a  machine  like  a  cash  register,  or  a  quarto  size  page  of  illustrations  or  print. 
With  the  vertical  objective  and  a  mirror  to  reflect  the  light  to  the  screen  the 
image  will  be  erect.  The  reflecting  mirror  is  silvered  on  the  front  to  avoid 
the  doubling  of  the  image. 


FIG.  108.     MODEL  5  DELINEASCOPE  FOR  OPAQUE  AND  LANTERN-SLIDE 

PROJECTION. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

With  the  arc  lamp  and  the  first  element  of  the  condenser  in  a  horizortal 
position  the  light  extends  directly  to  the  right  through  the  lantern  slide  or 
other  object  and  the  projection  objective,  or  projection  microscope,  or  it  may 
be  reflected  upward  through  the  vertical  projection  microscope  (fig.  175). 

For  opaque  projection,  the  arc  lamp  and  first  element  of  the  condenser  are, 
by  means  of  the  crank,  rotated  within  the  lamp-house  to  the  right  position  to 
direct  the  light  upon  an  opaque  object  in  a  vertical  or  in  a  horizontal  position 
as  desired. 

If  the  object  is  in  a  horizontal  position  the  light  from  it  is  reflected  to  a  mirror 
and  from  the  mirror  out  through  the  large  projection  objective.  It  will  appear 
correct  in  the  screen  image.  The  vertical  object  will  have  the  rights  and  lefts 
inverted.  Objects  or  surfaces  15x23  cm.  (6x9  in.)  can  be  projected  with 
this  instrument. 


194        PROJECTION  OF  IMAGES  OF  OPAQUE  OBJECTS       [Cn.  VII 

zontal  position  the  image  will  only  be  erect  with  the  screen  at  right 
angles  to  the  axis  of  the  objective,  no  mirror  or  prism  being  used. 
If  a  mirror  or  prism  is  used  to  project  upon  a  vertical  screen  then  a 
translucent  screen  will  give  a  mirror  image,  but  an  opaque  screen 
an  erect  image. 


FIG.  109.     DIAGRAM  SHOWING  THE  PARTS  AND  COURSE  OF  THE  RAYS  IN 

MODEL  4-5  DELINEASCOPE. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

This  diagram  shows  the  arc  lamp  and  first  element  of  the  condenser  in  posi- 
tion to  illuminate  a  vertical  object  in  opaque  projection.  Above  is  shown  in 
outline  the  course  of  the  rays  for  the  projection  microscope  or  the  magic  lantern. 

Commencing  at  the  left : 

OH    The  object  holder  for  objects  15  x  23  cm.,  6  x  9  in. 

H    Handle  for  operating  the  object  holder. 

X  The  horizontal  axis  on  which  rotates  the  arc  lamp  and  the  first  element 
of  the  condenser. 

OO    The  large  projection  objective  for  opaque  objects. 

WC    Water-cell  for  removing  the  radiant  heat. 

ID     Large  iris  diaphragm. 

L  Lantern  slide,  and  crank  for  turning  the  slide  up  in  the  vertical  position 
in  front  of  the  condenser  behind  the  objective. 

P     Platform  on  which  is  laid  the  lantern  slide. 

LO  Lantern-slide  objective  turned  to  one  side  to  allow  the  microscope  to 
get  to  the  horizontal  position. 

M  Mirror  to  reflect  the  horizontal  beam  of  light  through  the  vertical 
microscope. 


CH.  VII] 


TROUBLES  IN  OPAQUE  PROJECTION 


195 


§  296.    Troubles: 

i .  The  one  great  trouble  will  be  a  dim  screen  image.  This  can- 
not be  wholly  avoided.  It  can  be  made  tolerably  good: 
(i)  by  having  the  room  very  dark ;  (2)  by  using  a  powerful 
radiant;  (3)  by  having  a  projection  objective  of  large 
aperture;  (4)  by  magnifying  the  screen  image  very 
moderately  (5  to  10  diameters). 


FIG.  no.     MODEL  8  DELINEASCOPE  FOR  ALL  KINDS  OF  PROJECTION. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

In  this  instrument  there  is  provision  for  lantern-slide  projection  with  the 
slides  or  other  objects  in  a  vertical  or  in  a  horizontal  position. 

It  provides  for  opaque  objects  in  a  horizontal  position  and  lighted  directly 
by  the  radiant  (fig.  in),  and  for  objects  in  museum  jars  in  a  vertical  or  hori- 
zontal position. 

Finally  it  provides  for  micro-projection  with  the  objects  in  a  vertical  position 
or  in  a  horizontal  position,  and  for  the  drawing  of  objects  on  a  horizontal  or  on 
a  vertical  surface. 


TROUBLES  IN  OPAQUE  PROJECTION 


[CH.  VII 


FIG.  in.     DIAGRAM  OF  MODEL  8  DELINEASCOPE  SHOWING  THE  POSITION 
OF  THE  RADIANT  AND  THE  COURSE  OF  THE  LIGHT  RAYS  FOR  OPAQUE 
PROJECTION  WITH  THE  OBJECT  IN  A  HORIZONTAL  POSITION. 

(Cut  loaned  by  the  Spencer  Lens  Co.}. 

T    Table  for  opaque  objects. 

W    Wheel  by  which  the  table  is  raised  and  lowered. 

D     Diaphragm  above  the  table  for  flattening  out  the  page  of  a  book. 

B  Incandescent  bulb  which  always  gives  light  for  the  interior  of  the 
machine. 

C    Condensing  lenses  in  front  of  the  arc. 

O     Large  objective  for  opaque  projection. 

0T     Smaller  objective  for  vertical  projection. 

M  Mirror  for  throwing  light  downward  for  the  lantern-slide  compartment 
or  upward  through  the  vertical  attachment. 

Mt  Mirror  for  reflecting  a  perpendicular  beam  of  light  out  through  the 
lantern-slide  compartment;  shown  thrown  up  against  the  water-cell  in  this 
figure  (see  fig.  177) 

Ma  Mirror  used  in  connection  with  projection  of  the  vertical  side  of  an 
object. 

M^  Mirror  which  assumes  a  position  at  45°  when  the  microscope  is  used 
perpendicularly. 

P  Prism  which  is  thrown  into  the  prism  box  when  the  microscope  is  used 
in  a  perpendicular  position. 

S  Shelf  upon  which  the  lantern  slide  is  placed  previous  to  throwing  it  up 
into  the  optical  axis  by  the  handle. 

H    Handle  of  the  lever  for  raising  the  slide  into  position. 


CH.  VII]  TROUBLES  IN  OPAQUE  PROJECTION  197 

2 .  If  the  amperage  is  to  exceed  2  5  or  30,  it  is  better  to  use  an  arc 

lamp  with  inclined  or  vertical  carbons,  not  those  at  right 
angles  for  the  magnetic  blow  puts  the  right-angled  arc  out 
too  easily. 

3.  Do  not  have  the  carbons  in  contact  with  a  hand-feed  lamp 

when  the  current  is  turned  on.  Feed  them  together  after 
the  current  is  on,  then  they  can  be  separated  properly 
immediately  after  the  arc  is  struck. 

4.  Inverted  screen  image.     The  object  not  properly  placed  on 

the  support,  or  no  erecting  mirror  or  prism  is  used. 

5.  No  detail  in  the  screen  image.     The  object  may  be  too  light- 

absorbing,  or  the  light  may  not  be  sufficient. 
(See  Troubles  in  Ch.  I.). 


DO  AND  DO  NOT  IN  OPAQUE  PROJECTION        I€H.  VII 


§  297.     Summary  of  Chapter 

Do 

1.  Select  an  objective  of  large 
aperture  for  opaque  projection 

(§  275)- 

2 .  Use  a  light  of  great  brill- 
iance like  sunlight  or  the    arc 
light  (§  274,  277). 


3.  Make    the    screen    image 
only  six  to  ten  times  as  large  as 
the  object  (§  287). 

4.  Make  the  projection  room 
very  dark  (§  280). 

5.  Use  a  very  white  screen  or 
under  some  conditions  a  metal- 
lic screen  (§  286,  621). 

6.  From  25  to  50  amperes  of 
direct   current    are   needed   to 
give    good    opaque    projection 
(§  289). 

7.  If     lantern     slides     and 
opaque  objects  are  projected  at 
the    same    exhibition,    use    a 
neutral  tint   (smoky)   glass  to 
make  the  lantern-slide  image  as 
dim  as  the  image  of  the  opaque 
object  (§  282). 

8.  Use  a  condenser  for  opaque 
objects  somewhat  larger  than 
the  object  (see  fig.  114). 


VII: 

Do  NOT 

1.  Do  not  undertake  opaque 
projection  with  an  objective  of 
small  aperture. 

2.  Do      not      expect      good 
opaque  projection  unless  from 
20  to  50  amperes  of  direct  cur- 
rent, or  greater  amperages  of 
alternating   current   are   avail- 
able. 

3 .  Do  not  try  to  magnify  the 
object  too  much. 

4.  Do  not  try  to  project  in  a 
light  room.     It  must  be  dark. 

5.  Do  not  be  satisfied  with  a 
dirty,  non-reflecting  screen.     It 
must  be  white. 

6.  Do    not    expect    brilliant 
screen  images  with  a  weak  light. 


7.  Do  not  pass  quickly  from 
the    dim    pictures    of    opaque 
objects  to  the  brilliant  pictures 
of    transparencies.      Dim    the 
transparencies     down     to     the 
opaque  images. 

8.  Do  not  use  a  small  con- 
denser for  a  large  object. 


CH.  VII]        DO  AND  DO  NOT  IN  OPAQUE  PROJECTION 


199 


9.  Use  two  radiants  or  mir- 
rors for  avoiding  shadows  with 
solid  objects  (§  292). 


10.  Select  objects  which  re- 
flect well  for  opaque  projection 
(§  285). 

11.  If    very    light-absorbing 
objects  must  be  projected,  use  a 
white  background  (§285). 

1 2 .  Use  a  hand-feed  arc  lamp 
for  opaque   projection    (§  289, 
290). 

13.  Make  sure  that  the  wir- 
ing  is   adapted   to   the   heavy 
currents  needed  for  opaque  pro- 
jection (§  290). 

14.  Use  carbons  of  the  proper 
size    for    the    current    drawn 
(§  290,  753a). 

15.  Make  the  images  erect  by 
placing  the  object  up-side  down 
for  the  vertical  position,  or  with 
the  upper  edge  away  from  the 
screen     for     the     horizontally 
placed  objects  (§  293-294). 

1 6.  Use  a  mirror  or  prism  to 
avoid  a  mirror  image  on  a  ver- 
tical,  opaque    screen    (§    293- 
295). 


9.  Do  not  light  solid  objects 
so    that    there    will    be    deep 
shadows.     Use  two  radiants,  or 
mirrors,  or  arrange  so  that  the 
light  strikes  the  object  directly, 
not  obliquely. 

10.  Do  not  select  badly  re- 
flecting objects  for  opaque  pro- 
jection. 

1 1 .  Do  not  use  a  black  back- 
ground on  which  to  place  dark 
objects. 

12.  Do  not  use  an  automatic 
right-angle  carbon  arc  lamp  for 
the  heavy  currents  needed  for 
opaque  projection. 

13.  Do  not  run  any  risks  by 
using    the   heavy    currents   on 
wiring  not  adapted  to  it. 

14.  Do  not  use  small  carbons 
for  big  currents. 

15.  Do  not  get  the  images 
wrong  side  up  on  the  screen. 


1 6.  And  do  not  expect  too 
much  in  opaque  projection. 
Know  the  principles  involved; 
study  fig.  90-91. 


CHAPTER  VIII 
PREPARATION  OF  LANTERN  SLIDES 

§  310.    Apparatus  and  Material  for  Chapter  VIII: 

A  photographic  dark  room ;  Camera  with  suitable  objectives  and 
plate  holders  (fig.  116-119);  Lantern-slide  plates,  negative  plates 
of  various  kinds;  Chemicals  for  developing,  etc.;  Colors  and 
brushes  for  tinting  the  slides;  A  retouching  frame  (fig.  113); 
Cover-glasses  and  binding  strips  and  mats  for  the  slides ;  Markers 
and  labels  for  the  slides;  Cabinet  for  the  slides  (fig.  120).  • 

§  311.  For  the  historical  development  of  lantern  slides  see  the 
works  referred  to  in  Ch.  I,  §  2,  and  for  photographic  lantern  slides, 
The  Journal  of  the  Royal  Society  of  Arts,  Vol.  LIX  (1911),  pp. 

255-257- 

For  making  and  coloring  lantern  slides  see  the  works  inCh.  I,  §  2, 
and  Lambert,  Lantern-slide  making  and  coloring. 

The  Photo-Mineature  series  No.  9,  Lantern  Slides,  and  No.  83, 
Coloring  Lantern  Slides. 

SIZE  OF  LANTERN  SLIDES 

§  312.  Modern  lantern  slides  are  of  several  standard  sizes  as 
follows :  (See  §  3 1 2a) . 

A.  American  slides. — These  are  oblong  plates  82.5  x  102  mm. 
(3^  x  4  inches).     They  are  designed  to  go  into  the  lantern-slide 
carrier  with  the  long  side  horizontal  (§  35). 

B.  British  slides. — These  are  square,  being  82.5  x  82.5  mm. 
($14  x  3^  inches)  (§37). 

C.  French  slides. — These  are,  following  the  recommendations 
of  the  French  Congress  of   Photography  for   1889,   85x100  mm. 
(31Vs2  x  315/ie  inches).     That  is,  the  standard  is  practically  like  the 
American,  and  French  slides  can  be  used  in  American  lantern-slide 
carriers. 

D.  German  slides. — In  Germanic  countries,  slides  of  85  x  100 
mm.  are  much  used,  but  the  German  standard  is  often  given  as 
90  x  120  mm.  (39/i6  x  4^  inches).     Those  of  130  x  180  mm.  are 
likewise  employed. 

200 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  201 

E.  Italian  slides. — In  Italy  the  sizes  are  85  x  85  mm.,  85  x  100 
mm.  and  90  x  120  mm.,  that  is,  the  British  (B),  the  French  and 
American  (A,  C)  and  German  (D)  sizes. 

In  all  countries  those  of  larger  and  smaller  sizes  than  the  above 
standards  are  used  for  special  purposes;  and  provision  is  made 


Ca/Yxcuctcx     \raibarrrvj 


FIG.  1 12.     Ax  AMERICAN  LANTERN  SLIDE,  FULL  SIZE,  WITH  INSTRUCTIONS 

FOR  MAKING  LANTERN  SLIDES  DIRECT.     THE  SLIDE  is  PROPERLY 

"SPOTTED." 

everywhere  for  the  square  British  slides  of82.5x82.5  mm.  and  also 
for  the  oblong  form  of  82  or  85  x  100  mm.  of  the  French  and  Ameri- 
can manufacturers. 

Any  oblong  form  has  the  advantage  that  it  is  always  put  into 
the  carrier  with  its  long  side  horizontal  and  therefore  requires  only 
one  mark  or  spot  to  indicate  how  it  shall  be  inserted  for  an  erect 


202  PREPARATION  OF  LANTERN  SLIDES  [Cn.  VIII 

image  (fig.  6-8,  112).  For  a  square  form  two  marks  are  needed 
(fig.  13,  113). 

§  313.    Actual  size  of  the  free  opening  with  lantern  slides. — The 

sizes  given  above  are  the  measurements  from  the  extreme  edges 
of  the  plates.  The  actual  size  of  the  picture  to  be  projected  is 
always  less,  as  part  of  the  slide  is  covered  when  inserted  in  the 
carrier.  The  mat  between  the  slide  and  its  cover,  and  the  binding 
around  the  edge  lessen  the  size  a  variable  amount.  It  requires 
from  5  to  10  mm.  all  around  the  edge  for  the  binding  and  the  part 
covered  by  the  slide-carrier.  This  leaves  a  clear  opening  in  the 
lantern  slide  of  that  much  less.  The  smaller  the  slide  to  start  with 
the  less  will  be  the  proportionate  amount  of  clear  space  left  after 
the  mounting  of  the  slide. 

The  free  opening  of  the  American  slides  is  rarely  greater  than 
70  x  75  mm.  and  much  more  frequently  the  free  opening  is  con- 
siderably less. 

§  314.  Diameter  of  the  condenser  required  for  different  sized 
lantern  slides. — The  final  element  of  the  condenser  next  the  lantern 
slide  (fig.  i,  2,  114)  must  be  somewhat  greater  in  diameter  than 
the  diagonal  of  the  free  opening  of  the  lantern  slide  to  be  projected. 

The  accompanying  figures  show  the  British,  French  and  Ameri- 
can, and  German  standard  sizes  of  lantern  slides  with  the  minimum 
diameter  of  the  condenser  which  should  be  used  with  them  (fig. 
114). 


§  312a.  There  is  some  confusion  as  to  the  exact  outside  measurement  of 
lantern  slides.  For  example,  the  exact  size  of  the  British  square  slides  is 
3}4  x  $l/i  inches  (82.5  x  82.5  mm.)  In  the  two  French  works  consulted 
(Trutat,  p.  311,  and  Fourtier,  tome  ii,  p.  18)  the  British  size  is  given  as  80  x  80 
mm. 

In  Italy  the  size  is  given  as  85  x  85  mm.  In  the  German  work  of  Wimmer 
the  exact  size  is  given  (82.5  x  82.5  mm.).  Neuhauss  speaks  of  slides  85  x  85 
mm.  (p.  27). 

The  standard  French  slides  are  given  as  85  x  100  mm.  This  is  one  of  the 
standard  sizes  in  Germany  and  Italy.  Hence,  it  is  concluded  that  the  standard 
British  slide  is  meant  whenever  80  x  80,  82.5  x  82.5,  or  85.  x  85.  mm.  slides  are 
mentioned.  Also  that  the  standard  French  and  American  slide  of  3^x4 
inches  (82.5  x  100  mm.)  is  meant  whenever  slides  of  85  x  100  mm.  are  men- 
tioned. 


CH.  VIII] 


PREPARATION  OF  LANTERN  SLIDES 


203 


§  315.  Making  lantern  slides. — In  the  use  of  the  lantern  at  the 
present  day,  one  will  find  occasion  to  make  lantern  slides  by  all  of 
the  different  ways  that  have  ever  been  devised.  That  is,  they  may 
be  drawn  or  painted  wholly  by  hand ;  made  partly  by  photography 


FIG.  113.     BRITISH  LANTERN  SLIDE  OF  FULL  SIZE  WITH  TWO  "SPOTS." 

The  "spots"  are  on  the  upper  corners  in  the  English  slides. 

The  picture  shown  on  the  slide  is  of  a  retouching  stand  suitable  for  use  in 
coloring  slides. 

S    The  slide. 

R  A  reflector  to  throw  the  light  tip  through  the  slide.  This  may  be  a 
mirror  or  simply  white  paper. 

and  then  hand-colored;    made  wholly  by  photography,  or  trans- 
parent natural  objects  may  be  used. 

Natural  objects  of  the  right  transparency  may  be  mounted  on 
glass  slides  arid  used  in  the  lantern.  For  example,  seaweeds,  thin 
leaves,  skeletonized  leaves,  large  wings  of  insects;  crystals  on 


2O4 


PREPARATION  OF  LANTERN  SLIDES 


[CH.  VIII 


glass,  thin  sections  of  wood  or  animal  organs  mounted  on  glass, 
fibers  of  wood,  thin  cloth,  spiders'  webs,  etc.,  etc. 


FIG.  114.     STANDARD  BRITISH,  FRENCH,  AMERICAN  AND  GERMAN  LANTERN 

SLIDES  WITH  THE  CONDENSER  NECESSARY  TO  FULLY  ILLUMINATE 

THEM.     (ABOUT  HALF  NATURAL  SIZE). 

§  316.  Hand-made  lantern  slides. — Practically  no  one  now 
makes  the  beautiful  hand-painted  lantern  slides  of  former  times; 
but  for  outline  diagrams,  for  tables  and  for  short  statements,  it  is 
easier  and  cheaper  to  make  the  slides  direct  than  to  first  make  a 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  205 

diagram  or  table,  etc.,  and  then  have  a  photographic  lantern  slide 
made. 

In  preparing  these  slides  direct,  a  device  of  the  artists  of  earlier 
times  who  painted  lantern  slides,  is  used.  That  is,  the  slide  is 
cleaned  carefully  and  then  coated  with  a  thin  solution  of  some  hard 
varnish  or  with  gelatin  (fig.  112,  §  317).  After  the  varnish  has 
thoroughly  dried  one  can  use  a  pen  or  a  brush  upon  the  varnished 
surface  with  the  same  facility  as  upon  paper.  The  hand-made  slide 
is  then  mounted  as  usual  and  can,  of  course,  be  used  indefinitely. 

If  they  are  for  a  special  occasion — as  in  projecting  election 
returns,  games,  etc., — the  slides  are  used  without  a  cover-glass. 
They  may  be  easily  cleaned  off  with  turpentine  or  xylene  and  used 
over  and  over. 

§  317.     Coating  the  lantern-slide  glass  with  varnish. — One  of 

the  best  varnishes  for  this  purpose  is  composed  of  5%  dry  Canada 
balsam  or  gum  dammar  in  xylene  or  in  turpentine ;  or  10%  natural 
Canada  balsam  in  xylene  or  toluene.  Or  one  can  take  some  good, 
varnish,  especially  Valspar,  one  part  and  xylene,  toluene,  gasoline 
or  turpentine  nine  parts.  All  of  these  thin  solutions  should  be 
allowed  to  stand  until  they  are  clear,  and  only  the  clear  part  used. 
If  one  is  in  haste  it  is  possible  to  filter  the  thin  varnish  through  filter 
paper. 

For  coating  the  glass,  the  best  way  is  to  hold  the  clean  glass  flat 
by  grasping  the  edges  with  the  thumb  and  fingers.  Then  varnish 
is  poured  on,  and  the  glass  tilted  slightly  until  the  whole  surface 
is  covered.  The  excess  is  poured  off  one  corner  back  into  the 
bottle.  Then  the  glass  is  stood  on  edge  to  dry.  In  a  warm  dry 
room  15-20  minutes  will  suffice  for  varnish  in  xylene  or  toluene. 
If  turpentine  is  used  it  may  require  half  a  day  or  more.  When  the 
varnish  is  once  dry  the  glass  can  be  used  at  any  time. 

As  it  is  not  easy  to  tell  which  side  has  been  varnished,  a  slight 
mark  in  one  corner  of  the  varnished  surface  with  a  glass  pencil  or 
pen  will  enable  one  to  tell  quickly  and  with  certainty. 

§  318.     Coating  the  lantern-slide  glass  with  10%  gelatin. — For 

this,  some  clear  gelatin  is  made  into  a  10%  solution  in  hot  water, 


206  PREPARATION  OF  LANTERN  SLIDES  [Cn.  VIII 

and  filtered  through  filter  paper.  The  slides  are  coated  with  the 
gelatin  as  described  for  the  varnish.  When  the  gelatin  is  dry  the 
surface  receives  a  pen  or  brush  well.  Gelatin  slides  are  not  so 
satisfactory  as  the  varnished  slides. 


FIG.  115.     AMERICAN  LANTERN  SLIDE  OF  FULL  SIZE  WITH  GUIDE  LINES 
FOR  MAKING  SLIDES  DIRECT. 

The  thumb  tacks  at  the  four  corners  are  to  hold  the  slide  firmly  in  position 
while  writing  or  drawing  upon  it.  The  lined  area  represents  about  the  maxi- 
mum size  of  opening  projected  in  ordinary  work  (65  x  75  mm.),  (2^ x2 J/gin.). 

§  319.  Inks  and  pens. — One  can  use  any  ink  and  any  pen  on 
the  varnished  or  gelatinized  slides. 

For  making  tables,  etc.,  it  is  best  to  use  water-proof  India  ink 
and  a  fine  pen,  a  crow-quill,  steel  pen  is  excellent. 

§  320.  Drawing  diagrams  on  varnished  slides. — One  can  draw 
freehand  on  these  varnished  slides  as  well  as  upon  paper.  For 


PREPARATION  OF  LANTERN  SLIDES  207 

those  not  especially  skillful,  it  is  probably  better  to  draw  the  sketch 
first  and  then  trace  the  sketch  on  glass  as  follows:  Place  the 
lantern-slide  glass  on  the  drawing,  varnish  side  up,  and  arrange  it 
as  desired.  Select  very  thin  glass  for  this,  so  that  the  drawing  sur- 
face will  be  near  the  picture  to  be  traced.  Now  with  a  pen  or 
brush  trace  the  outlines.  One  can  also  use  colored  inks  if  desired. 

§  321.  Guide  for  table  making  and  for  writing. — For  making 
lantern-slide  tables  or  written  matter  direct  on  the  slide  it  is  best 
for  most  workers  to  have  a  guide  which  shall  show  the  maximum 
size  which  can  be  projected  (fig.  115).  If  one  has  no  special  guide, 
cross-section  paper  or  catalogue  cards  will  serve  well. 

To  hold  the  glass  in  position  while  writing  or  making  diagrams, 
thumb  tacks  at  the  corners  are  efficient  (fig.  115). 

§  322.  Ink  and  pen  to  use  on  unvarnished  glass. — For  tem- 
porary use,  as  in  reporting  games,  etc.,  the  glass  is  cleaned  and 
then  the  fingers  rubbed  over  it.  Now  with  a  ball-pointed  pen  one 
can  write  upon  the  glass.  The  lines  will  be  coarse,  but  that  will 
not  matter.  One  can  write  with  an  ordinary  pen  also,  but  not  so 
surely  as  with  a  ball-pointed  pen  (§  322a). 

The  ink  can  be  of  almost  any  kind.  The  black  India  ink  gives 
the  sharpest  images. 

A  special  ink  called  "glassine"  has  recently  been  put  on  the 
market.  It  is  in  six  colors,  white,  black,  red,  green,  blue  and  violet. 
The  ink  is  thick  and  with  it  one  can  write  on  untreated  glass  with 
any  pen,  although  a  ball-pointed  pen  is  here  also  an  advantage 
(§32  2b) .  The  ink  is  easily  washed  off  with  water  so  that  the  same 
glass  slide  can  be  used  over  and  over. 

§  322a.  The  writers  are  indebted  to  Dr.  E.  M.  Chamot  for  the  suggestion 
to  use  the  ball-pointed  pens  on  the  unvarnished  glass,  also  the  advantage  of 
rubbing  the  fingers  or  palm  over  the  cleaned  glass  to  prevent  the  ink  from 
spreading. 

According  to  Lewis  Wright,  p.  412,  one  can  write  on  glass  well  if  the  glass  is 
licked,  and  the  thin  coating  of  saliva  so  spread  upon  the  glass  is  allowed  to  dry. 
The  ink  will  not  spread,  and  the  saliva-coated  glass  takes  the  pen  well. 

§  322b.  "Glassine  announcement  slide  ink." — This  ink  is  made  by  the 
Thaddeus  Davids  Co.,  127  William  St.,  N.  Y.,  and  is  supplied  in  i  oz.  (30  cc.) 
bottles,  the  full  set  of  six  colors  costing  $1.00.  See  the  Moving  Picture  World, 
March,  1914. 


208  PREPARATION  OF  LANTERN  SLIDES  lCn.  VIII 

§  323.  Smoked  glass. — For  some  purposes  nothing  is  better 
than  smoked  glass  slides.  On  these  one  can  write  or  draw  with  a 
sharp  point  either  before  or  during  the  exhibition.  If  one  takes  the 
precaution  to  commence  writing  on  the  lower  edge  of  the  slide  and 
on  the  face  looking  toward  the  condenser  the  writing  or  diagram 
will  appear  right  side  up  on  the  screen  (see  §  3  5  for  proper  position 
of  lantern  slides  in  the  holder) . 

Smoked  slides  must  be  handled  carefully  or  the  surface  will  be 
spoiled. 

§  324.  Thin  sheets  of  mica  or  of  gelatin. — On  a  sheet  of  mica, 
of  gelatin  or  of  non-inflammable  cellulose  one  can  write  or  draw 
with  a  pen  or  brush,  using  any  colored  ink.  India  ink  is  best  for 
outlines  and  for  written  words,  letters,  or  numerals. 

As  these  sheets  are  very  thin  it  is  best  to  put  a  slide  made  upon 
one  of  them  between  two  glasses,  so  that  the  sheet  will  be  held  flat 
and  be  protected.  (For  other  methods  of  hand -made  slides  see 
Dolbear,  pp.  29-32). 


PHOTOGRAPHIC  LANTERN  SLIDES 

§  325.  Nearly  all  of  the  lantern  slides  now  used  are  made 
wholly  or  in  part  by  photography. 

Negative. — First,  there  is  made  a  negative  of  the  object  to  be 
represented  in  the  lantern  slide.  This  negative  may  be  on  any 
size  of  plate,  but  the  picture  should  be,  if  convenient,  of  the  proper 
size  for  a  lantern  slide.  That  is,  its  outside  dimensions  must  not 
exceed  75  x  70  mm.  (3  x  2.8  in.). 

This  negative  should  be  very  sharp  and  free  from  defects.  Any 
lack  of  sharpness  or  any  defects  will  come  out  with  distressing 
prominence  when  the  picture  is  magnified  by  the  lantern.  One 
must  then  use  a  good  objective  in  making  the  picture,  or  if  the 
objective  is  not  particularly  good  a  very  small  diaphragm  is  used. 
If  it  is  desired  that  print  shall  be  read  easily  by  all  in  the  room,  the 
lantern  slide  should  not  have  the  letters  smaller  than  six  point  type 
(see  fig.  216  for  sizes  of  type). 


CH.  VIII] 


PREPARATION  OF  LANTERN  SLIDES 


209 


§  326.    Printing  the  lantern  slide  from  the  negative. — If  the 

picture  on  the  negative  is  of  the  proper  size  for  a  lantern  slide,  it  is 
put  into  a  printing  frame  exactly  as  for  printing  with  paper.  Then 
in  the  dark  room  a  lantern-slide  plate  is  put  with  its  sensitive  side 
next  the  negative  and  arranged  so  that  the  picture  will  be  straight 
on  the  lantern  slide.  The  cover  of  the  printing  frame  is  put  on  and 
held  in  place  by  the  hands  or  by  the  springs.  The  exposure  may 
be  in  diffused  daylight,  or  about  30  cm.  from  any  good  artificial 
light  (incandescent  bulb,  Welsbach  gas  light,  kerosene  lamp). 


Base 


FIG.   116. 


CAMERA  FOR  MAKING  LANTERN  SLIDES  BY  MEANS  OF  AN 
OBJECTIVE. 


Base     The  base  of  the  camera  resting  on  the  table. 

Objective  The  photographic  objective  in  the  middle  segment  of  the  camera. 
The  objective  is  shown  as  if  the  enclosing  bellows  were  transparent. 

Front    The  front  of  the  camera  where  the  negative  is  placed. 

Reflector  A  white  sheet  of  paper  or  cardboard  placed  on  a  shelf  at  45°. 
This  reflector  serves  to  illuminate  the  negative. 

By  varying  the  relative  distances  of  ground  glass,  objective  and  negative, 
the  lantern  slide  can  be  larger  or  smaller  or  of  the  same  size  as  the  corresponding 
part  of  the  negative. 

The  exposure  required  varies  with  the  negative,  but  it  is  less  than 
for  most  developing  papers. 

§  327.  Developing  the  lantern  slide. — Any  good  developer  may 
be  used,  but  as  a  rule  the  directions  given  in  the  box  of  plates  are 
the  best  to  use  with  that  brand  of  plate.  One  should  develop  until 
the  picture  appears  clearly.  The  temptation  is  to  develop  too 
much  and  thus  make  the  slide  too  opaque.  Black,  like  printed 
letters,  should  be  opaque  in  the  correct  lantern  slide,  but  there 
should  be  all  gradations  from  that  to  clear  glass  in  the  whites. 


2IO 


PREPARATION  OF  LANTERN  SLIDES 


[Cn.  VIII 


Any  one  who  can  make  a  good  negative  and  a  good  paper  print 
from  it  can  make  a  good  lantern  slide.  The  lantern  slide  is  a 
positive  and  the  lights  and  shades  should  appear  as  in  the  object 
when  one  looks  through  the  slide  toward  the  light.  These  lantern 
slides  are  small  transparencies,  and  some  of  them  make  beautiful 
ornaments  when  used  as  transparencies  in  a  window. 

There  is  more  danger  of  getting  the  slides  too  opaque  than  not 
opaque  enough.  The  beginner  should  try  each  lantern  slide  with 


FIG.   117.     COPYING,  ENLARGING  OR  REDUCING  CAMERA. 

(From  the  Catalogue  of  Anthony  &  Co.). 

O     The  objective.     The  bellows  have  been  cut  away  to  show  it. 

/  Front  of  the  camera  with  frames  or  "kits"  for  negatives  of  various  sizes. 
For  making  enlargements  with  this  camera  the  objective  can  be  placed  in  the 
front. 

a  moderate  light  in  the  lantern.  If  the  picture  on  the  screen  is 
brilliant  and  shows  all  the  details  with  the  moderate  light,  it  will, 
of  course,  give  a  more  brilliant  picture  with  the  electric  light  of 
3000  to  4000  candle-power.  If  the  slide  is  too  opaque,  it  will  not 
come  out  well  with  the  moderate  light  and,  while  the  powerful 
electric  light  may  show  it  fairly  well,  so  much  radiation  will  be 
absorbed  and  transformed  into  heat  that  the  slide  is  liable  to  break 
if  left  in  the  lantern  a  considerable  time.  The  more  transparent 
slides  allow  the  radiant  energy  to  pass  through  them  and  naturally 
they  are  not  so  greatly  heated. 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  2 1 1 

§  328.  Negatives  as  lantern  slides. — Many  objects  appear 
equally  well  and  equally  clearly  when  projected  from  a  negative  as 
from  a  positive  or  transparency.  That  is,  there  will  be  white  lines 
and  white  letters,  etc.,  on  a  black  background.  This  was  a 
favorite  method  of  illustrating  in  the  older  works  on  physics  and 
projection.  For  examples,  look  at  the  pictures  in  Dolbear's  Art  of 


FIG.  118.     PHOTOGRAPHIC  CAMERA  UPON  A  BASEBOARD  HINGED  TO  A 

TABLE. 
(From  The  Microscope}. 

This  is  one  of  the  copying,  enlarging  and  reducing  cameras.  The  objective 
may  be  at  the  end,  in  a  cone,  or  in  the  middle  segment.  For  lantern-slide 
making  it  is  in  the  middle  segment  and  the  negative  at  the  end,  the  whole 
camera  being  directed  upward  toward  the  sky. 

By  reversing  the  position  of  the  camera,  and  placing  the  hinged  board  in  a 
vertical  position,  objects  in  liquids  and  any  object  in  a  horizontal  position  can 
be  photographed. 

NOTE. — The  arrangement  shown  in  fig.  118  with  a  baseboard  hinged  to  the 
table,  and  with  a  camera  which  could  be  placed  pointing  upward  or  downward 
was  devised  by  the  senior  author  in  1878  especially  for  photographing  objects 
in  liquids  or  objects  which  must  remain  in  an  inclined  or  horizontal  position. 
The  baseboard  carrying  the  camera  can  be  fixed  in  any  position  from  the 
horizontal  to  the  vertical.  (Proc.  Amer.  Assoc.  Adv.  Sc.  Vol.  XXVIII  (1879), 
p.  489;  Science,  Vol.  Ill,  p.  443,  and  Vol.  IV,  p.  5  (1884). 


212 


PREPARATION  OF  LANTERN  SLIDES 


[Cn.  VIII 


Projecting,   Deschanel's  Physics,  etc., 
and  fig.  141,  190,  211,  214. 

There  is  one  serious  drawback  to  such 
lantern  slides.  The  background  being 
nearly  opaque  stops  the  light  and  other 
radiant  energy  from  the  lamp,  and  the 
great  heat  developed  is 
liable  to  crack  the  slides 
(see  §  18,  845)- 


FIG.  119.  FOLMER  &  SCHWING'S  TILTING  CAMERA  AND  ADJUSTABLE  BACK. 

(From  the  Catalogue  of  Folmer  &  Schwing.     Cut  loaned  by  the  Eastman 

Kodak  Co.}. 

A  Tilting  camera  for  making  lantern  slides  or  other  transparencies  with  an 
objective,  or  for  photographing  objects  in  a  horizontal  or  inclined  position. 

B  Adjustable  back  for  the  tilting  camera.  The  adjustments  are  to  the 
right  or  left,  up  or  down  and  enable  one  to  center  accurately  any  desired  part 
of  the  negative  or  other  object  to  be  photographed.  The  rotary  motion  of  the 
back  enables  one  to  get  the  lines  on  the  negative  or  object  exactly  parallel  with 
the  edge  of  the  lantern  slide. 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  213 

§  329.  Printing  lantern  slides  by  the  aid  of  a  camera. — Unless 
the  negatives  from  which  lantern  slides  are  to  be  made  have  the 
part  to  be  shown  of  exactly  the  size  of  a  lantern  slide,  the  trans- 
parency or  positive  cannot  be  printed  by  contact.  Then  one  can 
use  a  photographic  camera  and  print  the  transparency  as  follows : 
The  negative  is  put  in  a  suitable  opening  or  in  the  proper  "kit"  or 
frame  in  the  end  of  a  copying  camera  (fig.  116-119),  and  the 
objective  in  the  second  segment.  The  picture  or  film  side  of  the 
negative  must  face  the  objective.  Then  the  end  of  the  camera 
holding  the  negative  is  elevated  sufficiently  to  get  a  sky  background 
through  the  window ;  or  the  camera  is  left  level  and  a  large  piece  of 
cardboard  or  white  blotting  paper  is  set  at  an  angle  of  about  45 
degrees  out  of  a  window  and  the  camera  pointed  toward  it.  In 
either  case  the  entire  lantern  slide  will  be  evenly  illuminated  and  a 
good  print  can  be  obtained. 

Now  focus  the  picture  of  the  negative  sharply  on  the  ground 
glass  of  the  camera  and  get  it  of  the  proper  size  by  pulling  out  or 
closing  up  the  bellows. 

Print  the  positive  by  putting  a  lantern-slide  plate  in  the  plate 
holder  in  the  usual  manner  and  exposing  it.  Then  develop  as 
usual. 

It  is  to  be  noted  that  the  film  surface  of  the  negative  and  the 
sensitive  surface  of  the  lantern-slide  plate  face  each  other  by  this 
method  exactly  as  for  contact  printing  (§  32ga). 

§  330.  Camera  for  lantern  slides. — If  one  is  to  make  many 
lantern  slides  it  is  a  great  convenience  to  have  available  a  special 

§  329a.  White  prints  on  a  black  ground. — By  using  an  ordinary  negative 
giving  black  lines  on  a  white  ground  one  can  get  white  lines  or  a  white  picture 
on  a  black  ground  by  applying  the  method  just  given  for  printing  lantern  slides 
by  means  of  a  camera  and  an  objective.  Place  the  negative  in  position,  but 
with  the  film  side  facing  away  from,  not  toward  the  objective  as  for  an  ordinary 
lantern  slide.  Use  a  lantern  slide  or  any  other  kind  of  plate  and  make  the 
picture  just  as  for  the  lantern  slide.  The  glass  picture  thus  produced  will  be  a 
positive  like  a  lantern  slide  but  it  will  have  all  the  parts  reversed  exactly  like  a 
negative.  If  now  this  picture  is  used  as  a  negative  and  printed  with  cyco, 
velox,  argo,  haloid  or  any  other  printing  paper  the  picture  will  appear  white  on 
a  dark  ground. 

Of  course,  any  lantern  slide  can  be  used  for  making  prints,  but  the  picture 
will  be  reversed  in  every  way,  the  lights  and  darks,  the  printing,  etc.  To  pre- 
vent the  inversion  of  the  printing  one  can  use  an  objective  and  camera  as 
described  in  Ch.  X,  §  512. 


214  PREPARATION  OF  LANTERN  SLIDES  [Cn.  VIII 

camera  known  as  a  "copying,  enlarging,  and  reducing  camera" 
(fig.  116-119).  As  seen  from  the  picture,  the  objective  is  placed 
in  the  middle  segment  if  lantern  slides  are  to  be  made  from  nega- 
tives, and  the  negative  is  placed  in  the  proper  sized  frame  or  "kit" 
at  the  end  of  the  camera.  No  light  then  reaches  the  negative 
except  on  the  face  looking  toward  the  light,  hence  there  will  be  no 
trouble  from  reflections. 

In  the  best  form  of  these  cameras  there  is  a  "back  with  revolving, 
rising  and  vertical  sliding  lantern-slide  attachment"  for  printing 
and  for  making  the  negatives  (fig.  119).  The  picture  can  be  got 
on  the  plate  in  the  exact  position  desired,  i.  e.,  lines  of  print,  etc., 
exactly  parallel  with  the  edge  of  the  plate.  By  means  of  a  camera 
one  can  print  lantern  slides  from  the  negatives  before  they  are  dry. 
This  is  sometimes  a  great  convenience. 

§  331.  Printing  lantern  slides  by  artificial  light. — With  contact 
printing  one  can  use  daylight  or  any  convenient  artificial  light — 
petroleum,  gas,  acetylene  or  electric.  For  printing  with  the 
camera,  however,  it  is  not  so  easy  to  get  the  negative  evenly 
illuminated.  A  good  way  to  evenly  illuminate  the  negative  is  to 
use  a  45  degree  cardboard  reflector  illuminated  with  one  or  two 
incandescent  lights,  preferably  with  frosted  bulbs  in  a  horizontal 
position.  Mantle  gas  lights  serve  well  for  illuminating  the  card- 
board. The  negative  is  set  vertically  some  distance  from  the  card- 
board. 

The  time  for  printing  lantern  slides  by  contact  or  by  the  aid  of  a 
camera  will  vary  with  the  negative  as  for  paper  prints;  much 
depends  on  the  intensity  of  the  light  and  on  the  rapidity  of  the 
plates  used. 

To  give  an  example  of  the  time  required  in  a  given  case  the 
following  table  is  added : 

The  same  objective  with  a  diaphragm  opening  of  F/8  was  used 
for  all,  and  the  same  negative  was  used  in  each  case.  All  the  plates 
were  from  the  same  box  and  the  same  developer  was  used  for  all, 
so  that  the  only  variable  was  the  light. 

1.  Sky  background,  diffused  light 10  seconds. 

2.  Cardboard  at  45  degrees,  under  the  sky 15  seconds. 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  215 

3.  Cardboard  at  45  degrees,  lighted  by  a  40  watt  mazda  lamp 
above  the  cardboard 30  seconds. 

4.  Cardboard  at  45  degrees  with  a  16  candle-power  frosted  bulb 
above  the  cardboard 120  seconds. 

For  contact  printing  with  the  same  negative,  30  cm.  (12  in.)  from 
the  light,  if  artificial,  the  following  times  sufficed:  Diffused  day- 
light, 2  sec.;  Mazda,  40  watt  lamp,  i  sec.;  Frosted  bulb,  16  c.p. 
lamp,  10  sec. ;  Petroleum  lamp,  10  sec.;  Gas  mantle,  5  sec. 

§  332.  Rapid  preparation  of  lantern  slides. — It  occasionally 
happens  that  one  needs  a  lantern  slide  at  very  short  notice.  In 
such  a  case,  the  negative  can  be  taken  and  fixed  in  the  hypo, 
rinsed  in  water,  and  put  into  the  camera  and  a  lantern  slide 
exposed  (§  329).  Then  the  negative  can  be  washed  as  usual. 
The  lantern  slide  is  then  developed  and  fixed,  and  washed  a  few 
minutes  in  water.  It  is  then  placed  a  few  moments  in  95%  alcohol 
or  denatured  alcohol  for  dehydration.  After  removal  from  the 
alcohol  it  is  dried  in  a  draught  or  in  the  current  of  an  electric  fan. 
Negatives  can  be  quickly  dried  in  the  same  way.  One  can  then 
make  contact  prints. 

§  333.  Typewritten  lantern  slides. — It  frequently  happens  that 
one  desires  to  project  some  statement  or  some  table.  This  can  be 
written  as  stated  above  (§  316,  321),  or  the  statement  or  table 
can  be  made  neatly  with  a  typewriter,  using  a  black  ribbon. 
Then  this  can  be  used  just  as  any  other  printed  matter  and  a 
photographic  lantern  slide  made  from  it. 

If  in  a  great  hurry  one  can  use  the  negative  form  of  lantern  slide 
and  dry  quickly  (§  332).  This  will  give  white  letters  on  a  black 
ground  (§  329a).  (For  film  slides  see  §  333a). 

§  333a.  Film  lantern  slides. — There  has  been  recently  introduced  by  the 
Eastman  Kodak  Co.,  a  method  of  producing  lantern  slides  on  celluloid 
films,  comparable  to  film  negatives.  The  celluloid  film  is  quite  thick.  There 
must  be  a  negative  as  for  glass  lantern  slides.  The  film  is  used  in  place  of  a 
lantern-slide  plate.  The  printing  is  like  printing  cyco,  velox  or  other  paper. 
When  the  lantern-slide  film  is  dry,  after  being  developed  and  washed  like  a  film 
negative,  it  is  varnished  and  placed  between  two  pieces  of  paper  with  the 
proper  opening  for  the  picture. 

Naturally,  these  film  slides  are  very  light  and  are  not  fragile.  Unfortunately 
the  substance  of  which  the  film  is  composed  is  inflammable,  and  therefore  the 


216  PREPARATION  OF  LANTERN  SLIDES  [CH.  VIII 

§  334.  Mounting  lantern  slides. — In  the  original  method, 
which  is  still  followed  to  a  certain  extent,  each  slide  was  mounted  in 
a  wooden  frame — that  is,  each  slide  had  its  own  carrier  which  was 
put  in  place  when  it  was  to  be  shown  (fig.  15). 

For  teaching  and  for  many  other  purposes  glass  lantern  slides 
are  not  now  put  in  separate  wooden  frames,  but  are  covered  with  a 
clear  glass  (cover-glass)  of  the  same  size  and  the  two  bound 
together  by  adhesive  paper.  They  are  far  less  bulky  in  this  way  of 
mounting,  although  they  are  not  as  well  protected  as  in  the  earlier 
form. 

In  mounting  them  the  slides  are  thoroughly  dried,  then  some 
form  of  opaque  mat  or  mask  is  put  over  the  picture  on  the  picture 
side  of  the  transparency  or  negative.  There  are  on  the  market 
masks  or  mats  of  various  shapes  and  sizes  of  opening.  These  may 
be  used  or  masks  may  be  made  by  using  strips  of  black  paper. 

When  the  mat  is  in  place  a  cover-glass  of  exactly  the  same  size 
as  the  lantern  slide  is  thoroughly  cleaned  and  placed  over  the 
picture  surface  of  the  slide.  Then  a  narrow  strip  of  adhesive  paper 
is  put  all  around  the  edge.  This  holds  the  slide  and  the  cover  in 
position,  and  prevents  the  sharp  edges  of  the  glass  from  cutting  the 
fingers  when  handling  the  slides.  The  mat  not  only  cuts  out  any 
part  which  is  not  to  be  shown,  but  it  separates  the  cover-glass 
slightly  from  the  picture  and  prevents  rubbing  or  other  injury  to  it. 
The  size  and  shape  of  the  opening  in  the  mat  to  give  the  best  effect 
depends  upon  the  picture  or  other  matter  on  the  lantern  slide.  The 
mat  is  a  kind  of  frame  and  like  any  other  frame  it  should  be  suited 
in  form  and  size  to  the  object  to  be  shown. 

§  335.     Marking    or    "spotting"    the    mounted    slides. — As 

pointed  out  in  Chapter  I  (§  23)  each  slide  should  have  some  kind  of 

Kodak  Company  recommend  that  the  film  slides  be  used  only  with  a  magic 
lantern  having  a  water-cell  (fig.  2,  3). 

Furthermore,  even  if  non-inflammable  film  were  used,  it  would  not  do  to 
leave  those  slides  in  a  lantern  without  a  water-cell  too  long  for  the  heat  would 
make  the  celluloid  buckle  and  get  out  of  shape  or  char  it,  although  of  course 
it  would  not  be  set  on  fire. 

The  lightness  and  small  space  required  for  such  slides  are  of  great  advantage , 
but  their  limitations  are  so  great  that  for  the  general,  and  rough  usage  of  ordi- 
nary lantern  slides  they  are  not  so  well  adapted  as  glass  slides. 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  217 

mark  on  it  so  that  the  operator  can  put  it  into  the  lantern  correctly 
without  closely  inspecting  each  slide. 

Unfortunately  there  is  no  general  system  of  marking  slides.  The 
method  recommended  by  the  British  Photographic  Club 
(Bayley,  p.  78)  is  to  put  two  white  spots  on  the  upper  edge  of  the 
slide  (fig.  113).  Two  spots  are  necessary  for  the  square  slides,  but 
for  oblong  slides  one  "spot"  or  mark  is  sufficient  (fig.  112). 

In  America  it  is  common  to  have  the  mark  or  spot  on  the  lower 
left  hand  corner  of  the  slide  (§  112),  then  when  the  slides  are  in  a 
pile  for  inserting  in  the  lantern  the  spot  will  be  turned  upward 
(fig.  8)  as  it  must  be  to  give  an  erect  screen  image.  In  the  British 
method  of  "spotting"  the  slides  would  have  the  spots  on  the  lower 
edge  when  piled  up  ready  for  insertion  in  the  lantern. 

§  336.  Coloring  lantern  slides. — Photographic  lantern  slides 
have  been  colored  from  their  first  production.  To  do  this  in  the 
best  manner  possible  requires  considerable  practise  and  natural 
artistic  ability,  but  any  one  can  color  lantern  slides  sufficiently  well 
to  add  to  clearness  in  teaching — for  example,  veins  blue,  arteries 
red,  etc.  All  that  is  needed  is  a  small  artist's  brush  and  some  of 
the  desired  color. 

Transparent  colors  in  sets  are  on  the  market  (see  Appendix),  or 
one  can  employ  the  aqueous  stains  used  in  histology.  It  takes 
some  experience  to  get  the  right  dilution  of  the  color  and  to  put  it  on 
neatly  with  the  brush.  The  slide  should  be  held  over  some  white 
paper  in  a  light  place  so  that  it  is  possible  to  see  exactly  what  is  being 
done.  The  frame  for  holding  slides  is  a  convenience  (fig.  113). 

If  one  wishes  to  become  expert  it  will  be  a  great  help  to  study  the 
works  of  reference  given  at  the  head  of  this  chapter,  for  they  give 
many  valuable  hints. 

One  very  important  thing  for  the  beginner  to  do  is  to  test  every 
slide  that  is  colored  in  the  lantern  to  make  sure  that  the  colors  look 
right  in  the  screen  image.  Sometimes  a  slide  that  looks  well  to  the 
naked  eye  in  daylight  will  not  look  well  when  projected  on  the 
screen.  It  is,  of  course,  the  screen  image  that  must  be  satisfactory. 

The  early  lantern  slides  were  mostly  colored  with  transparent 
oil  colors,  and  then  when  entirely  dry,  the  slide  was  mounted  in 
Canada  balsam,  and  a  cover-glass  put  on  exactly  as  microscopic 


218  PREPARATION  OF  LANTERN  SLIDES  [Cn.  VIII 

specimens  are  now  mounted.     This  gave  a  very  transparent  and 
vivid  picture. 

§  337.  Labeling  lantern  slides. — Besides  the  mark  or  spot  as 
guide  to  inserting  the  slides  in  the  carrier,  every  lantern  slide  should 
have  a  label  stating  what  it  is,  and  if  copied  from  some  book  or 
periodical  it  should  give  the  name  of  the  publication  from  which 
derived  and  the  number  of  the  figure. 

Slides  are  also  numbered  for  convenience  in  arrangement  at  the 
time  of  an  exhibition.  Some  workers  simply  number  the  slides 
and  have  no  label.  This  is,  of  course,  feasible  for  a  small  collection 
to  be  used  by  one  individual,  but  the  slides  are  practically  useless 
for  any  one  else  unless  they  are  labeled. 

Sometimes  slides  are  numbered,  and  a  catalogue  kept  with  cor- 
responding numbers  and  a  description  of  the  slide.  For  one 
unfamiliar  with  the  collection  the  numbers  and  the  cards  are  not 
easy  to  put  together.  Then  one  is  liable  to  have  more  than  one 
series,  and  the  series  are  liable  to  get  mixed.  With  a  label  on  each 
slide,  the  collection  can  be  made  use  of  by  any  one. 

§  338.  Storing  lantern  slides. — The  problem  of  storing  a  large 
collection  of  lantern  slides  is  a  serious  one.  A  still  more  serious 
problem  is  to  find  the  slides  needed  for  a  given  lecture  or  demon- 
stration. 

A  common  method  of  storing  is  to  have  a  cabinet  like  that  used 
for  the  card  catalogue  of  libraries,  and  to  put  the  slides  in  the  draw- 
ers as  the  catalogue  cards  are  filed. 

One  can  use  name  cards  to  designate  groups  of  slides  as  they  are 
used  to  group  catalogue  cards. 

In  order  to  store  and  make  them  most  easily  available  for  use,  Pro- 
fessor George  S.  Moler  of  the  department  of  Physics  in  Cornell 
University  has  devised  a  cabinet  which  holds  the  slides  in  a  single 
vertical  layer,  so  that  when  any  holder  is  pulled  out  the  slides  are 
all  exhibited,  and  one  can  see  exactly  what  the  slides  are  and  select 
those  desired. 

This  seems  to  the  writers  of  this  book,  by  all  odds,  the  most  prac- 
tical cabinet  yet  devised  for  safely  storing  slides  and  making  them 
available  with  the  least  trouble  and  the  least  waste  of  time  (fig.  120). 


CH.  VIII]  PREPARATION  OF  LANTERN  SLIDES  219 

§  339.  Troubles  in  making  lantern  slides. — These  are  the 
troubles  liable  to  be  met  in  photography.  They  must  be  over- 
come by  following  intelligently  the  directions  for  photographic 
work  in  general  and  for  lantern-slide  making  in  particular.  Study 
the  directions  coming  with  the  lantern-slide  plates  used. 

In  making  written  slides  or  diagrams  on  varnished  slides  the  pen 
will  not  work  well,  and  the  ink  will  crawl  if  the  varnish  is  not  dry. 


FIG.  120.     THE  MOLER  SECTIONAL  LANTERN-SLIDE  CABINET. 

(Cut  loaned  by  G.  S.  Moler). 

This  cabinet  holds  1200  lantern  slides.  It  consists  of  a  box  with  twenty 
vertical,  sliding  frames,  each  frame  holding  60  slides. 

In  the  picture  the  cabinet  is  shown  on  a  table.  One  of  the  frames  is  entirely 
removed  and  leans  against  the  table  leg.  One  frame  is  pulled  out  for  examin- 
ing the  slides  stored  in  it. 

In  coloring  lantern  slides  one  must  learn  to  use  colors  which  give 
the  correct  effect  with  the  artificial  light  used  in  projection.  A  tint 
which  does  not  seem  right  by  daylight  may  give  exactly  the  desired 
effect  by  lamp-light.  This  is  why  the  advice  is  given  to  test  the 
work  frequently  in  the  lantern. 

Remember  that  there  is  more  danger  of  getting  the  lantern 
slides  too  opaque  than  not  opaque  enough. 

Sometimes  when  being  exhibited  a  lantern  slide  shows  a  mist  or 
fog  spreading  over  it.  This  may  partly  or  wholly  disappear. 
This  is  a  real  fog,  and  comes  from  the  moisture  in  the  slide,  or  its 
mounting.  If  the  slides  are  thoroughly  dried  before  they  are  put 
into  the  lantern  this  fog  does  not  appear. 


22O 


PREPARATION  OF  LANTERN  SLIDES 


[Cn.  VIII 


§  340.     Summary  of  Chapter  VIII: 


Do 

1.  Use  the  standard  size  of 
lantern   slides   in   the   country 
where  you  live  (§  312). 

2.  Make  the    lantern    slides 
with  moderate  intensity,   then 
they  can  be  used  with  all  lan- 
terns, no  matter  what  the  source 
of  light  (§3  2  7). 

3.  Make    the    picture    small 
enough  so  that  all  desired  parts 
can  be  projected  (§  334). 

4.  Take   pains   in   mounting 
the  slides  so  that  the  frame  will 
appear   suited   to   the   subject 
(§  334). 

5.  In  making  slides  direct  on 
the  varnished  glass,  write  finely, 
neatly  and  clearly  (§  316). 

6.  Printed  or  written  matter 
on   the   slide   should   be   large 
enough  to  be  read  by  all  in  the 
room  (§  325). 

7.  Mark  or  spot  the  lantern 
slides  so  that  they  can  be  in- 
serted  in   the   holder   without 
hesitation  (§335). 

8.  Label  every  lantern  slide 
so  that  any  one  can  tell  what  it 
is  (§  337)- 

9.  Store  the  lantern  slides  so 
that  they  can  be  found  quickly 

(§338). 


Do  NOT 

1.  Do    not    use    odd    sized 
pieces  of  glass  to  make  lantern 
slides  on. 

2.  Do  not  make  the  lantern 
slides  so  opaque  that  only  the 
best   electric  lanterns   can   ex- 
hibit them. 

3.  Do  not  make  the  picture 
on   the   slide   too   large   to   be 
exhibited. 

4.  Do  not  mount  the  slides  in 
a  slovenly,  inartistic  manner. 


5.  Do  not  use  flourishes  in 
writing  on  the  varnished  slides. 

6.  Do  not  reduce  the  written 
or   printed   matter   so   that   it 
cannot  be  read  in  the  screen 
image. 

7.  Do   not   leave   the    slides 
unmarked    and    expect    every 
chance  operator  to  insert  them 
properly  at  railroad  speed. 

8.  Do  not  leave  the  lantern 
slides  unlabeled,  for  no  one  else 
can  make  the  best  use  of  them. 

9.  Do  not  store  the  slides  in  a 
miscellaneous  heap. 


CHAPTER   IX 
THE  PROJECTION  MICROSCOPE  AND  ITS  USE 

§  350.    Apparatus  and  Material  for  Chapter  IX: 

Suitable  room  with  screen,  for  projection;  Projection  Micro- 
scope; Sunlight  or  the  electric  arc  light;  Specimens  suitable  for 
projection  (§  399) ;  Tools  etc.,  as  for  Ch.  I. 

REFERENCES  AND  HISTORY 

§  351.  For  the  history  of  the  origin  and  development  of  the 
projection  microscope,  refer  to  the  appendix  at  the  end  of  the  book. 
In  this  history  will  be  given  many  references  to  the  original  sources 
of  information  upon  the  subject. 

For  works  dealing  with  modern  micro-projection,  the  reader  is 
advised  to  consult  the  works  given  in  §  2  of  Ch.  I.  He  is  especially 
advised  to  consult  the  catalogues  of  Zeiss  and  the  other  modern 
makers  of  projection  apparatus,  for  in  them  he  will  find  directions 
and  suggestions  for  making  the  best  use  of  the  most  modern  instru- 
ments. His  attention  is  also  especially  called  to  the  Journal  of 
the  Royal  Microscopical  Society  and  to  the  Zeitschrift  fur  wis- 
sentschaftliche  Mikroskopie.  See  also  the  Zeitschrift  fur  Instru- 
mentenkunde,  the  English  Mechanic  and  the  Scientific  American 
with  its  Supplement.  In  every  volume  of  these  periodicals  there 
are  almost  always  articles  bearing  directly  on  the  problems  in- 
volved in  Projection. 

GENERAL  CONSIDERATION  OF  THE  PROJECTION  MICROSCOPE 
§  352.  Similarity  of  all  projection  apparatus. — All  devices  for 
projection  are  fundamentally  alike  in  giving  images  of  brilliantly 
lighted  objects.  These  images  are  projected  upon  some  reflecting 
surface  or  screen  in  a  dark  room.  The  projection  microscope  simply 
gives  images  of  greater  enlargement  than  the  other  forms  of 
apparatus.  It  imperceptibly  merges  into  the  magic  lantern,  as 
the  magic  lantern  merges  into  the  camera  obscura.  (Compare 
fig.  121-122). 

221 


222 


THE  PROJECTION  MICROSCOPE 


[CH.  IX 


96euj| 


c\j 


FIG.  121.    PROJECTION  MICROSCOPE. 

i,  2     Feeding  screws  of  the  arc  lamp, 
j     Set  screw  for  the  upper  carbon. 

4  Set  screw  for  holding  the  stem  of  the  arc 
lamp  in  the  socket  on  block  i. 

5  Set  screw  for  the  lower  carbon. 

Hc+  The  horizontal,  upper  carbon.  It 
must  be  made  positive  (+)• 

L  The  source  of  light,  i.  e.,  the  crater  of 
the  upper  carbon. 

Vc —  The  vertical  or  lower  carbon.  It  is 
negative  ( — ). 

Axis,  Axis,  Axis  The  principal  optic  axis 
from  the  positive  crater  of  the  arc  lamp 
extending  through  the  condenser,  the  stage 
water-cell,  and  the  microscope  to  the  screen. 

/  Condenser  2  The  triple  condenser  for 
receiving  and  concentrating  the  light  from 
the  crater  of  the  arc  lamp. 

1  The  first  element  of  the  condenser  which 
renders  the  diverging  light  parallel.     It  con- 
sists  of   a   meniscus  next   the  light    and    a 
plano-convex  lens  (compare  fig.   105,  in). 

2  The  second  element   of  the  condenser 
which  concentrates  the  parallel  beam. 

W  Water-cell  between  the  two  plano-con- 
vex lenses  in  the  parallel  beam  of  light. 

As  a  projection  microscope  uses  ob- 
jectives of  shorter  focus  and  smaller 
diameter  than  the  magic  lantern, 
greater  care  must  be  exercised  in  get- 
ting all  the  elements,  radiant,  con- 
denser and  projection  objective,  cen- 
tered along  one  continuous  line  or 
axis,  and  in  having  the  different  ele- 
ments the  right  distance  apart. 


CH.  IX] 


THE  PROJECTION  MICROSCOPE 


223 


Micro-projection  is  simply  a  refinement  of  ordinary  magic 
lantern  projection.  If  one  understands  the  principles,  and  has 
mechanical  skill  to  apply  them,  there  is  no  great  difficulty  in  micro- 
projection.  But  if  ordinary  magic  lantern  projection  is  unsatisfac- 
tory in  untrained  hands,  micro-projection  in  such  hands  is  in- 
tolerable. 

This  is,  however,  such  a  powerful  aid  to  the  teacher  and  the 
lecturer  that  the  time  necessary  to  learn  to  use  it  properly  is  not  to 
be  counted.  With  micro-projection  the  beauties  of  structure  and 


Condenser 


H  C 


FlG.    122. 


MAGIC  LANTERN  FOR  COMPARISON  WITH  THE  PROJECTION 
MICROSCOPE  (See  fig.  2). 


form  are  made  visible  to  an  entire  audience  with  all  their  color, 
delicacy  and  exquisite  perfection. 

Furthermore,  the  teacher  or  lecturer  can  indicate  on  the  screen 
the  special  points  to  be  noted,  and  feel  confident  that  his  auditors 
see  the  special  features  and  do  not  get  confused  by  the  mass  of 
details,  as  when  looking  into  a  microscope.  Often  too,  the  most 
interesting  and  important  structures  in  a  specimen  are  not  so 
striking  as  some  less  important  detail,  and  the  important  points  are 
likely  to  be  missed  unless  pointed  out. 

§  353.  Limitation  of  the  Projection  Microscope. — Perfect  and 
useful  as  the  projection  microscope  is,  it  is  limited  in  its  powers. 
One  can  show  with  full  satisfaction  to  a  large  audience  (200  to 
1000)  only  those  details  which  an  experienced  observer  can  see  by 


224  MICRO-PROJECTION  FOR  LARGE  CLASSES         [Cn.  IX 

looking  directly  into  a  compound  microscope  supplied  with  a  low 
ocular  and  a  16  mm.  objective.  For  a  small  audience  near  the 
screen  higher  powers  are  satisfactory  (see  §  401). 

§  354.  Size  of  specimens  for  projection. — To  meet  the  require- 
ments of  teaching  and  demonstration  the  modern  scientific  man 
and  public  lecturer  should  be  able  to  commence  with  the  projection 
microscope  where  the  magic  lantern  leaves  off,  and  carry  the  pro- 
jection to  the  smallest  size  adapted  to  micro-projection;  that  is, 
from  a  specimen  60  mm.  in  diameter  to  one  of  half  a  millimeter  or 
less  in  size.  This  requires  an  opening  in  the  stage  slightly  larger 
than  the  largest  specimen,  that  is,  at  least  65  mm.  in  diameter. 

CHARACTER  AND  RANGE  OF  PROJECTION  OBJECTIVES  FOR  DEMON- 
STRATION TO  LARGE  CLASSES 

§  355.  Objectives  from  125  mm.  to  4  mm.  equivalent  focus  are 
especially  useful  in  micro-projection.  The  powers  of  125,  100,  75, 
50,  and  25  mm.  equivalent  focus,  and  in  some  cases  those  of  20  and 
1 6  mm.,  are  constructed  on  the  plan  of  photographic  objectives 
(fig.  123).  These  are  always  to  be  used  without  an  ocular,  and 
their  iris  diaphragms  are  wide  open. 

At  the  present  time  the  low  objectives  used  in  ordinary  micro- 
scopic observation  are  also  used  in  projection.  The  field  is  not 
flat,  as  with  the  micro-planar  and  other  forms  of  photo-micro- 
graphic  objectives,  but  they  are  much  cheaper  and  the  screen 
images  are  very  brilliant.  Formerly  many  of  the  objectives  used 
in  projection  were  made  especially  for  that  purpose.  They  gave 
very  brilliant,  flat  fields  over  a  narrow  angle,  but  they  were  neither 
satisfactory  for  ordinary  microscopic  observation  nor  for 
photography. 

Most  of  the  projection  with  the  microscope  is,  however,  accom- 
plished with  objectives  of  .about  the  following  range:  50  mm., 
1 6  mm.,  and  8  mm.  With  these  in  a  triple  nose-piece  or  revolver, 
the  projection  microscope  can  accomplish  great  things,  especially 
if  assisted  occasionally  by  amplifiers.  For  an  audience  of  2  50  to  500 
and  a  screen  distance  from  7.5  to  10  meters  (25  to  33  ft.)  the  mag- 
nifications will  range  from  about  150  to  3000  diameters  (§  391). 


CH.  IX]          MICRO-PROJECTION  FOR  LARGE  CLASSES 


225 


For  a  larger  audience  and  a  correspondingly  larger  room  the 
screen  distance  might  be  made  15  to  20  meters  (50  to  65  ft.),  and 
the  magnification  raised  from  250  at  the  lower  limit  up  to  about 
5,000  diameters  at  the  upper  limit.  The  smaller  room  enables 
one  to  get  more  brilliant  screen  images,  and  to  use  a  wider  range  of 
objects  (see  table  of  magnifications  §  391).  In  the  smaller  room 
the  screen  should  be  at  least  4  meters  (12-13  feet)  square,  and  in 
the  larger  room  5-6  meters  (15-20  feet)  square. 


B 


FIG.  123.     DIAGRAMS  SHOWING  THE  CONSTRUCTION  OF  OBJECTIVES  FOR 
MICRO-PROJECTION  AND  FOR  PHOTOGRAPHY. 

(From  the  Catalogues  of  Zeiss,  Leitz,  and  the  Bausch  &  Lomb  Optical  Co.). 

A     Microsummar  of  Leitz. 

B     Microplanar  of  Zeiss. 

C    Microtessar  of  the  Bausch  &  Lomb  Optical  Co. 

When  used  for  micro-projection  the  diaphragm  is  wide  open  and  no  ocular  is 
employed. 

In  the  diagram  of  the  Microtessar,  F  represents  the  front  lens,  d  the  dia- 
phragm, and  B  the  back  combination  of  the  objective.  The  arrow  indicates 
the  direction  of  the  light. 

In  articles  and  books  upon  projection,  it  is  advocated  sometimes, 
that  oil  or  water  immersion  objectives  as  high  as  i  .5  or  2  mm.  should 
be  used  for  class  demonstration. 

There  is  no  doubt  that  brilliant  images  with  short  screen  dis- 
tances can  be  obtained  with  high  power  objectives,  but  such  pro- 
jection is  only  applicable  for  small  numbers;  and  if  fine  details  are 
to  be  seen,  the  observer  must  be  very  close  to  the  screen.  Further- 
more, no  screen  image  in  its  finest  details  is  equal  to  that  which 
one  gets  in  looking  directly  into  a  compound  microscope.  (For 
high  power  projection  see  §  401). 

If  it  is  high  magnification  that  is  desired,  it  is  vastly  better  to  use 
lower  objectives  with  an  amplifier  (§  356,  fig.  126).  The  lower 


226  MICRO-PROJECTION  FOR  LARGE  CLASSES         [Cn.  IX 

objective  with  larger  lenses  admits  much  more  light,  hence  the 
screen  image  will  be  brighter.  For  example,  suppose  it  were 
desired  to  obtain  the  magnification  which  is  given  by  a  2  mm.  objec- 
tive, it  would  be  much  better  to  use  a  4  mm.  objective  and  an 
amplifier  doubling  the  size  of  the  real  image.  This  would  make  the 
screen  image  of  the  same  magnification  as  the  2  mm.  would  give, 
and  it  would  be  far  brighter  and  show  a  larger  field.  In  like 
manner  and  for  the  same  reason,  it  is  better  to  use  an  8  mm.  objec- 
tive and  an  amplifier,  than  a  4  mm.  objective  without  the  amplifier 
(but  see  §  401). 


ABC  D 

FIG.  124.     FIGURES  SHOWING  THE  GENERAL  CONSTRUCTION  OF  MICROSCOPE 

OBJECTIVES. 

A  Low  power  objective  of  a  single  combination  (50-30  mm.  equivalent 
focus). 

B,  C  Medium  power  objectives  with  two  combinations  (25-12  mm. 
equivalent  focus).  Sometimes  the  front  combination  is  composed  of  two  and 
sometimes  of  three  lenses  as  shown. 

D     High  power  objective  (8  to  2  mm.  equivalent  focus).     Usually  the  front 
combination  is  of  a  single  lens,  the  others  of  two  or  three  lenses  as  shown. 
Many  high  power  objectives  have  but  three  combinations. 
(D  is  from   Voigtlander's  Catalogue). 

The  writers  have  found  that  in  projection  for  actual  class  demon- 
strations, objectives  of  higher  power  than  4  mm.  are  unsatisfactory. 
We  believe  also  that  the  purpose  of  class-room  projection  is  not 
the  demonstration  and  study  of  minute  details  which  require  that 
the  observer  should  be  close  to  the  screen  image,  but  the  general 
outlines  and  broad  features  which  can  be  seen  clearly  at  a  distance 
when  suitably  magnified. 


CH.  IX]         MICRO-PROJECTION  FOR  LARGE  CLASSES  227 

The  fresh  blood  corpuscles  of  man,  for  example,  are  about  7.57,1 
in  diameter.  To  see  these  as  discs  on  a  screen  at  a  distance  of  10 
meters  would  require  a  magnification  of  4,000  and  preferably  of 
8,000  diameters.  With  such  a  high  magnification  the  sharpness  of 
the  outline,  and  the  distinction  between  the  corpuscles  and  the 
medium  in  which  they  float  is  almost  lost,  and  there  is  nothing  but 
a  vague  haze  with  shadowy  outlines.  If  one  goes  up  closer  to  the 
screen  to  see  the  images  well,  one  will  be  sorely  disappointed,  for 
they  are  vague  in  outline  and  wholly  unsatisfactory  as  compared 
with  the  appearance  gained  by  looking  directly  into  a  microscope 
(§  355a). 


§  355a.  Visibility  of  objects  or  their  magnified  images. — It  has  been  found 
by  careful  observation  and  experiment  that  the  most  sensitive  part  of  the  eye 
is  in  the  f ovea  centralis  or  yellow  spot ;  and  that  in  order  to  see  two  points,  by 
the  fovea,  as  separate,  they  must  be  far  enough  apart  so  that  the  visual  angle 
is  one  minute.  If  the  visual  angle  is  less  than  one  minute,  two  points  appear  to 
most  eyes  as  one. 

The  question  now  is,  how  far  separated  must  the  parts  of  an  object  be  in 
millimeters  or  inches  in  order  that  the  form  of  the  object  can  be  distinguished. 
To  answer  this  it  is  necessary  to  know  the  actual  length  of  the  one  minute  of  arc 
when  the  eye  is  at  different  distances. 

To  determine  the  length  of  one  minute  of  arc  in  any  case,  the  eye  is  con- 
sidered to  be  at  the  center  of  a  circle  and  the  object  at  the  circumference,  and 
no  matter  how  great  the  visual  distance,  the  object  must  subtend  one  minute 
of  the  arc  of  the  circle  of  which  the  visual  distance  is  the  radius  in  order  to  have 
its  parts  distinguishable. 

To  determine  the  actual  length  in  millimeters  or  inches  of  one  minute  of  arc 
in  any  circle,  it  is  only  necessary  to  remember  that  the  circumference  of  a  circle 
is  6.2832  times  its  radius  and  that  it  is  divided  into  360  degrees  or  21,600 
minutes  (fig.  125). 

If,  now,  the  radius  of  the  circle,  or  the  distance  of  the  eye  from  the  object  is 
i  meter,  the  circumference  of  the  circle  will  be  6.2832  meters  or  6,283.2  milli- 
meters. As  there  are  21,600  minutes  in  the  circumference,  the  length  of  one 
minute  is  6,283.2  mm.  -4-  21,600  =  .2908  mm.  or  approximately  .3  mm.  That 
is.  with  the  eye  at  one  meter  distance,  the  parts  of  an  object  should  be  separated 
.3  mm.  to  be  seen  as  distinct  points. 

For  the  standard  distance  of  distinct  vision  (25  cm.),  used  in  microscopic 
magnification,  the  object  must  be  >^th  this  size  or  .075  mm.;  and  for  a  dis- 
tance of  i  o  meters  it  must  be  10  times  as  great  or  3  millimeters,  and  for  6  meters, 
the  distance  used  for  testing  vision,  it  must  be  .3  x  6  =  1.8  mm. 

A  greater  separation  of  the  points  is  desirable  for  the  most  accurate  deter- 
mination, but  those  given  above  are  the  minimum  for  most  observers. 

Now  to  apply  the  above  to  the  magnification  necessary  for  a  screen  image  of 
the  human  blood  corpuscle  which  has  a  size  of  7.5;*  (.0075  millimeters;  .000295 
inch).  To  give  the  necessary  sized  screen  image  of  .3  mm.;  .075  mm.  and  3 
mm.  at  distances  of  i  meter,  Xth  meter,  and  10  meters,  it  is  only  necessary  to 
divide  the  size  of  the  screen  image  in  each  case  by  the  size  of  the  object  (7.5^ 
or  .0075  mm.). 


228 


MICRO-PROJECTION  FOR  LARGE  CLASSES          [Cn.  IX 


FIG.  125.     DIAGRAM  SHOWING  VISUAL  ANGLE. 

The  nodal  point  or  optic  center  of  the  eye  is  placed  at  the  center  of  the  circle, 
and  the  rays  from  the  extremities  of  the  object  which  cross  at  this  nodal  point 
show  the  visual  angle. 

It  is  clearly  seen  from  the  diagram  that  the  object  must  increase  in  length 
in  direct  proportion  to  its  distance  from  the  eye  if  the  visual  angle  remains 
constant. 

Visual  Angle  The  angle  between  the  lines  extending  from  the  extremities 
of  the  visible  object  and  crossing  at  the  nodal  point  (n)  of  the  eye. 

Axis  The  straight  line  extending  along  the  principal  optic  axis  of  the  eye 
to  the  visible  object  on  one  side  and  to  the  retina  on  the  other  side  of  the  nodal 
point  («). 

n     Nodal  point  or  optic  center  of  the  eye. 

ri  Retinal  image.  The  size  of  the  retinal  image  of  a  given  object  depends 
upon  the  visual  angle  and  the  visual  angle  depends  upon  the  distance  of  the 
object  from  the  nodal  point. 


For  i  meter  (.3  mm.  -f-  .0075  =  400  diameters  magnification). 
For  y$  meter  (.075  -5-  .0075  =  100  diameters  magnification). 
For  10  meters  (3  -r-  .0075  =  4,000  diameters  magnification). 
For  anything  like  a  satisfactory  view  of  the  corpuscles,  it  would  be  desirable 
to  double  these  magnifications. 


CH.  IX]  MICRO-PROJECTION  WITH  AMPLIFIERS 


229 


§  356.  Amplifiers. — An  amplifier  is  a  concave  lens  or  combina- 
tion producing  divergence  instead  of  convergence  of  light  rays, 
hence  placing  an  amplifier  in  the  path  of  the  image-forming  rays 
from  the  objective  produces  a  larger  image  (fig.  126),  and  there  is 
little  loss  in  light.  It  should  be  made  as  great  in  diameter  as  the 
large  tube  (fig.  121)  of  the  microscope  will  receive  to  avoid  cut- 
ting down  the  field,  and  should  be  mounted  in  a  short  tube  which 
can  be  easily  slipped  into  a  cloth-lined  collar  screwed  into  the 
end  of  the  microscope  tube  (fig.  133). 

The  amplifiers  most  generally  useful  are  of  -5  and  -10  diopters. 
The  average  increase  in  magnification  given  by  the  -5  diopter 
amplifier  is  1.7  and  that  given  by  the  -10  diopter  is  2.5  (see  §  356a). 


Object 


Objective 


Microscooe  Tube  122  x  46  mm.          ^^      "^-  — —  __^ 

FIG.  126.    AMPLIFIER  FOR  PROJECTION.  **"***»< 

Object    The  object  to  be  projected. 

Objective    The  projection  objective. 

Axis     Optic  axis  of  the  apparatus. 

A  mplifier  The  concave  lens  diverging  the  rays  from  the  objective  and  thus 
increasing  the  screen  image. 

Images  The  ones  with  broken  lines  show  the  images  with  a  -5  diopter  and  a 
-10  diopter  lens.  The  full  lines  show  the  image  which  the  objective  alone 
would  give. 

The  microscope  tube  is  122  mm.  (4.8  in.)  long  and  48  mm.  (1.9  in.)  in 
diameter. 


§  356a,  403a.  Diopter,  Dioptre,  Dioptry. — For  spectacle  lenses  especially 
this  is  the  unit  of  strength.  It  is  the  strength  of  a  lens  of  i  meter  principal 
focus. 

As  the  focal  length  of  a  lens  varies  inversely  as  its  power,  the  focal  length  of  a 
lens  of  2  diopters  is  one-half  as  great  as  the  standard,  hence  it  has  a  focal  length 
of  y2  meter;  and  one  of  10  diopters  has  a  focal  length  of  i/io  meter  and  so  on. 

For  lenses  having  a  strength  less  than  the  standard  of  I  meter  the  focal 
length  will  also  be  inversely  as  the  power,  and  hence  a  Y*  diopter  lens  will  have 
a  focus  of  2  meters  and  a  i/ioth  diopter  lens  has  a  focus  of  10  meters.  In 
general,  the  less  the  dioptry  or  strength  the  longer  is  the  focus,  and  the  greater 
the  dioptry  or  strength  the  shorter  is  the  principal  focus. 

Convex  lenses  with  a  real  principal  focus  are  indicated  by  the  plus  sign  (+). 


230 


MICRO-PROJECTION  WITH  OCULARS  [CH.  IX 


FIG.  127.    HUYGENIAN  OCULAR  IN  SECTION. 
(From  The  Microscope). 

F.  L.     Field  lens.     This  aids  the  objective  in  forming  a  real  image. 

D  Diaphragm  in  the  ocular.  It  is  at  this  level  that  the  real  image  is 
formed  in  ordinary  microscopic  observation. 

E.  L.  The  eye  lens.  In  projection  this  acts  like  an  objective  and  projects 
upon  the  screen  an  image  of  the  real  image  (see  fig.  207). 

Axis    The  optic  axis  of  the  microscope. 

E.  P.     Eye-point  or  Ramsden's  circle. 


§  357.  Projection  oculars. — Any  ocular  may  be  used  for  pro- 
jection. The  lower  powers,  x  2,  X3,  x  4,  x  6,  (§  357a)  are 
better  than  the  higher  powers,  for  they  cut  down  the  field  less,  there 
is  less  loss  of  light,  and  there  is  not  an  inordinate  magnification. 


Concave  lenses  having  a  virtual  focus  are  indicated  by  the  minus  sign  (  —  )  . 

If  the  dioptry  of  a  lens  is  given,  to  find  the  principal  focus:  divide  i  meter 
by  the  dioptry.  For  example,  the  dioptry  of  the  amplifiers  mentioned  above 
(§  356)  is  —  5  for  one  and  —  10  for  the  other.  Their  foci  are  then  I  meter, 

-5 
i  meter.     That  is,  they  are  concave  lenses  of  1/5  and  i/io  of  a  meter  focus. 

-10 

On  the  other  hand,  to  find  the  dioptry  of  a  lens  whose  principal  focus  is  known, 
divide  i  meter  by  the  principal  focus  and  the  result  will  represent  the  dioptry 
of  the  given  lens.  Taking  the  same  case  as  before  where  the  amplifiers  have 


principal  foci  of  1/5  and  i/io  meter, 


=  10.     As  the  lenses  are 


known  to  be  concave,  the  minus  sign  is  placed  before  the  dioptry:     —  5,  —  10 
diopters. 

The  increase  in  magnification  given  by  the  amplifiers,  —  5,  —  10  was  found 
to  average  1.7  for  the  —  5  and  2.5  for  the  —  10.  The  average  was  obtained  by 
considering  all  the  screen  distances  and  all  the  different  objectives  shown  in  the 
table,  §  391.  See  also  §  392a. 


CH.  IX]  MICRO-PROJECTION  WITH  OCULARS  231 

In  using  the  ordinary  oculars  a  small  tube  must  be  screwed  into 
the  large  microscope  tube  as  for  ordinary  observation  (fig.  147, 

197)- 

Special  oculars  have  been  designed  for  projection.     Some,  like 
those  of  Zeiss  (fig.  128)  give  sharp  brilliant  images,  but  the  field 
is  very  small.     Williams,  Brown  and  Earle  have  a  very  large  pro- 
No.  2. 


FIG.  128.     PROJECTION  OCULARS  OF  ZEISS. 
(From  Zeiss'  Catalogue,  No.  30). 

A  section  has  been  removed  to  show  the  construction.  Both  are  of  the 
negative  form. 

The  eye  lens  is  in  a  smaller  tube  with  spiral  movement  to  enable  the  operator 
to  focus  the  image  of  the  diaphragm  of  the  ocular  sharply  on  the  screen. 
Below  are  shown  in  face  view  the  upper  ends  of  the  oculars  with  their  graduated 
circles.  By  noting  the  position  in  any  experiment  it  is  easy  to  set  the  position 
exactly  the  same  if  the  experiment  is  to  be  repeated. 

No.  2,  No.  4  These  numbers  indicate  that  the  ocular  magnifies  the  image 
two  or  four  times  (see  §  391)'. 

jection  ocular  of  the  Huygenian  form  which  magnifies  about  twice. 
On  account  of  the  loss  of  light  and  the  restriction  of  the  field  of 
view,  the  writers  of  this  book  do  not  advocate  the  use  of  oculars  for 
ordinary  micro-projection,  but  see  §  401. 

§  357a.  Designation  of  oculars. — At  the  present  time  an  ocular  is  usually 
designated  by  the  increase  in  magnification  it  gives  a  microscopic  image  when 
the  microscope  is  used  in  the  ordinary  way.  For  example,  if  the  objective  alone 
would  give  an  image  10  times  as  long  as  the  object,  then  an  ocular  x  2  should 
double  that  size,  thus  giving  an  image  magnified  20  times,  and  an  ocular  x  4, 
an  image  magnified  40  times  and  so  on. 


232 


MICRO-PROJECTION  WITH  OCULARS 


ICn.  IX 


§  358.  Micrometer  ocular  for  demonstration. — It  is  so  difficult 
for  most  students  to  understand  the  workings  of  the  ocular  micro- 
meter, that  it  is  of  great  help  to  them  to  use  a  micrometer  ocular 
like  fig.  130  to  131  on  the  projection  microscope,  then  the  object 
and  micrometer  lines  can  be  projected  together  by  suitably  adjust- 
ing the  eye-lens  of  the  ocular.  A  stage  micrometer  might  also  be 
used  as  object  and  the  students  shown,  all  together,  how  to  deter- 
mine the  ocular  micrometer  valuation  (see  Gage,  The  Microscope). 


Ocular  lo.  2 


FIG.  129.     COMPENSATION  OCULARS. 

(From  Zeiss'  Catalogue,  No.  jo). 

A  section  has  been  removed  to  show  the  construction. 
The  numbers  2,  4,  8,  /2, 18,  indicate  the  magnification  of  each 
ocular  (see  §  357a,  39ia). 

§  359.  Substage  condensers. — The  writ efs  believe,  from  their 
experience  and  experiments  in  photometry  under  the  different 
conditions,  that  it  is  better  to  use  for  illumination  only  the  large 
condenser  (fig.  121). 

The  use  of  a  substage  condenser  is  for  either  one  of  two  purposes : 
(i)  to  enable  the  position  of  the  object  and  the  projection  objective 

The  average  increase  in  magnification  given  by  the  different  oculars  with  the 
different  objectives  and  screen  distances  shown  in  the  table  (§  377)  is  as  follows: 

Projection  ocular  X2  gives  a  magnification  of 1.99 

X4  " 3.69 

Compensation  "     x2  " 2.05 

X4  " 4.21 

Huygenian  X4  " 4.2 1 

From  these  figures  it  is  seen  that  the  increase  in  magnification  for  projection 
work  can  be  closely  enough  approximated  by  multiplying  the  image  given  by 
the  objective  alone  by  the  number  designating  the  ocular,  i.  e.,  2  or  4. 

If  very  precise  results  are  desired,  one  must  use  a  stage  micrometer  and  pro- 
ceed as  described  in  §  391  a. 


CH.  IX]        PROJECTION  WITH  SUBSTAGE  CONDENSER 


233 


to  be  different  from  what  it  would  be  with  the  main  condenser  only ; 
or  (2)  to  make  the  aperture  of  the  illuminating  cone  correspond 
with  that  of  the  objective. 

The  positional  reason  (i)  can  only  have  weight  when  combined 
apparatus  is  used,  that  is,  when  a  magic  lantern  objective  as  well 
as  microscopic  objectives  are  used  without  changing  the  distance 
between  the  main  condenser  of  the  microscope  or  the  magic  lantern 
objective. 


FIG.   130.     OCULAR  MICROMETER  WITH  MOVABLE  SCALE. 

(Cut  loaned  by  the  Spencer  Lens  Co.}. 

This  is  a  Huygenian  ocular  with  a  5  mm.  scale  divided  into  twenty  ^  mm. 
intervals.  The  pitch  of  the  screw  moving  the  scale  is  ^  mm.,  therefore  one 
complete  revolution  of  the  drum  moves  the  scale  one  interval  or  ]/$  mm.  The 
drum  is  divided  into  100  graduations  thus  enabling  one  to  measure  looth  of 
an  interval  on  the  micrometer  scale.  This  ocular  micrometer  combines  the 
advantages  of  the  ocular  micrometer  with  fixed  scale  and  the  filar  micrometer. 
To  complete  the  measurement  of  an  object  not  exactly  between  any  two 
micrometer  lines  the  drum  need  be  revolved  only  partly  around. 

With  reference  to  the  aperture  (2)  it  is  one  of  the  fundamental 
laws  of  microscopic  vision  that  the  brilliancy  and  clearness  of 
details  depend  largely  upon  the  aperture  of  the  light  which  illumin- 
ates the  object,  and  which  passes  through  the  objective  to  form  the 
retinal  or  the  screen  image.  As  the  numerical  aperture  of  objec- 
tives varies  greatly  it  is  necessary,  if  the  clearest  and  most  brilliant 
images  are  to  be  produced,  to  light  the  object  with  a  numerical 
aperture  equal  to  that  of  the  objective.  Where  substage  con- 
densers are  used  arrangements  must  be  made  for  this. 


234 


PROJECTION  WITH  SUBSTAGE  CONDENSER        [Cn.  IX 


FIG.  131.     FILAR  MICROMETER  OCULAR. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

This  filar  micrometer  ocular  is  of  the  Ramsden  type  and  consists  of  a  positive 
ocular  with  a  movable  hair  line  and  two  reference  lines  at  right  angles  to  each 
other  as  shown  in  A .  The  movable  line  must  be  carried  over  the  entire  length 
of  the  object  to  be  measured  by  rotating  the  drum. 

A  Field  of  the  filar  micrometer  showing  the  movable  and  the  cross  lines, 
and  the  comb.  The  teeth  serve  to  measure  the  total  revolutions  of  the  drum. 


FIG.  132. 


ILLUMINATING  OBJECTS  OF  VARIOUS  SIZES  IN  MICRO-PROJEC 

TION    WITH    THE    MAIN    CONDENSER    ONLY. 


The  object  must  be  put  in  the  cone  of  light  at  a  point  where  it  will  be  fully 
illuminated. 

For  high  powers  it  will  be  at  or  very  near  the  focus  (/).  For  larger  objects 
and  low  powers  the  object  is  at  2  or  j,  or  even  closer  to  the  condenser  face. 

Arc  Supply     The  right-angled  carbons  of  the  arc  lamp. 

LT  L2     The  first  and  second  elements  of  the  triple  condenser. 

Water- Cell  The  water-cell  for  absorbing  radiant  heat.  It  is  in  the  parallel 
beam  between  the  first  and  second  elements  of  the  condenser. 

Axis     The  principal  optic  axis  on  which  all  the  parts  are  centered. 

If  only  the  main  condenser  is  used  (fig.  121),  the  cone  of  light 
from  the  condenser  must  be  sufficient  to  fill  the  aperture  of  the 
projection  objective.  This  requires  that  the  second  element  of  the 


CH.  IX]       PROJECTION  WITH  SUBSTAGE  CONDENSER  235 

main  condenser  (fig.  132  1^)  have  a  focus  of  150  to  200  mm.  (6  to  8 
inches).  With  such  a  main  condenser  one  can  do  successful  pro- 
jection with  objectives  from  125  to  4  mm.  focus.  The  aperture 
will  not  be  completely  filled  in  the  8,  6  and  4  mm.  objectives,  but 
brilliant  screen  images  are  obtained  even  with  them  for  a  7.5  meter 
(25  ft.)  screen  and  12  amperes  of  direct  current.  One  can  also  use 
a  -5  diopter  amplifier  when  good  specimens  are  projected.  (For 
the  position  of  the  objective  and  specimen  see  §  376). 

With  a  substage  condenser  there  is  a  great  loss  of  light  from 
reflection  and  absorption  so  that  the  increased  aperture  hardly 
compensates  for  it,  and  the  increased  detail  is  lost  for  the  observers 
are  too  far  from  the  screen  to  see  them  (see  §  35ga). 

For  special  demonstrations  and  for  drawing  where  the  observers 
are  very  close  to  the  screen,  the  substage  condenser  and  also  an 
ocular  are  advantageous,  and  for  fine  details,  necessary  (see  §  401, 
477)- 

SUITABLE  ROOM  AND  SCREEN  FOR  MICRO-PROJECTION 

§  360.  From  the  small  size  of  the  objective  for  micro-projection 
the  image  on  the  screen  cannot  be  made  as  bright  as  with  the  magic 
lantern,  hence  it  is  necessary  in  micro-projection  to  have  a  room 
that  can  be  made  very  dark ;  and  the  devices  for  cutting  out  stray 
light, — bellows,  objective  hood  and  shield — must  be  efficient  (fig. 
133,  139)- 


§  359a.  i.  Wright,  p.  212,  says:  "The  iris  of  the  substage  condenser  is 
opened  or  closed  until  the  best  effect  is  produced."  This  can  mean  only  that 
not  the  whole  cone  of  light  is  used  in  some  cases. 

2.  To  determine  the  amount  of  aperture  of  the  objective  used  in  projection, 
take  a  thick  piece  of  smoked  mica  or  combine  brown  and  blue,  or  deep  red  and 
blue,  or  red  and  green  glass  and  put  them  over  the  front  of  the  objective  to 
soften  the  light.  Or  one  might  hold  one  of  these  light  softeners  just  in  front 
of  the  eye.  Then  in  any  given  case  look  along  the  microscope  tube  directly 
toward  the  light,  and  the  aperture  of  the  objective  actually  filled  by  the  enter- 
ing cone  of  light  can  be  seen.  If  the  entire  aperture  is  used,  the  back  lens  of 
the  objective  will  be  filled  with  light;  if  only  a  part  of  the  aperture,  then  there 
will  be  a  central  brilliant  circle  and  a  dark  zone  of  glass  surrounding  it  (fig.  151). 

It  must  be  remembered  too  that  the  large  specimen  cooler  (fig.  121,  134) 
cannot  be  used  with  a  substage  condenser ;  and  in  our  opinion  this  overbalances 
any  advantage  that  the  substage  condenser  might  yield  for  demonstrations  to 
large  classes. 


236  MICRO-PROJECTION  WITH  DIRECT  CURRENT     [CH.  IX 

The  screen  must  be  as  reflecting  as  possible.  Nothing  has  ever 
yet  exceeded  in  satisfactory  quality  a  smooth,  dull,  white,  wall. 
For  a  full  discussion  of  screens  see  Ch.  XII,  §  621. 

MICRO-PROJECTION  WITH  THE  DIRECT  CURRENT  ARC  LAMP  AS  THE 

LIGHT  SOURCE 

§  361.  Arc  lamp  and  wiring  for  the  same. — The  direct  current 
arc  light  is  the  only  fully  satisfactory  artificial  light  known  at 
present  for  micro-projection.  Hence  it  will  be  taken  as  the 
standard,  as  with  the  magic  lantern  (Ch.  I).  Furthermore,  as  the 
upper  carbon  is  always  made  positive  and  hence  is  the  source  of 
light,  this  carbon  is  made  horizontal  and  the  crater  faces  the  con- 
denser and  is  in  the  optic  axis.  That  is,  for  micro -projection 
we  take  the  right-angled  arc  lamp  as  the  standard  (fig.  3,  121). 

The  wiring,  rheostat  and  ammeter  are  as  with  the  direct  current 
magic  lantern  radiant,  (figs.  2,3,  133).  The  rheostat  should  be  an 
adjustable  one.  The  ammeter  can  be  omitted,  but  it  is  more 
important  than  with  the  magic  lantern,  for  the  conditions  of 
micro-projection  must  be  made  as  nearly  perfect  as  possible.  With 
the  ammeter  one  can  tell  instantly  whether  the  proper  amount  of 
current  is  flowing.  If  there  is  sufficient  current  the  light  should  be 
satisfactory,  or  if  it  is  not  satisfactory  it  will  be  due  to  some  fault 
in  optical  adjustment.  The  ammeter  is  urged  upon  all  users  of  the 
projection  microscope  because  the  tendency  is  to  run  in  more  and 
more  current  if  the  projection  is  unsatisfactory,  hoping  by  pure 
brute  strength,  so  to  speak,  to  overcome  difficulties  due  to  improper 
adjustment.  In  case  one  cannot  afford  an  ammeter,  then  the  next 
best  thing  is,  when  installing  the  apparatus,  to  measure  the  current 
flowing  through  the  arc  with  the  different  settings  of  the  adjustable 
rheostat,  and  to  mark  these  values  on  the  rheostat  dial.  One  can 
then  set  the  rheostat  at  the  proper  amperage  for  the  given  projec- 
tion; but  as  the  voltage  on  the  line  is  subject  to  variation,  one 
cannot  be  sure  that  the  proper  current  is  flowing  at  any  given 
moment  unless  an  ammeter  is  present  to  indicate  the  amount. 
With  many  lighting  circuits,  the  fluctuations  in  voltage  are  very 
small,  and  one  can  be  reasonably  sure  of  getting  the  current  indi- 


CH.  IX]      MICRO-PROJECTION  WITH  DIRECT  CURRENT  237 

cated  on  the  dial  of  the  rheostat.  When  current  is  drawn  from  an 
overloaded  power  line,  however,  the  voltage  fluctuations  are  often 
so  great  that  an  ammeter,  as  well  as  an  adjustable  rheostat, 
is  a  necessity. 

§  362.  Fine  adjustment  for  the  arc  lamp. — For  micro-projec- 
tion it  is  absolutely  necessary  to  have  fine  adjustments  on  the  arc 
lamp  so  that  the  position  of  the  crater  can  be  changed  slightly 
during  an  exhibition.  In  the  burning  of  the  carbons  there  is  a 
slight  shift  in  position  of  the  crater  even  with  soft-cored  carbons. 
The  crater  may  be  in  perfect  alignment  to  start  with,  and  by  the 
shifting  as  the  carbon  burns  away  it  may  get  far  enough  outside 
the  longitudinal  axis  on  which  the  apparatus  is  placed  to  spoil  the 
light  on  the  screen.  This  is  emphatically  true  for  high  powers  (16 
mm.  and  higher).  If  now  there  are  fine  adjustments  on  the  lamp 
(fig.  3,  146),  by  which  the  crater  can  be  slightly  raised  or  lowered 
or  turned  toward  the  right  or  left,  compensation  for  this  shifting 
can  be  made,  and  the  most  brilliant  part  of  the  crater  kept  strictly 
in  the  axis  where  it  must  be  to  give  satisfactory  illumination. 
Furthermore,  it  is  necessary  to  have  an  independent  adjustment 
for  one  or  both  of  the  carbons,  so  that  one  or  both  carbons  may  be 
moved  independently.  This  is  because  the  carbons  are  liable  to 
wear  away  somewhat  unequally,  and  some  one  of  the  mal-positions 
shown  in  fig.  2 4,  .2  5  would  occur  if  the  carbons  were  not  adjustable. 

§  363.  Condenser. — The  triple  form  with  a  meniscus  next  the 
radiant  (fig.  121,  132)  is  very  satisfactory  for  micro-projection, 
although  many  use  the  double  form  (fig.  146)  with  success.  As 
the  objectives  used  for  projection  with  the  microscope  are  of  short 
focus  and  rather  large  aperture  the  final  element  of  the  condenser 
used  to  bring  the  light  to  a  focus  should  not  be  of  too  great  focal 
length.  A  focus  of  150—200  mm.  (6-8  in.)  is  a  good  average  for 
the  condenser  with  the  objectives  usually  employed  (125  to  4  mm., 
§  355)-  See  §  401  for  condenser  with  substage  condenser. 

§  364.  Water-Cell  to  prevent  overheating. — For  micro-projec- 
tion a  water-cell  in  connection  with  the  large  condenser  is  a  neces- 
sity. It  absorbs  most  of  the  radiant  energy  in  the  infra-red  part 


238 


MICRO-PROJECTION  WITH  DIRECT  CURRENT      [Cn.  IX 


FIG.  133.     PROJECTION  MICROSCOPE  WITH  AMPLIFIER. 

This  picture  shows  the  projection  microscope  arranged  for  use  in  a  lecture 
room. 

Commencing  at  the  left : 

The  supply  wires  to  the  table  switch. 

A  The  ammeter  to  indicate  the  amount  of  current.  It  is  along  one  wire 
(in  series). 

R     The  adjustable  rheostat.     It  is  along  one  wire. 

10-20  These  figures  indicate  that  the  rheostat  is  adjustable;  the  lowest 
current  allowed  to  flow  being  10  amperes  and  the  highest  20  amperes.  The 
arrow  indicates  the  direction  to  turn  the  knob  to  increase  the  current. 

The  arc  lamp  in  the  lamp-house.  This  is  the  three-wire,  automatic  arc  lamp 
of  the  Bausch  &  Lomb  Optical  Co. 

The  wiring  is  shown  to  be: 

One  wire  from  the  negative  pole  of  the  switch  to  the  pole  for  the  lower  carbon. 

One  wire  passes  from  the  positive  pole  of  the  switch  to  the  middle  binding 
post  of  the  motor  mechanism  of  the  automatic  lamp.  The  current  for  the 
motor  does  not  traverse  the  rheostat. 

One  wire  passes  from  the  positive  pole  of  the  switch  to  the  ammeter,  to  the 
rheostat  and  from  the  rheostat  to  the  positive  (+)  binding  screw  of  the  arc 
lamp. 


CH.  IX]     MICRO-PROJECTION  WITH  DIRECT  CURRENT  239 

The  metal  lamp-house  is  semi-transparent  as  it  was  in  position  during  only 
a  part  of  the  exposure  for  the  photograph. 

The  condenser  and  water-cell  are  connected  to  the  stage  by  a  bellows  to 
exclude  stray  light. 

The  microscope  shows  the  objectives  on  a  revolving  nose-piece  and  behind 
them  a  metal  shield  to  keep  stray  light  from  the  screen. 

An  amplifier  is  shown  in  place,  at  the  end  of  the  large  tube  of  the  microscope. 

The  arc  lamp,  condenser,  stage  and  microscope  are  each  on  an  independent 
block  which  moves  along  the  optical  bench  on  the  single  baseboard.  The 
vertical  white  lines  on  the  baseboard  indicate  the  position  of  the  various  blocks 
for  the  optical  combination  here  shown. 

On  the  front  legs  of  the  table  is  the  adjustable  drawing  shelf  upon  which  are 
demonstration  preparations. 

The  scale  of  this  picture  is  shown  by  the  10  centimeter  rule  just  above  the 
table  drawer  at  the  right. 

of  the  spectrum  and  thus  helps  to  avoid  the  overheating  which 
would  result  if  all  of  this  energy  remained.  The  best  position  of 
the  water-cell  is  between  the  first  and  second  elements  of  the  con- 
denser, where  the  rays  are  practically  parallel  (fig.  121).  For 
further  discussion  of  the  avoidance  of  heating  the  specimens,  see 
§852. 

§  365.  Stage  for  specimens. — The  stage  should  be  of  ample 
size,  and  should  have  an  opening  sufficiently  large  for  the  largest 
specimens  to  be  used  in  micro-projection,  that  is,  not  less  than  65 
mm.  (2^4  in.)  square. 

§  366.  Mechanical  stage. — If  serial  sections  are  to  be  used 
with  the  apparatus  then  the  stage  should  be  supplied  with  a 
mechanical  stage  of  great  range,  that  is  about  50  x  65  mm.  This 
is  about  the  maximum  range  for  the  sections  mounted  on  slides 
50  x  75  mm.  (2x3  in.)  (fig.  135,  136). 

§  367.  Stage  cooling  device. — While  the  large  water-cell  in 
connection  with  the  condenser  absorbs  practically  all  the  long 
waves  of  radiant  energy  that  can  be  absorbed  by  water,  it  is  very 
desirable,  and  for  many  specimens  necessary,  to  have  some  device 
for  carrying  off  the  heat  developed  in  the  specimen  itself  by  the 
absorbed  light.  The  most  practical  stage  cooling  device  is  a  stage 
water-cell.  The  one  found  very  efficient  and  satisfactory  in  every 
way  is  shown  in  fig.  121,  134.  The  specimen  rests  directly  against 
the  glass  side  of  the  water-cell  and  is  cooled  by  conduction.  Many 


240 


MICRO-PROJECTION  WITH  DIRECT  CURRENT     [Cn.  IX 


J 


- 


B 


D 


u/\ 


C 


FIG.  134.  FACE  VIEW  OF  THE 
STAGE  OF  THE  PROJECTION 
MICROSCOPE  AND  SECTION- 
AL VIEW  OF  THE  STAGE 
WATER-CELL. 

(About  half  size) . 

(From  The  Microscope,  Ninth 
Edition,  1904). 

A  Sectional  view  of  the 
stage  with  the  stage  water- 
cell. 

5  Metal  part  of  the  stage 
in  section. 

5  w.     Stage  water-cell. 

gsf  Glass  front  of  the  stage 
water-cell.  The  microscopic 
specimen  rests  directly  upon 
the  glass  front,  and  heat  from 
the  specimen  is  conveyed 
away  by  conduction. 

B  Face  view  of  the  metal 
part  of  the  stage  of  the  projec- 
tion microscope  (fig.  121), 
and  the  optic  bench.  In  this 
case  the  base  (£)  with  V's  is 
of  cast  iron  as  is  also  the 
block  (£>).  Both  were  pre- 
pared on  a  lathe  (Compare 

fig-  158,  159). 

E  End  view  of  the  guide 
piece  with  V's. 

D     Apparatus  block. 

C  Post  of  the  stage  in  the 
block  socket.  Two  set  screws 
hold  the  post  in  place.  It  is 
better  to  use  but  a  single 
screw  for  this. 

The  stage  proper  has  a  very 
large  opening,  and  the  water- 
cell  inserted  in  this  opening 
permits  of  the  demonstration 
of  specimens  up  to  65  mm.  in 
diameter. 

specimens  like  those  of 
the  nervous  system 
stained  with  Weigert's 
hematoxylin,  or  by  the 
Golgi  method  absorb  a 
great  deal  of  the  light 


CH.  IX]     MICRO-PROJECTION  WITH  DIRECT  CURRENT  241 

falling  upon  them,  and  hence,  following  the  law  of  the  conserva- 
tion of  energy,  all  this  absorbed  light  is  transformed  into  heat. 
The  darker  the  specimen  the  more  light  is  absorbed,  and  the  quicker 
it  will  be  spoiled  by  overheating.  The  stage  water-cell  against 
which  the  specimen  rests  conducts  this  heat  away,  in  part,  and 
makes  it  possible  to  exhibit  the  specimen  a  longer  time  (see  §  852). 


FIG.  135.     MECHANICAL  STAGE  OF  GREAT  RANGE. 

(Cut  loaned  by  the  Spencer  Lens  Co.). 

This  can  be  clamped  to  any  rectangular  microscope  stage  and  as  no  part  of 
the  clamp  extends  above  the  stage  the  full  range  of  85  by  65  mm.  is  available 
and  slides  50  x  75  mm.  (2x3  inches)  can  be  examined  to  the  edges.  This  is 
of  the  greatest  convenience  in  examining  serial  sections,  and  also  in  projecting 
them  on  the  screen. 

§  368.  Microscope-tube,  and  focusing  device. — If  a  tube 
for  receiving  the  objective  is  used  it  should  be  a  large  one,  (fig. 
121,  145).  The  small  tubes  used  on  most  microscopes,  and  on  all 
when  using  an  ocular,  cut  down  the  field  too  greatly  (fig.  137,  147). 
The  tube  should  be  short,  that  is,  about  9  to  10  cm.  (4  in.)  long,  and 
4  to  5  cm.  (2  in.)  in  diameter.  There  should  be  coarse  and  fine 
adjustments  as  for  the  ordinary  microscope  (fig.  121). 

§  369.  Mounting  of  objectives  of  low  power. — For  the  lowest 
powers  (125  to  75  mm.  equivalent  focus)  it  is  better  to  have  no 
tube  at  all,  but  to  have  a  black  shield  about  15  cm.  (6  in.)  in  diam- 


242  BLACK  APPARATUS  FOR  MICRO-PROJECTION       [Cn.  IX 

eter  into  the  center  of  which  is  screwed  the  objective  (fig.  138),  then 
the  field  is  not  at  all  restricted.  The  low  power  objectives  can  be 
focused  easily  by  moving  their  supports  back  and  forth  along  the 
optical  bench  by  hand  (fig.  158-159). 


FIG.  136.     MECHANICAL  STAGE  OF  WIDE  RANGE. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

This  mechanical  stage  can  be  attached  to  any  microscope  with  square  stage, 
and  it  permits  the  use  of  large  slides.  The  right  to  left  scale  is  80  mm.  and  the 
front  to  back  one  58  mm.  The  actual  range  available  depends  on  the  size  of 
the  stage  of  the  microscope. 

BLACKENED  APPARATUS 

§  370.  The  light  necessary  for  micro-projection  is  so  dazzling 
that  it  should  be  kept  strictly  within  the  projection  apparatus  by 
means  of  a  proper  lamp-house  and  bellows,  so  that  the  only  light 
which  finally  reaches  the  screen  is  that  which  passes  through  the 
projection  objective.  But  this  ideal  condition  cannot  be  wholly 
realized  in  practice,  hence  the  necessity  of  making  the  outside  of 
the  entire  apparatus,  from  lamp-house  to  microscope  tube,  dull 
black.  Then  any  escaping  light  will  not  be  reflected  from  polished 
surfaces  and  scatter  light  into  the  room,  on  the  one  hand,  or  blind 
the  eyes  of  the  operator  and  annoy  the  auditors  on  the  other. 
Projection  apparatus  found  in  institutions,  in  many  cases,  have  a 
finish  of  polished  brass  or  nickel.  If  the  operator  cannot  focus 


CH.  IX]        BLACK  APPARATUS  FOR  MICRO-PROJECTION  243 

properly  and  has  ill  success  in  general,  there  is  no  wonder,  as  he  has 
blinding  reflections  constantly  in  his  eyes.  With  a  dull  black  finish 
for  all  outside  surfaces,  if  the  apparatus  is  properly  built,  this 
defect  will  be  abolished.  Polished  black  finish  will  not  answer,  for 
it  reflects  almost  perfectly.  The  finish  must  be  dull,  dead,  or  luster- 
less,  then  the  light  will  be  mostly  absorbed,  and  so  small  a  part 
reflected  that  no  inconvenience  is  produced. 

§  371.    Blackening  the  interior  of  projection  apparatus. — As 

with  the  exterior  of  projection  apparatus,  so  the  interior  of  all  the 
parts  should  be  dull  black  to  avoid  internal  reflections  and  conse- 
quent confusion.  This  is  especially  true  of  the  objective  mount- 
ings, the  tube  of  the  microscope  and  the  amplifier  tube.  Lewis 


FIG.  137.     DIAGRAM  TO  SHOW  THE  SIZE  OF  IMAGE  WITH  THE  SAME  OBJEC- 
TIVE AND  DIFFERENT  LENGTH  AND  DIAMETER  OF  MICROSCOPE  TUBE. 

Objects     The  different  lengths  of  object  shown. 

Objective     The  projection  objective. 

Microscope  Tube  Microscope  tubes  with  diameters  of  48,  30  and  23  milli- 
meters. 

/,  2     Rays  which  are  stopped  by  the  largest  tube. 

Ray  3     The  marginal  ray  allowed  to  pass  by  the  largest  (48  mm.)  tube. 

Ray  4     The  extreme  marginal  ray  allowed  to  pass  by  the  30  mm.  tube. 

Ray  5     The  marginal  ray  allowed  to  pass  the  23  mm.  tube. 

Axis     The  optic  axis. 

Images  The  one  in  full  lines  is  for  the  smallest  tube.  The  others  in 
broken  lines  for  the  tubes  of  larger  size. 

By  tracing  back  to  the  specimen  it  is  seen  that  the  larger  tubes  show  corre- 
spondingly more  of  the  object,  the  projection  objective  remaining  the  same. 


244          BLACK  APPARATUS  FOR  MICRO-PROJECTION          [Cn.  IX 


FIG.  138.     PROJECTION  WITH  PHOTOGRAPHIC 
OBJECTIVES  OF  75  TO  125  MM.  Focus. 


Commencing  at  the  left: 

The  supply  in  this  case  is  from  the  house  circuit  for  a  current  of  five  amperes. 

There  is  first  a  separable  attachment  plug  in  the  lamp  socket.  On  the  table 
is  a  separable  extension.  This  is  to  serve  as  a  safe  switch  for  turning  the  cur- 
rent on  and  off. 

R  Small  rheostat  for  five  ampere  currents.  It  is  in  series,  along  one  wire. 
In  this  case  it  is  the  positive  wire,  if  direct  current  is  used,  and  goes  to  the  bind- 
ing post  of  the  upper  or  horizontal  carbon. 

The  other  wire  extends  between  the  binding  post  of  the  arc  lamp  and  the 
separable  extension. 

The  arc  lamp  with  small  carbons,  in  the  metal  lamp-house.  The  lamp- 
house  appears  transparent  as  it  was  in  place  during  only  a  part  of  the  exposure. 

Following  the  lamp-house  is  the  triple  condenser  and  water-cell  (fig.  122). 

The  stage  with  the  stage  water-cell  and  the  mechanical  stage  of  great  range 
(fig.  121,  135). 

Support  for  the  photographic  projection  objective. 

All  the  parts  are  supported  by  posts  and  blocks  and  all  move  independently 
on  the  baseboard  with  track.  The  vertical  white  lines  on  the  baseboard 
indicate  the  proper  relative  positions  of  the  different  blocks. 

At  the  extreme  right  is  shown  the  adjustable  drawing  shelf  attached  to  the 
legs  of  the  table.  On  this  shelf  is  the  projection  microscope  with  three  objec- 
tives in  the  revolving  nose-piece. 

The  shield  behind  the  objectives  is  to  prevent  stray  light  from  reaching  the 
screen.  Demonstration  preparations  are  also  shown  in  the  slide  box  on  the 
shelf. 

The  projection  table  with  the  drawer  for  holding  apparatus  is  shown  with 
the  legs  partly  removed.  The  entire  table  drawn  to  scale  is  shown  in  fig.  182. 
In  this  picture  the  scale  is  shown  by  the  10  centimeter  rule  just  above  the 
drawer  at  the  right. 


CH.  IX]      BLACK  APPARATUS  FOR  MICRO-PROJECTION  245 

Wright  (p.  194)  in  speaking  of  the  necessity  of  a  dull  finish  in  the 
interior  of  objectives  says  :  "I  may  add  here  that  some  really  good 
lenses  [objectives]  when  used  with  brilliant  light  such  as  projection 
demands,  give  a  "mist"  over  the  image  purely  from  flare,  or  reflec- 
tion in  the  lens  mount,  and  which  is  removed  by  careful  blacken- 
ing." 

Finally,  there  may  be  a  bright  spot  or  "ghost"  in  the  screen  image 
from  the  internal  reflections  of  a  shiny  microscope  tube,  especially 
if  the  tube  is  small.  If  an  ocular  is  used  this  ghost  usually  dis- 
appears. It  can  also  be  avoided  by  having  the  interior  of  the 
microscope  tube  a  dull  black  (§  37ia). 


Objective  Hood 


FIG.  139.     PROJECTION  OBJECTIVE  WITH 
BLACK  METAL  HOOD. 

§  372.  Hoods  for  projection  objectives. — Usually  the  ends  of 
objectives  are  tapering  and  finished  in  polished  nickel,  making  them 
veritable  mirrors.  As  the  image  of  the  source  of  light  spreads  more 
or  less  beyond  the  opening  of  the  front  lens  upon  this  mirror  surface 
the  dazzling  light  is  reflected  into  the  face  of  the  operator,  and  also 
more  or  less  around  the  room.  The  operator  is  likely  to  be  so 
blinded  by  the  reflections  that  he  cannot  see  to  focus  properly. 


§  37 la.  When  necessary,  a  person  can  give  polished  surfaces  a  dull  finish 
himself.  A  camel's  hair  artist's  brush  should  be  employed  for  the  finer  work. 
For  the  dull  finish,  dead-black  japalac  thinned  somewhat  with xylene  (xylol of 
the  Germans)  toluene  or  turpentine  answers  well. 

Dull  black  may  be  prepared  by  adding  to  thin  shellac  varnish  plenty  of  good, 
dry  lamp-black.  After  thorough  shaking,  this  should  be  filtered  through 
gauze  to  take  out  any  coarse  particles.  If  the  shellac  is  too  thick  the  resulting 
finish  is  more  or  less  shiny,  but  if  the  proper  mixture  is  used  the  surface  will 
be  very  dull,  but  not  so  smooth  as  the  japalac. 

As  the  black  surface  wears  off  by  use,  the  bright  surfaces  underneath  are 
exposed,  and  occasionally  one  should  go  over  the  apparatus  and  reblacken  all 
bright  spots. 


246  BLACK  APPARATUS  FOR  MICRO-PROJECTION       [Cn.  IX 

The  light  scattered  in  the  room  is  liable  also,  if  the  room  is  finished 
in  a  light  tint,  to  diminish  the  brilliancy  of  the  screen  image  by 
lessening  the  contrast. 

To  avoid  the  troubles  just  considered,  the  objective  should  have 
a  perforated  hood  over  its  front.  The  perforation  should  be  of  the 
diameter  of  the  front  lens.  The  free  surface  of  the  hood  over  the 
front  of  the  objective  should  be  perfectly  flat,  and  should  be  fmshed 
in  dull  black  (fig.  139-140).  Such  a  hood  is  also  of  the  greatest 
use  in  enabling  one  to  center  the  light  (§  375>  372a)- 


FIG.   140.     END  VIEW  OF  A  HOODED  OBJECTIVE  SHOWING  THE  LIGHT 
CENTERED  AND  OFF  CENTER. 

In  A  the  image  of  the  crater  is  directly  over  the  opening  in  the  hood'  and 
therefore  gives  the  greatest  light  for  projection. 

In  B  the  crater  image  is  at  the  right  and  only  a  small  amount  of  light  enters 
the  objective. 

In  both  A  and  B  the  negative  or  lower  carbon  is  shown  by  cross  lines.  It  is 
above  owing  to  the  inverting  action  of  the  condenser. 

§  373.  Light  shield  beyond  the  objective. — There  should  be  a 
flat  or  concave  shield  beyond  the  objective  to  prevent  any  stray 
light  reaching  the  screen  from  the .  apparatus  except  what  passes 
through  the  objective  (see  fig.  133,  138). 

CENTERING  THE  PARTS  OF  THE  PROJECTION  MICROSCOPE  ON  ONE 
LONGITUDINAL  Axis 

§  374.  For  micro-projection  it  is  absolutely  necessary  that  all 
the  parts  or  elements  should  be  on  one  straight  longitudinal  axis 
like  beads  on  a  rod.  With  the  large  lenses  used  in  magic  lantern 

§  372a.  If  one  does  not  have  the  metal-hooded  objectives  (fig.  139), 
ordinary,  nickel-plated  objectives  can  be  greatly  improved  by  painting  the 
bright  surfaces  with  dull  black  (§  37 la).  The  objectionable  reflections  can 
also  be  prevented  by  tying  black  velvet  or  blackened  asbestos  paper  around  the 
objectives. 


CH.  IX]  CENTERING  FOR  MICRO-PROJECTION  247 

objectives  a  slight  variation  from  perfect  alignment  would  do  no 
particular  harm,  but  the  lenses  are  so  small  in  micro-projection 
objectives  that  a  very  slight  displacement  from  the  axis  would 
throw  the  light  outside  the  objective  and  spoil  the  projection. 

The  fundamental  principles  and  precise  directions  for  centering 
projection  apparatus  are  given  in  Ch.  I.  §  51-58. 

§  375.  Final  centering  of  the  projection  objective. — After  the 
lamp  and  condenser  are  centered  as  nearly  as  possible  and  are  at  the 
right  distance  apart  (§  55,  56,  376),  move  the  stage  up  toward  the 
condenser  so  that  there  is  plenty  of  room  between  it  and  the  objec- 
tive. Use  some  dust  or  smoke  to  find  where  the  cone  of  light  from 
the  condenser  comes  to  a  focus  (fig.  132,  323). 

Now  move  the  microscope  on  its  mounting  toward  the  condenser. 
If  the  objective  is  centered,  then  the  point  of  light  at  the  focus  will 
enter  the  front  lens  through  the  hole  in  the  objective  hood  (fig.  140). 
If  it  is  not  centered  then  it  will  appear  at  one  side  or  even  entirely 
outside  the  objective.  Use  the  fine  adjusting  screws  of  the  arc 
lamp  and  change  the  position  of  the  image  of  the  crater  sufficiently 
to  direct  the  cone  of  light  into  the  front  lens  of  the  objective.  In 
case  the  objective  is  greatly  out  of  center  it  may  be  found  necessary 
to  change  the  position  of  the  entire  microscope. 

§  376.  Distance  of  the  objective  from  the   condenser. — The 

objective  should  be  at  a  distance  which  will  bring  the  crossing  point 
of  the  rays  in  the  cone  from  the  condenser  within  the  objective,  as 
for  the  magic  lantern  objective  (fig.  122).  As  the  center  of  the 
objective  is  but  slightly  beyond  the  front  lens,  the  following  method 
has  been  found  to  give  excellent  results.  The  objective  is  drawn 
up  toward  the  condenser  until  the  image  of  the  crater  is  shown 
within  the  opening  upon  the  black  hood  in  front  of  the  objective 
(fig.  140).  As  the  image  is  inverted  the  lower  or  negative  carbon 
will  appear  above  in  the  image.  If  now  the  stage  with  a  specimen 
is  moved  up  toward  the  objective  until  the  microscopic  object  on 
the  stage  is  in  focus,  the  image  on  the  screen  will  be  very  brilliant. 
One  should  make  slight  adjustments  toward  and  away  from  the 
condenser  to  get  the  most  brilliant  image.  It  will  be  found  that 


248  CENTERING  FOR  MICRO-PROJECTION  [Cn.  IX 

the  greatest  brilliancy  is  when  there  is  a  slight  yellowish  tinge  to  the 
light.  It  will  be  pure  white  if  one  moves  the  stage  and  objective 
slightly  nearer  the  condenser,  but  it  will  not  be  so  brilliant.  Guid- 
ing marks  should  be  made  on  the  apparatus  at  the  best  position 
for  the  different  objectives  used  (fig.  133,  138). 

§  377.  Table  of  Candle-Power  and  Current  with  Direct  Current 
Arc  and  Right-Angled  Carbons : 

Size  of  Carbons  Amperes  Candle-Power 

6  mm.  2  200 

6  mm.  3  400 

6  mm.  4  650 

6  mm.  5  900 

8  mm.  7.5  1,500 

ii  mm.  10  2,200 

ii  mm.  12.5  2,900 

ii  mm.  15  3,700 

ii  mm.  17.5  4,500 

13  mm.  20  5,400 

13  mm.  25  7,500 

15  mm.  30  9,500 

§  378.     Increase  in  size  of  the  crater  with  increase  of  amperage. 

— As  the  size  of  the  crater  and  hence  its  image  increases  with  the 
increased  amperage,  the  gain  for  actual  micro-projection  is  not  so 
great  as  would  appear,  for  the  larger  crater  image  will  be  larger 
than  the  lenses  of  objectives  of  high  power,  hence,  much  light  is 
wasted  (fig.  141). 

The  heating  is  also  much  increased  by  the  higher  amperage. 
It  has  been  found  by  experience  with  everything  in  the  best  possible 
condition  that  12  amperes  is  sufficient  for  most  micro-projection. 
A  current  above  20  amperes  is  a  pure  waste,  as  well  as  a  source  of 
danger  to  the  specimens  and  apparatus  by  overheating.  The  light 
given  by  10  amperes  properly  utilized  yields  far  better  results  than 
that  from  20  amperes  only  partly  utilized.  For  the  candle-power 
with  different  amperages  see  §  377. 


CH.  IX] 


USE  OF  PROJECTION  MICROSCOPE 


249 


USE  OF  THE  PROJECTION  MICROSCOPE 

§  379.  Objectives  in  a  revolving  nose-piece. — For  most  projec- 
tion a  battery  of  three  objectives  would  be  sufficient.  These 
should  be:  (i)  a  low  power  objective  to  show  entire  specimens 
(one  of  40  to  50  mm.  focus  is  good) ;  (2)  an  intermediate  objective 
of  16  to  18  mm.  focus;  and  (3)  a  high  power,  that  is,  one  of  10  to  4 
mm.  equivalent  focus  (§  355). 


10 


20 


FIG.  141.     SIDE  AND  FRONT  VIEWS  OF  THE  CRATER  AND  CARBONS 

BURNING  WITH  10  AND  WITH  20  AMPERES  OF  DIRECT 

CURRENT  (Natural  size). 

This  picture  is  to  show  the  increase  in  size  of  the  crater  with  the  larger  cur- 
rent. (See  also  fig.  292-293). 

(In  making  the  photographs,  the  lamp  was  burning  with  the  amperage  indi- 
cated, and  an  instantaneous  exposure  was  made  with  a  diaphragm  of  F/32. 
The  current  was  then  turned  off  and  the  carbons  exposed  90  seconds  with  a 
diaphragm  of  F/8.  This  brought  out  the  carbons,  and  gives  the  appearance 
gained  by  the  eye  when  suitably  screened  and  looking  at  the  burning  lamp.) 


250  USE  OF  PROJECTION  MICROSCOPE  [Cn.  IX 

The  three  objectives  selected  should  be  in  a  revolving  nose-piece 
(fig.  142)  so  that  one  can  pass  quickly  from  one  power  to  another. 
The  lecturer  and  operator  must  always  keep  in  mind  that  for  an 
audience  giving  their  entire  attention,  a  delay  of  even  a  quarter  of 
a  minute  seems  a  very  long  time,  hence  every  precaution  should  be 
taken  to  avoid  delays. 

§  380.     Preparation  of  the  carbons  for  an  exhibition. — The 

carbons  supplied  for  projection  are  soft-cored,    and    sharpened 
somewhat  like  a  lead  pencil.     This  end  form  is  unlike  that  assumed 


FIG.  142.     TRIPLE  NOSE-PIECE  OR  REVOLVER  FOR  QUICKLY  CHANGING 

OBJECTIVES. 

(From  the  Catalogue  of  Viogtlander  und  Sohri). 

in  the  actual  use  of  the  carbons  (fig.  141),  and  until  the  carbons 
have  burned  for  some  time,  one  will  not  get  the  best  light  from 
them.  Hence  it  is  wise  to  get  the  carbons  formed  by  burning  them 
in  the  lamp  for  five  minutes  or  so  before  using  them  for  a  lecture  or 
an  exhibition. 

Soft-cored  carbons  are  a  necessity  for  micro-projection,  for  the 
crater  remains  more  uniform  and  it  does  not  wander  around  the 
end  of  the  carbons  and  thus  get  out  of  line  of  the  general  axis  so 
frequently  as  would  be  the  case  with  solid  carbons  (§  38oa). 


§  380a.  Cored  and  solid  carbons. — Some  workers  with  the  projection 
microscope  use  a  large,  cored  carbon  above  (i.e.,  for  the  positive)  and  a  solid 
carbon  for  the  negative  one.  For  example,  in  a  projection  outfit  from  Zeiss 
the  upper  carbon  was  19  mm.  in  diameter  and  soft-cored.  The  lower  or  nega- 


CH.  IX]  USE  OF  PROJECTION  MICROSCOPE  251 

§  381.  Screen  image  of  the  carbons. — One  of  the  good  ways  of 
learning  to  get  the  carbons  in  the  correct  relative  position  is  to 
study  their  image  on  the  screen.  For  this  use  an  objective  of 
50  or  100  mm.  focus.  By  moving  the  objective  somewhat 
beyond  the  focus  of  the  condenser  an  image  of  the  burning  car- 
bons will  be  projected  on  the  screen  and  one  can  tell  the  exact 
appearance  of  the  crater  and  the  relative  position  of  the  car- 
bons. The  glowing  upper  carbon  ought  to  show  the  crater 
well  and  appear  to  face  directly  toward  the  observer.  As  this  is 
an  image  of  the  real  image  of  the  carbons  formed  by  the  condenser 
the  screen  image  will  appear  right  side  up.  If  the  negative  or  lower 
carbon  is  not  in  the  correct  position  it  will  shade  the  image  (see 
fig.  24,  25). 

§  382.  Centering  the  light  and  getting  the  objective  at  the 
correct  distance  from  the  condenser  for  an  exhibition. — In  using 
any  of  the  objectives  on  the  revolving  nose-piece  it  is  always  to  be 
kept  in  mind  that  the  centering  is  most  easily  accomplished  by 
drawing  the  objective  toward  the  condenser  until  the  image  of  the 
crater  and  the  tip  of  the  negative  carbon  appear  in  the  opening  and 
upon  the  objective  hood  (fig.  140). 

Now  if  this  image  is  not  so  that  the  brightest  part  is  over  the 
opening  in  the  objective  hood,  use  the  fine  adjustment  of  the  arc 
lamp  and  get  the  image  of  the  crater  directly  in  the  opening.  The 
screen  image  will  then  be  evenly  and  brilliantly  lighted.  In  case 
cne  side  is  more  brilliantly  illuminated  than  the  other,  one  can 
make  the  illumination  even  by  the  fine  adjustments  of  the  arc  lamp 
(fig.  3,  146). 

One  can  sometimes  improve  the  illumination  slightly  by  looking 
at  the  screen  image  and  moving  the  microscope  slightly  nearer  or 
farther  from  the  condenser,  but  as  a  rule,  when  the  image  of  the 

live  carbon  was  13  mm.  in  diameter  and  solid.  In  Ewon's  lamp  the  upper  or 
positive  carbon  is  cored  and  18  mm.  in  diameter;  the  lower  carbon  is  12  mm.  in 
diameter  and  solid. 

Experience  leads  us  to  recommend  cored  carbons  below  as  well  as  above. 
For  the  size  of  carbons  for  different  amperages  see  §  377,  753a. 

For  alternating  current  both  carbons  are  of  the  same  size,  and  most  workers 
recommend  that  they  be  cored. 


252  USE  OF  PROJECTION  MICROSCOPE  [Cn.  IX 

crater  and  the  negative  carbon  are  most  sharply  defined  on  the 
objective  hood  the  light  on  the  screen  will  be  the  best  attainable. 
Occasionally,  during  an  exhibition,  it  will  be  necessary  to  use  the 
fine  adjustments  on  the  arc  lamp  (fig.  146)  to  get  the  crater  back  in 
exact  alignment  as  the  crater  changes  position  slightly  on  the  wear- 
ing away  of  the  carbons.  As  the  carbons  sometimes  wear  away 
unevenly  it  is  necessary  to  have  a  mechanism  by  which  one  carbon 
can  be  moved  without  affecting  the  other,  otherwise  there  would 
result  some  one  of  the  malpositions  shown  in  fig.  24,  25. 

§  383.  Specimens  for  projection. — The  specimens  giving  the 
best  images  with  the  projection  microscope  are  those  which  are  best 
for  ordinary  observation,  that  is  those  with  the  most  definite  out- 
lines and  sharpest  details.  They  must,  of  course,  be  more  or  less 
transparent.  For  staining,  any  color  which  gives  definite  details 
can  be  used,  but  one  must  remember  that  the  red  colors  are  trans- 
parent to  the  longer,  visible  waves  of  light  and  hence  red-colored 
objects  can  remain  on  exhibition  much  longer  than  hematoxylin, 
osmic  acid  or  other  dark  stained  objects  which  are  more  opaque  to 
the  long  waves  in  the  red  end  of  the  spectrum  (fig.  307). 

No  matter  how  large  the  water-cell  or  the  cooling  stage,  a  thick, 
darkly  stained  specimen  will  be  spoiled  after  a  time  by  the  trans- 
formation of  the  absorbed  light  into  heat  (§  852). 

§  384.  Masks  for  microscopic  slides. — The  light  used  in  pro- 
jection is  of  necessity  so  brilliant  that  the  scattered  light  from  the 
microscopic  glass  slide  is  very  liable  to  dazzle  the  eyes  of  the 
operator  when  he  looks  at  the  slide  in  arranging  it  for  the  projection 
of  the  object  or  objects  thereon.  If  one  has  a  series  for  example,  it 
is  very  difficult  to  select  with  ease  and  certainty  just  the  sections 
that  are  to  be  shown  with  this  scattered  light  in  the  eyes.  It  must 
always  be  remembered,  too,  that  a  very  short  time  seems  long  to  a 
waiting  audience;  and  that  it  lessens  their  confidence  in  the  lec- 
turer to  have  too  much  blundering  in  showing  the  specimens  he 
wishes  them  to  see. 

All  this  difficulty  can  be  easily  avoided  by  properly  masking  the 
preparations  to  be  shown  (fig.  143,  148). 


CH.  IX] 


USE  OF  PROJECTION  MICROSCOPE 


253 


§  385.  Kind  and  color  of  paper  for  the  masks. — The  best  paper 
to  use  is  one  that  allows  only  a  moderate  amount  of  light  to  pass, 
and  that  cuts  out  the  green-blue  end  of  the  spectrum. 

The  color  found  best  for  this  is  an  aqueous  solution  of  the 
microscopic  stain  known  as  "Orange  G."  For  the  quality  of 
paper,  a  white  linen  bond  paper  of  moderate  weight  is  used.  It  is 
stained  by  soaking  it  a  few  minutes  (10-30)  in  a  saturated  aqueous 
solution  of  the  "Orange  G."  It  is  then  hung  up  to  dry. 


CO 


S\   [:^%:-M>:-:;S'vSx^Vr:->/\v-:;vx: 


S  us 

ser  11 

si  60 

SCC15U 
1900 


FIG.  143.     SLIDE  OF  SERIAL  SECTIONS  WITH  MASK. 

The  sections  to  be  demonstrated  are  left  uncovered. 
Sus     (Sus  scrofa,  the  pig). 

Ser.  ii     This  shows  that  the  slide  is  from  the  I  ith  series  of  pig  embryos. 
si  60.     The  6oth  slide  of  series  1 1 . 

Sec  /5M  This  indicates  that  the  sections  of  this  embryo  were  cut  15  microns 
(.015  mm.,  .00058  in.)  thick. 

ipoo     The  year  in  which  the  series  was  prepared. 
ii  60     At  the  left;   series  1 1 ,  slide  60. 

Paper  thus  colored  allows  a  moderate  amount  of  light  to  pass, 
and  allows  practically  all  of  the  long  waves  of  red  and  infra-red  to 
pass,  so  that  it  will  not  burn  very  quickly  in  the  focus  of  the  con- 
denser. If  black  paper  were  used  it  would  burn  almost  instantly 
in  the  focus.  Of  the  many  yellows  and  oranges  tried  for  masks  the 
"Orange  G"  proved  most  satisfactory. 

§  386.  How  to  employ  the  masks. — The  paper  is  cut  of  the 
right  size  for  the  slide  and  then  square  or  round  holes  are  made  in 
it  to  give  a  clear  field  for  the  different  objects  to  be  shown  on  the 
slide,  then  it  is  pasted  on  the  cover-glass  (fig.  143).  It  is  put  on 


254  USE  OF  PROJECTION  MICROSCOPE  [Cn.  IX 

the  cover-glass  and  not  on  the  slide  for  the  reason  that  if  it  were  put 
on  the  slide  it  would  almost  entirely  overcome  the  good  effect  of  the 
stage  cooling  cell,  as  it  would  hold  the  slide  away  from  the  glass 
surface,  so  that  the  heat  could  not  be  carried  off  by  conduction. 
If  it  is  on  the  cover-glass,  the  slide  can  then  rest  directly  against 
the  stage  water-cell. 

If  one  ever  wishes  to  remove  the  mask  it  is  easily  done  by  putting 
a  piece  of  wet  blotting  paper  upon  it  till  thoroughly  softened.  It 
can  then  be  peeled  off,  and  the  cover-glass  cleaned  with  a  wet  cloth. 

§  387.  Field  of  view  in  the  screen  image. — Except  with  objec- 
tives corrected  in  the  manner  of  photographic  objectives  the  screen 
image  will  not  be  equally  sharp  over  the  entire  field  where  the  large 
tube  and  where  no  tube  is  used  (fig.  138,  145).  To  obviate  this, 
oculars  may  be  used,  or  iris  diaphragms  to  cut  off  the  outer  margin 
which  is  not  sharp.  This  margin  also  shows  color  from  the 
chromatic  aberration  of  the  condenser.  But  demonstrations  in 
histology  and  embryology,  at  least,  depend  largely  in  their  effec- 
tiveness upon  the  relations  of  parts  shown  in  a  large  field.  The 
part  to  be  shown  with  greatest  distinctness  is  brought  into  the 
middle  of  the  field  as  with  ordinary  microscopic  observation. 

The  importance  of  a  large  field  in  which  the  relation  of  parts  can 
be  shown,  can  be  illustrated  by  a  simple  experiment.  For  example, 
let  a  well  known  friend  cover  his  face  with  a  mask  having  only  eye- 
holes, or  with  a  hole  to  show  a  part  of  the  cheek  or  forehead.  It 
would  be  hard  to  recognize  him  from  that  limited  view  alone. 

§  388.  Objectives  needed  for  different  sizes  of  field. — In  fig. 
144  there  is  given  a  graphic  representation  of  different  sizes  of  field 
or  object  which  one  might  wish  to  project,  and  the  objective  or 
objectives  with  which  it  can  be  done.  It  will  be  seen  that  the 
larger  the  field  the  longer  must  be  the  focus  of  the  projection 
objective.  In  this  figure  it  is  assumed  that  no  ocular  is  used  and 
that  the  field  is  not  restricted  by  the  tube  of  the  microscope,  hence 
for  the  largest  fields  the  objective  must  be  mounted  in  a  shield 
without  tube  (fig.  138).  In  fig.  137  is  shown  how  the  field  may  be 
cut  down  by  using  microscope  tubes  of  different  diameter.  See 
also  the  table  of  magnification  and  field  (§  391). 


CH.  IX] 


USE  OF  PROJECTION  MICROSCOPE 


255 


1  mm.  Field 
Objectives  Ur 


70  mm.  Field 


(120mm. 

Objectives]  <oo  mm. 

(   80mm. 


o 

2-1.5  mm.  Field 
Objectives]  I""; 

2.5  mm.  Ffeld 
Objectives  (125mm 


O 


10mm. 


5  mm.  Field 
Objectives  ill 

10  mm.  Field 
Objectives  | 


15  mm.  Field 
Objectives  { 2°  mm. 


25-20  mm.  Field. 
Objectives! mm 


30  mm.  Field 
Objectives  1 50 


FIG.   144.     SIZES  OF  FIELD  AND  OBJECTIVES  NECESSARY  TO  PROJECT 
OBJECTS  OF  THESE  SIZES. 


256 


USE  OF  PROJECTION  MICROSCOPE 


[Cn.  IX 


§  389.  Sharpness  of  the  screen  image. — It  is  a  mistake  to  think 
that  it  is  necessary  that  the  screen  image  should  be  photograph- 
ically sharp.  As  well  said  by  Lewis  Wright,  p.  191:  "A  certain 
breadth  or  coarseness  of  line  is  a  positive  advantage  in  the  image  to 
be  viewed  many  feet  [meters]  away."  Of  course,  the  image  should 
be  focused  as  sharply  as  possible,  but  a  line  or  structure  that 
appears  perfectly  distinct  at  a  considerable  distance  may  appear 
indistinct  when  the  observer  is  close  to  the  screen.  If  the  operator 
is  at  a  considerable  distance  (15  to  20  meters,  50  to  65  ft.)  from  the 
screen,  he  will  find  good  opera-glasses  a  help  in  getting  the  screen 
images  properly  focused. 


Objective 


FIG.  i44a.     DIAGRAM  TO  SHOW  THE  POSITIONS  OF  THE  SAME  OBJECT  AND 

THE  SlZE  OF  THE  SCREEN  IMAGE  FOR  OBJECTIVES  OF  50,  2O  AND  IO  MM. 

Focus. 

In  this  figure  it  is  assumed  that  the  object  in  each  case  is  practically  at  the 
principal  focal  distance  from  the  objective  and  that  the  screen  distance  is  the 
same  for  all.  As  the  size  of  the  image  varies  inversely  with  the  distance  of  the 
object  from  the  objective  it  is  seen  that  the  screen  must  be  larger  for  an 
objective  of  short  than  for  one  of  long  focus  in  accordance  with  the  general 
law  of  the  relative  size  of  the  object  and  image  (§  392a). 


CH.  IX]  MAGNIFICATION  IN  MICRO-PROJECTION 


257 


Any  one  can  get  a  pretty  correct  idea  of  the  screen  image  and  the 
details  visible  at  different  distances  by  putting  the  first  page  of  a 
newspaper  up  in  a  well  lighted  place  and  then  moving  back  from 
it.  Close  up,  the  ordinary  print  can  be  read,  farther  away  the 
ordinary  headlines,  and  still  farther  the  title  of  the  newspaper  or 
some  gigantic  headline.  Meantime  the  ordinary  print  and  the 
ordinary  headlines  have  merged  into  a  gray  haze. 

§  390.  Position  of  the  object  on  the  stage. — For  many  micro- 
scopic specimens  it  makes  no  difference  how  the  specimen  is  placed 
upon  the  stage,  except  that  for  high  powers  the  cover-glass  must 
be  next  the  objective.  If  a  specimen  must  have  a  given  part,  end 
or  border  at  the  top  in  the  screen  image,  then  with  an  objective  only 
or  an  objective  and  an  amplifier,  the  object  must  be  put  on  the 
stage  so  that  the  part  is  down  which  is  to  appear  at  the  top  in  the 
screen  image.  With  an  objective  and  ocular  the  object  should  be 
placed  on  the  stage  as  the  image  is  to  appear  on  the  screen.  For 
getting  the  screen  image  exactly  like  the  object  see  §  36,  512. 

§  391.  Magnification  and  Screen  Image  of  Various  Objectives 
as  Found  by  Actual  Measurement  (see  §  3Qia). 


5M.  (i6£t.) 
Screen  Distance 

7.5  M.  (25  ft.) 
Screen  Distance 

10  M.     (33  ft.) 
Screen  Distance 

Objective 

Field  of 
Objective 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  o 
Field 

No  TUBE  (Fig.  138) 

125  mm. 

55mm. 

39 

2.25  M. 

61 

3-56  M. 

80.6 

4.50  A 

100 

50 

48 

2.70 

74 

3.16 

98.4 

.4.20 

70 

50 

72 

3-50 

107 

5.10 

143 

7.10 

60 

42 

85 

3-15 

123 

3-70 

168 

5.20 

50 

38 

101 

3-30 

147 

4-50 

202 

6.10 

35 

26 

142 

3-50 

2IO 

5-io 

285 

7.60 

30 

20 

167 

3-30 

250 

4.90 

330 

6.70 

20 

II 

253 

2.85 

370 

4.20 

495 

5-40 

MAGNIFICATION  IN  MICRO-PROJECTION  [Cn.  IX 


LARGE  TUBE  (Fig.  121) 


5  M.  (i6ft.) 

7.5  M.  (25  ft.) 

10  M.     (33ft.) 

Screen  Distance 

Screen  distance 

Scieen  Distance 

Objective 

Field  of 
Objective 

Magni- 
fication 

Screen 
Image  of 
Field 

Screen 
Magni-    j      image  of 
fication    ;         Field 

Magni- 
fication 

Sc:een 
Image  o 
Field 

No  TUBE  (Fig.  138) 

70  mm. 

31  mm. 

72 

I 

.85  M. 

107 

3-34  M. 

143 

4.26  M 

60 

25 

85 

2.10 

127 

3.00 

1  68 

4.10 

50 

22 

101 

2-30 

155 

3-50 

205 

4-50 

35 

14 

142 

2-35 

218 

3.00 

285 

4-45 

30 

12 

167 

2.00 

250 

3-35 

330 

4.10 

20 

8 

253 

2.10 

380 

3.00 

500 

4.10 

16 

5-75 

322 

•75 

488 

2.85 

650 

3-73 

12.5 

4-5 

385 

.80 

590 

2.65 

750 

3.38 

10 

3-7 

454 

•75 

7OO 

2-59 

9OO 

3-33 

8 

2.5 

640 

.80 

940 

2-35 

1280 

3-20 

6 

2.18 

760 

.80 

1  1  2O 

2.44 

1460 

3-i8 

4 

1.42 

1080 

.70 

1600 

2.27 

2180 

3.10 

2 

0.42 

2600 

.10 

3820 

1.70 

5080 

2.00 

MAGNIFICATION  AND  SCREEN  IMAGE  OF  VARIOUS  OBJECTIVES  AS  FOUND 
BY  ACTUAL  MEASUREMENT. 

First  is  given  the  magnification  of  the  objective  only,  using  the  large  tube 
of  the  microscope  (fig.  121) ;  then  are  given  the  magnification,  etc.,  with  am- 
plifiers (fig.  126)  and  wii;h  oculars.  With  the  latter  the  draw-tube  is  in  place 
(fig.  147,  I72,§39ia). 


5  M.  (i6ft.) 
Screen  Distance 

7.5  M.  (25  ft.) 

Screen  Distance 

10  M.  (33ft.) 
Screen  Distance 

Objective 

Ampli- 
fier 

Ocular 

Micro- 
scope 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

1  6  mm. 

i     ii 
i     n 
i     « 

-3d 
-iod 

Proj.    x2 
"       x4 
Comp.  x2 

X4 
Huyg.  X4 

5.75  mm. 

4.2 
4.2 

1.45  ' 
1.32  ' 

2.35  ' 
1  .60  " 
1.  60  ' 

322 
550 
800 
640 
1170 
650 
1320 
1310 

1.75  M. 
2.30  " 
3-20  " 

•95  " 
1.70  " 

i-55  " 
2-35  " 
2.25  " 

488 
820 
1  1  6O 
950 
I900 
1000 
2000 
2250 

2.85  M. 

3-56    " 
5.00    " 
1.40    " 
2.65    " 
2-35    " 
3-40   " 

3-35  " 

650 
1  100 
1650 
1320 
2610 
1350 
2880 
27OO 

3-78  M. 

4-85    " 
6.89    " 
I.9I    " 

3-50  ;; 
3.10 
4.60  " 

4-45  " 

12.  5  mm. 

11     u 
«     « 

-  5d 
-iod 

Proj.    x2 

X4 

Comp.   x2 
x4 
Huyg.  X4 

4.5    mm. 
3-5 
3-5 
1.25    ' 

1-25    ' 

2.00     ' 
1.50     " 
1.40     ' 

385 
650 
910 
730 
1300 

750 
1520 
1520 

i.8oM. 
2.30  " 
3-30  "• 
•95  " 

.70  " 

•50  " 

2-35  " 
2.25  " 

590 
990 
1430 
1075 
2000 
1150 
2350 
2350 

2.67  M. 
3.60  " 
5.20  " 
i-35  " 
2-45  " 
2.30  " 

3-50  " 
3-37  " 

750 
1280 
1900 
1450 
2700 
1550 
3200 
3150 

3.40  M. 

4-85  " 
6.50  " 
1.86  " 
3-30  " 
3-05  " 
4-75  " 
4-45  " 

CH.  IX]  MAGNIFICATION  IN  MICRO-PROJECTION 


259 


5  M. 

(i6ft.) 

7-5  M 

(25  ft. 

10  M. 

(33  ft.) 

Screen 

Distance 

Screen 

Distance 

Screen 

Distanca 

Objective 

Ampli- 
fier 

Ocular 

Micro- 

Magni- 
fication 

Sci  een 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 

Field 

10  mm. 

3.70  mm. 

454 

1.75  M. 

700 

2.52  M. 

900 

3.65  M. 

" 

-5d 

2.70 

766 

2.20    " 

IH5 

3-40 

1540 

4.70 

11 

-lOd 

2.70 

IIOO 

3.20    " 

1630 

4-50 

2225 

6.50 

Proj.    X2 

1.05 

870 

•95  " 

1300 

1.40 

1750 

1.90 

X4 

1.05 

1600 

1.70  " 

2330 

2.40 

3100 

3-30 

Comp.   X2 

1.70 

870 

1.50  " 

1330 

2.25 

1780 

3-05 

"       X4 

1.25 

1820 

2.35  " 

2720 

3-50 

3680 

4.70 

Huyg.  X4 

1-25     ' 

1800 

2.25  " 

2700 

3-30 

3680 

4-55 

8  mm. 

2.5  mm. 

640 

i.SoM. 

940 

2-55  M. 

1280 

3-31  M. 

-  5d 

1.  80    " 

1  120 

2.30  " 

1650 

3-24 

2250 

4.36 

-lod 

i.  80    " 

l600 

3-20  " 

2430 

4-35 

3250 

6.00 

Proj.    X2 

0.7 

1280 

.90  " 

I960 

1.40 

2610 

1.89 

"       *4 

0.7      " 

2360 

1.70  " 

3521 

2-35 

4820 

3-35 

Comp.   x2 

1.  10      " 

1380 

1.50  " 

2030 

2.25 

2770 

3-i8 

x4 

0.82    " 

2750 

2.30  " 

4I2O 

3-6o 

5560 

4.90 

Huyg.  X4 

0.78    ' 

2750 

2.25  " 

4050 

3-35 

5560 

4-50 

6  mm. 

2.18  mm. 

760 

i.8oM. 

1  120 

2.50  M. 

1460 

3.50  M. 

-  5<i 

i-7 

1270 

2.25  " 

1950 

3-30  ' 

2570 

4-50  " 

-lod 

i-7 

1850 

3-25  " 

2760 

4-95  ' 

3700 

6.50  " 

Proj.    x2 

0.60 

1500 

•95  " 

2250 

1.41  ' 

3100 

1.91  " 

X4 

0.60 

2720 

1.65  " 

4200 

2-55  ' 

5700 

3-37  " 

Comp.   x2 

0-93 

1550 

1.50  " 

2400 

2.30  ' 

3200 

3-05  " 

«"       x4 

0.70 

3120 

2-35  " 

4800 

3-55  ' 

6500 

4-75  " 

Huyg.  X4 

0.67 

3100 

2.25  " 

4700 

3-37  ' 

6500 

4-50  " 

4  mm. 

i  .4.2  mm. 

1080 

1.70  M. 

1600 

2.40  M. 

2180 

3.ioM. 

~55 

1.05 

1910 

2.30  " 

2720 

3-20  " 

3800 

4-30    ' 

-lod 

1.05 

2750 

3-25  " 

4160 

4-50  " 

5650 

6.50    ' 

Proj.    x2 

0.40 

2250 

•92  " 

3460 

1.40  " 

4500 

1.90    ' 

«     « 

"       *4 

0.40 

4120 

1.65  " 

6800 

2-75  " 

9000 

3-6o    ' 

Comp.  X2 

0.62 

2350 

1.50  " 

3500 

2.28  " 

4830 

3.10    ' 

*4 

0.47 

4820 

2-37  " 

7200 

3-60  " 

9800 

4.80    ' 

Huyg.  x4 

0-45 

4770 

2.25  " 

7100 

3-37  " 

9820 

4-50    ' 

2  mm. 

0.42  mm. 

2600 

i.ioM. 

3820 

i.yoM. 

5080 

2.00M. 

-  5d 

0.42 

4440 

1-95  " 

6560 

2.85    ' 

8900 

3.65  " 

-lod 

0.42 

7220 

2.60  " 

9480 

4-50    ' 

12550 

7.00  " 

Proj.    X2 

0.185 

4940 

0-93  " 

7400 

1.40    ' 

10500 

2.37  " 

*4 

0.18 

9160 

1.67  " 

13800 

2.50    ' 

18750 

3-70  " 

Comp.  X2 

0.28 

5120 

1.50  " 

6666 

2.28    ' 

10600 

3.00  " 

XT"        X4 

0.215 

10500 

2.20    " 

16150 

3-35    ' 

21250 

5.00  " 

Huyg.  x4 

O.2O 

10750 

2.20    " 

15500 

3-45    ' 

21000 

5.10  " 

260 


MAGNIFICATION  IN  MICRO-PROJECTION 


[Cn.  IX 


2.5  M.  (8ft.) 
Screen  Distance 

Objective 

Ampli- 
fier 

Ocular 

Microscope 
Field 

Magni- 
fication 

Screen 
Image  o 
Field 

>  mi 

n. 

-  5d 
-lod 

Proj.    X2 
X4 
Comp.  X2 
X4 
Huyg.  X4 

0.42  m 
0.42    ' 
0.42    ' 
0.185  ' 
0.18    ' 
0.28    ' 
0.215  ' 

O.2O     ' 

11. 

1300 
2130 
3080 
2350 
4400 
2440 
5000 
4960 

0.56  M 
0.97  ' 
1.38  ' 
0.44  ' 
0.78  ' 
0.71  ' 

1.  10    ' 
I.IO   ' 

§  391a.  In  preparing  this  table  the  apparatus  shown  in  fig.  121,  138  was 
used.  The  second  element  of  the  condenser  giving  the  cone  of  light,  had  a  focus 
of  30.3  cm.  (8  in.),  and  the  stage  was  moved  up  in  the  light  cone  (fig.  132)  to 
give  the  largest  and  brightest  field  possible  for  the  given  objective.  No  sub- 
stage  condenser  was  used  except  for  the  2  mm.  oil  immersion. 

A  stage  micrometer  in  millimeters,  tenths  and  one-hundredths  was  used  as 
object.  The  screen  image  of  one  or  more  of  the  micrometer  divisions  was 
measured  with  a  metric  rule  and  the  magnification  obtained  by  dividing  the 
size  of  the  image  by  the  known  size  of  the  object.  For  example:  if  the 
micrometer  is  in  one-tenth  millimeters  (o.i  mm.)  and  the  screen  image  of  two 
spaces  (0.2  mm.)  measures  20  centimeters  or  200  mm.  the  magnification  of  the 
screen  image  must  be  200  divided  by  0.2  =  1000.  That  is,  the  image  is  one 
thousand  times  the  size  of  the  object,  therefore,  the  magnification  of  the  pro- 
jection apparatus  in  that  case  is  1000.  The  size  of  the  field  of  the  projection 
apparatus  is  found  by  the  use  of  the  micrometer  as  follows :  The  micrometer 
is  arranged  on  the  stage  so  that  the  image  shows  one  of  the  lines  on  one  edge 
of  the  field  (the  circle  of  light).  Then  one  simply  counts  the  spaces  to  the 
other  edge  of  the  field.  For  example,  suppose  that  it  requires  14  of  the  o.i 
mm.  spaces,  then  the  size  of  the  field  is  1.4  mm.  and  an  object  larger  than  this 
cannot  be  projected  entire  with  this  objective. 

To  get  the  size  of  the  screen  image  of  this  field  a  tape  measure  or  meter  stick 
is  used  and  the  diameter  of  the  circle  of  light  on  the  screen  is  measured. 

This  method  of  finding  the  size  of  the  field  of  the  projection  apparatus,  the 
magnification  and  the  size  of  the  screen  image,  depends  upon  direct  observation 
and  is  applicable  to  any  projection  outfit  whether  an  objective  only  or  an  objec- 
tive and  an  amplifier  or  an  objective  and  an  ocular  are  used  (see  also  §  392a). 
The  amplifiers  used  had  a  free  opening  of  36' mm.  (i -^  in.),  and  were  placed  at 
the  end  of  the  large  tube  (fig.  133)  at  a  distance  of  about  n  cm.  (4^  in.)  from 
the  objective. 


CH.  IX] 


MAGNIFICATION  IN  MICRO-PROJECTION 


261 


§  392.    Magnification  and  Screen  Image  of  Various  Objectives 
as  Found  by  Calculation  (see  §  39 2 a). 


5M.  (i6ft.)                   7-5  M.  (25ft.) 
Screen   Distance    ;  !        Screen  Distance 

10  M.  (33  ft.) 
Screen  Distance 

Objective 

Field  of 
Objective 

Magni- 
cation 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

Magni- 
fication 

Screen 
Image  of 
Field 

7.20  M. 
440  " 
7-50  " 

125  mm. 

100     " 

70     ' 
60     ' 
50     ' 
35 
30 

20       ' 

90  mm. 

55 
75 

No  TUBE  (Fig.  138) 

80 
IOO 

138 

166 
200 
286 

333 
500 

40.0 
50.0 

71-5 
834 

IOO.O 

143.0 
166.6 
250.0 

3.60  M. 

2.2O 

3-75 
2.50 
3-57 
3*50 
3.80 
3-72 
3-34 

2-75 

60 

75 

107 
125 
150 
214 
250 
375 

5-40  M. 

3-30  ;; 
5.62  " 

3-75  " 
5-35  " 

5.25  ;; 
5-70 
5.56  " 

5-00  " 
4-13  " 

50 
50 
42 
38 
26 

20 
II 

5.00  " 
6.90  " 
7.00  " 
7.60  " 

7-43  " 
6.66  " 
5-50  " 

LARGE  TUBE  (Fig.  121) 


70  mm. 

31  mm. 

71-5 

2.22  M. 

107 

3-34  M. 

138 

4.27  M. 

60 

25   ' 

834 

2.08 

125 

3.12  " 

1  66 

4.16  " 

50 

22 

IOO.O 

2.20 

150 

3.30  " 

200 

4.40  " 

25 

14 

143.0 

2.00 

214 

3.00  " 

286 

4.00  " 

30 

12 

166.6 

2.00 

250  3.00  " 

333 

4.00  " 

25 

8   ' 

250.0 

2.00 

375 

3-oo  " 

500 

4.00  " 

16 

5-75' 

312.5 

•79 

468 

2.69  " 

625 

3-59  " 

12.5 

4-50' 

400.0 

.80 

600 

2.70  " 

800 

3-60  " 

10 

3.70' 

500.0 

•85 

• 

750 

2.78  " 

IOOO 

3-70  " 

8 

2.50' 

625.0 

•56 

937 

2-34  " 

1250 

3.12  " 

6 

2.18' 

833-3 

.82 

1250 

2.72  " 

1666 

3-63  " 

4 

1.42' 

1250.0 

•77 

1875 

2.66  " 

2500 

3-55  " 

2 

0.42  ' 

2500.0 

•05 

3750 

1-57  " 

5000 

2.69  " 

§  392a.  This  table  was  derived  by  calculation  from  the  optical  law  that: 
The  size  of  the  image  is  to  the  size  of  the  object  as  the  distance  of  the  image 
is  to  the  distance  of  the  object  from  the  center  of  the  projecting  lens  or  objec- 
tive (fig.  209).  In  each  case  the  objective's  principal  focus  is  marked  upon  it 
by  the  maker,  and  the  distance  of  the  screen  from  the  objective  is  known. 
Referring  to  the  diagram  (fig.  121)  it  is  seen  that  the  focus  of  the  objective 
represents  approximately  the  distance  of  the  object  from  the  center  of  the 
objective  when  the  screen  distance  is  relatively  great.  The  focus  of  the  objec- 
tive and  the  screen  distance  being  known  their  ratio  is  easily  found.  For 
example,  with  the  20  mm.  objective  and  a  5  meter  screen  distance,  the  object 
will  be  20  mm.  from  the  center  of  the  objective  (fig.  209)  and  the  screen  image 
is  5  meters  (5000  mm.)  distant,  then  the  ratio  is  250  to  I  (5000/20)  and  it 
follows  from  the  optical  law  given  above,  that  the  magnification  in  this  case 
is  250. 

The  field  in  each  case  was  determined  by  the  use  of  a  stage  micrometer  as 
with  §  39 1  a.  From  fig.  209  it  is  evident  that  the  screen  image  of  the  entire 


262  ORDINARY  MICROSCOPE  FOR  PROJECTION       [Cn.  IX 

MICRO-PROJECTION  WITH  AN  ORDINARY  MICROSCOPE 

§  393.  Magic  lantern  with  optical  bench  and  ordinary  micro- 
scope.— If  one  has  a  magic  lantern  with  an  optical  bench,  the 
bellows  and  lantern-slide  objective  may  be  removed  and  an  ordinary 
microscope  put  in  place.  The  microscope  is  made  horizontal  and 
firmly  clamped  to  a  suitable  block  (fig.  145,  187).  This  block 
should  be  furnished  with  cleats  or  grooved  so  that  it  will  slide  on 
the  rods  or  guides  of  the  magic  lantern,  and  be  of  sufficient  height 
to  put  the  objective  and  tube  of  the  microscope  in  the  optic  axis. 
The  mirror  and  the  substage  condenser  may  be  removed  or  turned 
aside  and  the  object  lighted  by  the  cone  directly  from  the  large 
condenser  as  in  fig.  145  or  the  condenser  and  ocular  may  be  left 
in  place  (fig.  187). 


field  is  magnified,  hence  to  get  the  size  of  the  screen  image,  the  size  of  the  field 
is  multiplied  by  the  magnification  of  the  apparatus  in  any  giyen  case.  In  the 
case  of  the  20  mm.  objective  the  entire  field  measures  8  mm.,  hence  its  screen 
image,  with  a  magnification  of  250,  should  be  8  x  250  =  2000  mm.  or  2  M. 

If  one  compares  the  tables  obtained  by  actual  measurement  and  that 
obtained  by  calculation  it  will  be  seen  that  they  do  not  exactly  agree.  This'is 
due  to  two  things :  first,  the  rated  focus  of  the  objective  is  only  an  approxima- 
tion, and  second,  the  measurement  of  the  diameter  of  the  screen  image  is  not 
very  exact  from  the  difficulty  of  deciding  just  where  to  begin  and  where  to 
leave  off  in  measuring  to  get  the  magnification  and  for  determining  the  size  of 
the  field  or  the  screen  image  of  the  field. 

The  table  of  calculated  values  is  only  for  the  objective  without  the  use  of 
amplifiers  or  oculars. 

If  one  knows  the  magnification  of  the  objective  for  a  given  screen  distance 
the  magnification  obtained  when  using  an  amplifier  or  an  ocular  with  the 
objective  may  be  obtained  approximately  as  follows: 

For  — 5d  amplifier  multiply  the  magnification  of  the  objective  only,  by  1.70 

For  — lod  amplifier  multiply  the  magnification by  2.50 

For  x2  projection  ocular  multiply  the  magnification by  2.00 

For  X4  projection  ocular  multiply  the  magnification  by  3.70 

For  x2  compensation  ocular  multiply  the  magnification by  2.05 

For  X4  compensation  ocular  multiply  the  magnification. .by  4.20 

For  X4  Huygenian  ocular  multiply  the  magnification by  4.20 

As  the  field  of  the  projection  apparatus  is  cut  down  by  the  use  of  an  amplifier 
or  an  ocular  one  must  determine  the  size  of  the  field  by  the  use  of  a  micrometer 
as  with  the  objective  alone.  The  screen  image  can-  then  be  calculated  by 
multiplying  the  observed  size  of  the  field  by  the  magnification  of  the  combined 
objective  and  ocular  or  amplifier.  It  will  be  seen  that  the  objective  with  an 
ocular  X2  or  X4  does  not  give  a  magnification  exactly  twice  or  four  times  as  great 
as  the  objective  alone.  The  oculars  are  rated  for  the  ordinary  distance  of 
distinct  vision  (254mm.,  10  in.)  and  the  relation  does  not  hold  strictly  for  the 
much  greater  screen  distances  (§  357a). 


CH.  IX]         ORDINARY  MICROSCOPE  FOR  PROJECTION 


263 


FIG.  145.     ORDINARY  MICROSCOPE  FOR  PROJECTION. 

This  figure  is  to  show  how  an  ordinary  microscope  can  be  used  for  projection 
if  one  has  an  arc  lamp  and  condenser. 

Commencing  at  the  left : 

The  supply  wires  coming  to  the  table  switch. 

From  the  negative  pole  of  the  switch  one  wire  proceeds  to  the  negative  bind- 
ing post  of  the  arc  lamp,  i.  e.,  to  the  one  for  the  lower  carbon. 

From  the  positive  pole  of  the  switch  extend  two  wires  for  the  automatic  lamp 
of  the  Bausch  &  Lomb  Optical  Co.  One  wire  goes  to  the  binding  post  of  the 
automatic  mechanism  (the  middle  post).  This  means  that  the  automatic 
mechanism  receives  current  which  does  not  go  through  the  rheostat.  The 
other  wire  from  the  positive  pole  of  the  switch  goes  to  the  ammeter  (A),  and 
from  the  ammeter  to  the  rheostat  (R),  and  from  the  rheostat  to  the  positive 
binding  post  for  the  arc  lamp,  i.  e.,  for  the  upper  carbon. 

The  arc  lamp  is  shown  through  the  metal  lamp-house.  The  lamp-house 
appears  transparent  as  it  was  left  in  position  during  only  a  part  of  the  exposure. 

Following  the  lamp-house  is  the  triple  condenser  and  water-cell. 

The  microscope  is  bent  over  in  a  horizontal  position  to  bring  the  axis  of  the 
objective  in  line. 

The  microscope  is  clamped  to  a  block  which  raises  it  to  the  right  level. 


264  ORDINARY  MICROSCOPE  FOR  PROJECTION          [Cn.  IX 

As  here  shown  the  substage  condenser  and  mirror  have  been  removed,  and 
also  the  draw-tube  and  ocular  (see  fig.  147,  192  for  the  ordinary  microscope 
with  substage  condenser,  draw-tube  and  ocular  in  position). 

The  lamp,  condenser  and  microscope  are  on  independent  blocks  and  can  be 
moved  to  any  desired  position  on  the  baseboard. 

A     The  ammeter  to  indicate  the  amount  of  current. 

R  Adjustable  rheostat.  This  rheostat  is  adjustable  between  10  and  20 
amperes.  The  arrow  indicates  the  direction  of  increase  in  current. 

5  Adjustable  drawing  shelf  attached  to  the  front  legs  of  the  table.  In  this 
picture  the  shelf  supports  the  stage  of  the  projection  microscope  (fig.  121),  and 
a  box  of  demonstration  specimens. 

The  scale  of  the  picture  is  indicated  by  the  10  cm.  rule  just  above  the  table 
drawer  at  the  right. 

If  the  tube  of  the  microscope  is  large  it  is  an  advantage,  but  with 
the  small  tube  one  can  do  much.  If  the  ocular  is  not  to  be  used, 
then  it  is  better  to  remove  the  draw-tube  so  that  only  the  main 
tube  remains.  One  should  be  sure  that  the  interior  of  the  tube  is 
dull  black  (§  370). 

§  394.     Magic  lantern  with  rods,  and  an  ordinary  microscope.— 

If  the  magic  lantern  has  the  simple  construction  with  rods  and  feet 
(fig.  32,  33,  36)  an  ordinary  microscope  can  be  used  with  it  as 
follows :  Remove  the  rods,  bellows  and  projection  objective,  and 
support  the  arc  lamp  and  the  condenser  on  a  block  which  will  lift 
them  high  enough  so  that  the  microscope  in  a  horizontal  position 
will  be  in  the  optic  axis.  Place  all  on  a  baseboard  with  guides 
(fig.  146).  Clamp  the  microscope  to  a  suitable  block  with  grooves 
or  cleats  to  enable  one  to  move  the  block  accurately  along  the 
guides.  When  properly  centered  this  form  of  apparatus  works 
well. 


§  394a.  For  a  water-cell  one  of  the  plane-sided  glass  boxes  found  on  the 
market  can  be  used,  or  a  cell  can  be  prepared  in  the  laboratory  as  follows: 
Select  some  good  plane  and  clear  glass.  For  the  ends  of  the  box  make  two 
strips  about  2^2  cm.  (i  in.)  wide  and  about  10  cm.  (4  in.)  long.  For  the  sides 
use  two  sheets  about  10  cm.  (4  in.)  wide  and  n  cm.  (4^"  in.)  long;  and  for 
the  bottom  a  rather  thick  sheet  or  strip  about  n  cm.  (4^2  in.)  long  and  3  cm. 
(i  >4  in.)  wide.  The  pieces  of  glass  are  then  put  together  by  placing  the  bottom 
on  a  level  table  and  the  other  pieces  in  position  and  held  in  place  by  a  string 
or  by  narrow  strips  of  gummed  paper. 

The  joints  are  then  gone  over  carefully  with  an  artist's  brush  dipped  in 
Ripolin  white  paint  or  Valspar  varnish.  Each  coat  should  be  allowed  to  dry 
thoroughly  before  adding  the  next,  that  is,  for  two  to  five  days.  Finally  one 
can  add  water  to  see  if  the  joints  are  all  tight.  If  not,  dry  the  glass  box  and 
then  add  more  of  the  Ripolin  paint  or  Valspar  varnish. 


CH.  IX]         ORDINARY  MICROSCOPE  FOR  PROJECTION 


265 


FIG.  146.  USE  OF  THE  SIMPLE 
MAGIC  LANTERN  CONDENSER 
AND  LAMP  AND  AN  ORDINARY 
MICROSCOPE  FOR  PROJECTION. 

This  is  a  magic  lantern  with 
iron  legs  and  rods  for  the  support 
and  guidance  of  the  parts  (fig. 
33).  The  slide-carrier  bellows 
and  lantern  objective  with  the 
guide  rods  have  been  removed, 
leaving  only  the  condenser,  arc 
lamp  and  lamp-house.  The  short 
tubes  for  the  lamp  are  supported 
at  the  left  by  the  ordinary  legs 
of  the  apparatus.  In  front  a 
support  of  wood  is  used  when 
necessary.  As  the  whole  lamp  and 
condenser  would  be  too  low  for 
the  axis  of  the  microscope  it  is 
raised  on  a  block  (Block-)  to  the 
proper  height.  There  is  a  base- 
board on  which  all  the  apparatus 
is  placed,  and  at  the  left  there  is 
a  track  made  of  rods  or  tubes  as 
in  fig.  158, 159  on  which  the  block 
supporting  the  microscope  can 
be  moved  back  and  forth  in  line 
of  the  axis.  For  a  water-cell,  a 
glass  box  made  as  described  in 
§  394a  is  set  on  a  block  in  the 
path  of  the  cone  from  the  con- 
denser. 

Commencing  at  the  left : 

Arclamp  The  hand-feed, 
right -angle  carbon  arc  lamp. 

s.  s     Set  screws. 


This  is  also  an  excellent  meth- 
od of  making  small  glass  boxes  for 
experimental  work  where  water  is 
the  liquid  medium.  Such  boxes 
also  have  been  used  continuously 
for  months  for  observing  the 
growth  of  aquatic  plants.  If  one 
side  is  made  of  cover-glass,  then 
high  powers  of  the  microscope 
can  be  used  to  study  the  growth 
on  the  inside  face  of  the  cover- 
glass. 

We  are  indebted  to  Prof.  Romyn 
Hitchcock  for  the  method  of  mak- 
ing water-cells  by  the  aid  of 
Ripolin  paint. 


266  ORDINARY  MICROSCOPE  FOR  PROJECTION       [Cn.  IX 

F.  S.     Feeding  screws  for  the  carbons. 

V.  A.     Vertical  fine  adjustment  for  centering  the  crater. 

L.  A.     Lateral  fine  adjustment  for  centering  the  crater. 

Wi     Supply  wire    to  the  upper  carbon. 

Wa     Supply  wire  from  the  lower  carbon  through  the  rheostat  (R). 

R     Rheostat  in  the  wire  from  the  lower  carbon. 

Rods    The  short  tubes  or  rods  supporting  the  lamp. 

Lt,  L2    The  left  and  right  supports  or  legs  of  the  lamp-rods. 

Block^  The  block  on  the  baseboard  to  elevate  the  arc  lamp  and  condenser 
to  the  axis  of  the  microscope. 

Lamp-House     The  metal  enclosure  of  the  arc  lamp. 

V    Ventilator  of  the  lamp-house. 

Condenser  The  two-lens  condenser.  It  is  supported  by  the  front  end  of  the 
lamp-house. 

/,  2     The  two  plano-convex  lenses  forming  the  condenser. 

Water-cell  The  glass  vessel  with  plane  sides  filled  with  water  and  placed  in 
the  path  of  the  cone  of  light  from  the  condenser  to  absorb  the  radiant  heat. 

Microscope  An  ordinary  microscope  turned  in  the  horizontal  position.  The 
draw-tube  and  ocular  have  been  removed,  also  the  substage  condenser. 

Stage     The  stage  of  the  microscope. 

5*5    The  substage  condenser  sleeve.     The  condenser  has  been  removed. 

Axis,  Axis    The  principal  optic  axis  of  the  condenser  and  the  microscope. 

O     Objective. 

H    Handle  for  carrying  the  microscope. 

c,  f    Coarse  and  fine  focusing  adjustments. 

cl     Clamp  for  holding  the  microscope  to  the  block. 

FM    Foot  of  the  microscope. 

Block,     The  wooden  block  supporting  the  water-cell. 

Block3  The  block  to  which  the  microscope  is  clamped.  It  moves  back  and 
forth  on  the  track  (tr). 

tr    The  rods  on  the  baseboard  serving  for  a  track. 

Base  Board     The  board  on  which  all  the  apparatus  is  placed. 

§  395.  Stray  light,  and  a  water-cell. — For  a  water-cell,  any  glass 
vessel  with  plane  sides  can  be  used,  and  it  can  be  put  between  the 
condenser  and  the  stage  of  the  microscope  instead  of  between  the 
lenses  of  the  condenser  as  in  fig.  4,  167.  For  cutting  off  stray 
light  one  can  use  a  black  cardboard  shield,  or  a  black  disc  may  be 
perforated  and  hung  on  the  end  of  the  tube  of  the  microscope 
beyond  the  focusing  mechanism.  For  bellows  between  the  con- 
denser and  stage,  use  a  sheet  of  asbestos  paper. 

§  396.  The  directions  for  using  the  ordinary  microscope  in 
projection  are  precisely  as  for  the  special  microscope  shown  in  fig. 
121,  and  discussed  in  the  first  part  of  this  chapter.  As  there  is  no 
stage-cooling  device  one  must  be  careful  not  to  overheat  the  speci- 
mens. 


CH.  IX]          ORDINARY  MICROSCOPE  FOR  PROJECTION  267 


FIG.  147.     PROJECTION  WITH  THE  MICROSCOPE  IN  A  VERTICAL  POSITION. 

W    Supply  wires  from  the  outlet  box  (fig.  3). 

r     Rheostat  of  the  theater-dimmer  type. 

t  w     Wires  to  the  arc  lamp  from  the  switch. 

/  a    Fine  adjustment  screws  projecting  behind  the  lamp-house. 

h  f    Hand-feed  screws  for  the  carbons  of  the  arc  lamp. 

/  h  Lamp-house.  It  is  of  sheet  iron,  but  was  left  in  position  only  a  part  of 
the  time,  hence  it  appears  transparent. 

g     Observation  window  opposite  the  crater. 

C    Triple  condenser  with  water-cell  (fig.  121). 

a  a  Principal  optic  axis.  The  mirror  of  the  microscope  reflects  the  light 
vertically  along  this  axis  and  through  the  microscope,  then  the  mirror  or  prism 
over  the  ocular  reflects  it  horizontally  again. 

m  Mirror  or  prism  over  the  ocular  to  reflect  the  light  horizontally  to  the 
screen. 

sh     Shield  to  cut  off  stray  light. 

b     Baseboard  with  track  for  an  optical  bench. 

a  5     Adjustable  shelf  for  drawing. 


268 


PROJECTION  OF  HORIZONTAL  OBJECTS  [Cn.  IX 


PROJECTION  OF  HORIZONTAL  OBJECTS 

§  397.  As  with  the  magic  lantern,  so  with  the  projection  micro- 
scope some  objects  must  be  left  in  a  horizontal  position  for  projec- 
tion. This  requires  that  the  microscope  be  in  a  vertical  position. 
As  the  light  source  is  for  giving  light  in  a  horizontal  direction 
(fig.  121),  it  is  necessary  to  use  a  mirror  or  prism  to  reflect  the 
horizontal  light  upward  through  the  vertical  microscope  and  then 
another  mirror  or  prism  above  the  microscope  to  reflect  the  vertical 
light  horizontally  to  the  screen.  This  is  shown  in  fig.  147,  175. 

The  ordinary  mirror  of  the  microscope  serves  very  well  for  mak- 
ing the  light  vertical,  but  for  reflecting  it  horizontally  to  the  screen 
a  prism  or  a  plane  mirror  silvered  on  the  face  is  best,  as  it  gives  a 
single  image,  not  a  double  image  as  would  the  ordinary  glass  mirror 
silvered  on  the  back. 

§  398.     Avoidance  of  stray  light  with  a  vertical  microscope.— 

This  is  easily  accomplished  by  using  a  vertical  piece  of  blackened 
cardboard  just  beyond  the  microscope  as  shown  in  fig.  147.  If 
light  escapes  from  the  sides  one  can  use  pieces  of  black  cardboard 
or  asbestos  to  enclose  the  microscope  more  completely.  Ordi- 
narily, however,  the  single  black  shield  beyond  the  microscope 
will  answer. 

§  399.  Sample  Objects  Suitable  for  Projection  with  the  Differ- 
ent Objectives  (see  also  §39ga). 

PHOTOGRAPHIC  TYPE  OF  OBJECTIVES   (Micro-Planars,  etc.) 
No  Tube  (fig.  138) 


Magnification  with:  — 

Object 

Size  of  Object 

Objective 

5  Meter 
Screen 

7.5  Meter 
Screen 

10  Meter 
Screen 

Brain  Section    
Cerebellum  and  Brain 
Stem 

55  to  90  mm. 
50  to  75  mm. 

35  to  50  mm. 

25  to  40  mm. 

20  to  35  mm. 
15  to  25  mm. 

10  to  20  mm. 

5  to  1  1  mm. 

125  mm. 
100  mm. 

70  mm. 
60  mm. 

50  mm. 
35  mm. 

30  mm. 
20  mm. 

39 

48 

72 
85 

IOI 

142 

167 

253 

61 

74 

107 
123 

147 

210 

250 
370 

80.6 
98.4 

143 
1  68 

202 

285 

330 
495 

Longitudinal    Section 
of  40  mm.  Embryo  . 
Section  of  Eye  
Section     of     Injected 
Kidney  

36  Hour  Chick  Entire 
Transection  of  Human 
Esophagus  
Appendix  (Homo)    .  . 

CH.  IX]  OBJECTS  FOR  MICRO-PROJECTION  269 

Large  Tube  (fig.  121) 

Magnification  with: — 


Object 

Size  of  Object 

Objective 

5  Meter 
Screen 

7.5  Meter 
Screen 

10  Meter 
Screen 

Pyloric  Stomach  .... 
Medulla  and  Olives 
Scalp                    

20  to  30  mm. 
1  5  to  25  mm. 
12  to  22  mm. 

70  mm. 
60  mm.    ' 
50  mm. 

72 

85 
101 

107 
I27 
155 

H3 

1  68 
205 

Human  Spinal  Cord 
Thyroid 

10  to  14  mm. 

8  to  12  mm. 

35mm. 
30  mm. 

I42 
167 

218 

250 

285 

33° 

Adrenal    

5  to    8  mm. 

20  mm. 

253 

380 

500 

ORDINARY  MICROSCOPIC  OBJECTIVES 
Large  Tube  (fig.  121) 


Section  of  Lung  or 
Artery    
Neural  Plate  of  Am- 
blystoma  
Transection  of 
Trachea 

4  to  5    mm. 
2  to  4.5  mm. 
2  to  3.7  mm. 

16 
12.5 
10 

mm. 
mm. 
mm. 

322 
385 
4S4 

488 
590 
7OO 

650 
750 
9OO 

Striated  Muscle  Longi 
and  Transections 
Nerve  Cells  in  Spinal 
Cord  
Goblet  Cells  of  Intes- 
tine, Mucicarmin 
Stain  

i  to  2.5  mm. 
i  to  2     mm. 

i  to  1.2  mm. 

8 
6 

4 

mm. 
mm. 

mm. 

640 
760 

1080 

940 
1  120 

1600 

1280 
1460 

2180 

Silvered  Endothelium 

0.2  to  0.4  mm. 

2 

mm. 

2600 

3820 

5080 

§  399a.  The  preparations  listed  in  the  above  table  are  simply  examples  of 
objects  which  can  be  shown  entire  with  the  different  objectives  without  oculars. 
In  practice  any  good  microscopic  preparation  and  many  living  things  can  be 
shown  with  the  projection  microscope. 

For  the  complete  understanding  of  any  specimen  it  is  necessary  to  see  it  as 
a  whole  and  then  by  using  higher  and  still  higher  powers  (§391)  to  get  views  of 
finer  and  finer  details. 

In  demonstrating  the  finer  details  one  can  show  but  a  very  small  specimen  or 
a  small  part  of  a  large  specimen.  For  large  specimens  it  is  a  great  advantage  to 
have  objectives  of  different  powers  on  a  revolving  nose-piece  so  that  it  takes 
only  a  moment  to  turn  from  one  to  the  other.  If  only  the  large  condenser  is 
used  (fig.  12 1 )  the  objective  remains  practically  stationary,  but  the  specimen 
must  be  on  a  movable  stage  so  that  it  can  be  farther  from  the  objective  or 
nearer  to  it  depending  upon  the  focal  length  of  the  objective  (fig.  132). 

If  one  uses  substage  condensers  the  stage  remains  stationary  and  a  long 
focus  substage  condenser  is  used  for  low  powers  and  a  short  one  for  high  powers 
and  the  objective  is  placed  at  approximately  its  focal  distance  from  the  object. 

It  must  be  remembered  that  many  living  things  are  soon  destroyed  by  the 
intense  light  necessary  for  projection."  While  the  circulation  of  the  blood  seems 
an  ideal  demonstration  with  the  projection  microscope  it  is  found  in  practise  to 
be  a  very  poor  way  to  demonstrate  it.  If  this  is  tried  the  microscope  in  a  ver- 
tical position  (fig.  147)  is  convenient.  The  screen  distance  should  not  be  very 
great  (3  to  5  meters,  10  to  16  ft.).  In  the  author's  experience  the  demonstra- 
tion of  blood  circulation  under  a  microscope  is  vastly  superior  to  anything  that 
can  be  done  with  a  projection  microscope. 


270  EXHIBITION  WITH  PROJECTION  MICROSCOPE       [Cn.  IX 

CONDUCT  OF  AN  EXHIBITION  OR  DEMONSTRATION  WITH  THE 
PROJECTION  MICROSCOPE 

§  400.  What  is  said  in  Ch.  I,  §  21-40  is  entirely  applicable  to 
the  projection  microscope  by  substituting  microscopic  specimens 
for  the  lantern  slides.  Only  from  the  greater  difficulty  and  pre- 
cision demanded  in  using  the  projection  microscope,  it  is  impera- 
tive that  the  operator  be  prepared,  hence  the  greater  necessity  of 
making  certain  that  everything  is  in  absolute  order  before  the 
lecture  begins. 

If  any  of  the  projection  objectives  (i.  e.,  those  of  125  to  20  mm. 
focus)  have  iris  diaphragms,  open  these  as  widely  as  possible. 
Never  try  to  project  with  the  iris  of  the  objective  partly  closed. 


FIG.   148.     SLIDE-TRAY  WITH  MASKED  PREPARATIONS  TO  BE  USED  ix 
PROJECTION.     (About  ^  size). 

Three  series  are  here  represented  on  different  sized  slides. 

The  sections  to  be  shown  are  not  covered  with  the  masking  paper.  The 
numeral  on  the  side  give  the  number  of  the  series  (ser.  90,  ser.  17,  ser.  15).  On 
each  slide  is  also  the  number  of  the  slide  in  the  series  as  ser.  15,  slide  57,  63,  67, 
etc. 


CH.  IX]       EXHIBITION  WITH  PROJECTION  MICROSCOPE          271 

An  experiment  with  the  iris  partly  closed  and  then  wide  open  will 
show  the  necessity  of  observing  this  rule. 

The  microscopic  slides  should  be  in  order  and  properly  masked 
(§384)  and  marked  in  some  way  so  that  the  operator  can  tell  which 
edge  up  they  should  be  placed  on  the  stage. 

It  is  also  a  great  advantage  to  have  marked  on  the  microscopic 
specimen  the  objective  or  objectives  that  should  be  used  in  pro- 
jecting it  to  bring  out  the  structural  details  which  it  is  desired  to 
show. 


FIG.  149.     SLIDE  Box  TO  HOLD  PREPARATIONS  FOR  DEMONSTRATION. 
(Cut  loaned  by  the  Spencer  Lens  Company). 

For  ease  in  getting  hold  of  the  slides  to  be  exhibited,  either  a 
shallow  tray  can  be  used  or  a  slide  box  (fig.  148,  149).  As  with 
lantern  slides,  it  is  advantageous  to  have  the  microscopic  specimens 
so  placed  that  they  can  be  grasped  easily,  and  put  on  the  stage  as 
desired  without  hesitation. 

Some  teachers,  including  the  senior  author,  have  found  it 
advantageous  to  manage  the  projection  themselves,  giving  the 
explanations  from  the  position  of  the  lantern. 

The  best  way  to  point  out  the  parts  in  the  screen  image  to  be 
especially  noted  is  to  have  a  slender  pointer  about  two  meters  (six 
feet)  long,  like  the  upper  two-thirds  of  a  bamboo  fishing  rod,  and 
to  hold  this  out  in  the  beam  of  light.  The  shadow  appears  on  the 
screen  sharply,  and  one  can  point  out  details  with  the  same  clear- 
ness as  by  using  a  pointer  on  the  screen.  It  is  easier  also,  because 
the  speaker  does  not  get  his  eyes  dazzled  by  looking  into  the  light 
beam,  as  so  often  happens  when  standing  near  the  screen  in  the 
usual  lecture  position. 

SPECIAL  DEMONSTRATIONS  WITH  HIGH  POWERS 
§  401.     Substage   condenser  in  projection. — As  indicated  in 
§  359  the  authors  of  this  book  believe  that  projection  for  large 
audiences  and  with  low  objectives  is  best  accomplished  without 


272  HIGH  POWER  MICRO-PROJECTION  [Cn.  IX 

substage  condensers,  and  without  oculars;  but  they  realize  that 
in  laboratory  work  and  for  some  special  lectures  to  small  classes  it 
is  of  the  highest  advantage  to  be  able  to  show  pictures  of  photo- 
graphic sharpness  in  all  details.  For  this  it  is  necessary  to  use,  first 
of  all,  a  substage  condenser  which  will  give  a  light  cone  of  sufficient 
aperture  for  the  details;  and  secondly  there  must  be  a  proper 
screen,  i.  e.,  the  screen  must  be  very  white  and  very  smooth,  but 
not  shiny  (§  409,  621).  White  cardboard  answers  well.  Finally 
there  must  be  an  ocular  used,  and  the  observers  must  be  near 
enough  the  screen  to  see  the  fine  points. 


FIG.  i5oA.     ACHROMATIC,  SUBSTAGE  CONDENSER  WITH 

CENTERING  SCREWS. 
(From  Zeiss'  Catalogue). 

There  has  been  a  segment  of  the  condenser  cut  away  to  show  the  construc- 
tion. 

The  centering  screws  (c-s,  c-s)  enable  the  operator  to  get  the  condenser  in 
the  optic  axis  of  the  microscope.  The  iris  diaphragm  for  this  condenser  is 
between  the  lower  and  middle  combinations,  not  below  the  condenser  as  with 
the  Abbe  form. 

This  form  of  condenser  is  especially  desirable  for  projection  and  for  photo- 
micrography. 

The  substage  condenser  for  micro-projection  must  either  be  of  a 
special  form  to  use  with  the  main  condenser  of  the  apparatus  or 
special  means  must  be  employed  to  utilize  the  light  cone  from  the 
main  condenser  when  the  ordinary  substage  condenser  is  used. 

This  is  because  the  substage  condenser  ordinarily  used  on  micro- 
scopes is  designed  for  approximately  parallel  beams  of  light,  not 
for  those  markedly  converging  or  diverging.  By  examining  the 
figures  of  the  light  cone  from  the  main  condenser  it  will  be  seen 


CH.  IX] 


HIGH  POWER  MICRO-PROJECTION 


273 


that  the  cone  of  light  is  converging  to  the  focal  point  and  diverging 
beyond  that  point  (fig.  122,  132  and  320-323).  If  the  converging 
cone  is  used  the  substage  condenser  brings  it  to  a  focus  too  soon 
and  if  the  diverging  cone,  then  the  substage  condenser  brings  it 
to  a  focus  too  far  beyond  it. 

§  402.  Methods  of  rendering  converging  or  diverging  light 
parallel. — There  are  two  principal  ways  of  utilizing  the  light  cone 
from  the  main  condenser. 


Objective 


Objective 


FIG.    1506.     ABBE    SUBSTAGE    CONDENSER    SHOWING    PARALLEL    AND 
CONVERGING  INCIDENT  LIGHT. 

In  this  form  of  condenser  the  iris  diaphragm  is  below  both  condenser  lenses 
(compare  fig.  150). 

With  parallel,  incident  light  the  condenser  focuses  the  light  just  above  the 
condenser,  with  converging  light  the  focus  is  within  the  upper  lens  and  the 
light  is  diverging  on  leaving  the  upper  lens. 

o,  o     Object. 

Objective    The  front  lens  of  the  projection  objective. 

A.  Rendering  the  converging  cone  of  light  approximately 
parallel  by  means  of  a  concave  lens.  As  it  is  desirable  to  use  all  the 
light  in  the  cone,  the  concave  lens  is  put  in  the  cone  where  its 
diameter  is  slightly  less  than  the  diameter  of  the  substage  con- 
denser, that  is  about  25  mm.  (i  in.).  The  trial  glasses  used  by  the 
oculist  are  excellent  for  the  purpose.  A  fork  with  stem  is  desirable, 
and  this  is  placed  in  the  socket  for  the  mirror  stem.  This  brings 
the  fork  carrying  the  spectacle  lens  near  the  substage  condenser. 
Concave  spectacle  lenses  of  10  to  20  diopters  (100  to  50  mm.,  4  to  2 
in.  focus)  have  been  found  excellent.  The  microscope  for  projec- 
tion is  so  placed  that  the  fork  carrying  the  concave  lens  is  about 


274  HIGH  POWER  MICRO-PROJECTION  [CH.  IX 

2>£  to  3  cm.  (i  to  i>2  in.)  from  the  focus  of  the  converging  cone. 
The  concave  lens  will  render  the  converging  light  approximately 
parallel,  and  this  cylinder  of  light  is  small  enough  to  enter  the 
substage  condenser.  By  a  small  manipulation  of  the  screw  of  the 
substage  condenser  bringing  it  slightly  nearer  the  specimen  or 
slightly  farther  from  it  the  most  brilliant  screen  image  can  be  pro- 
duced. A  slight  change  in  the  position  of  the  substage  condenser 
often  works  wonders. 

Objective  ft  Back  of  C         Objective 

Objective 


FIG.  151.  RELATION  OF  THE  APERTURE  OF  THE  LIGHT  FROM  THE  CON- 
DENSER TO  THE  APERTURE  OF  THE  OBJECTIVE. 

(From  Nelson,  Jour.  Roy.  Micr.  Soc.). 

A  The  cone  of  light  from  the  condenser  just  fills  the  aperture  of  the  objec- 
tive (B). 

B     Back  lens  of  the  objective  entirely  filled  with  light. 

C  The  cone  of  light  from  the  condenser  is  not  great  enough  to  fill  the  aper- 
ture of  the  objective  (D). 

D     Back  lens  of  the  objective  lighted  by  the  condenser  (C). 

The  dark  ring  shows  the  aperture  of  the  objective  not  lighted  by  the  con- 
denser. 

B.  Rendering  the  diverging  cone  of  light  approximately  parallel 
by  the  use  of  a  convex  lens.  If  a  convex  lens  is  placed  in  the  path 
of  the  diverging  cone  at  its  focal  distance  from  the  focus  of  the  main 
condenser,  the  light  will  be  rendered  parallel.  In  order  to  have  a 
cylinder  of  light  of  the  right  size  to  enter  the  substage  condenser  a 
convex  lens  of  the  proper  focal  length  and  diameter  must  be  used. 
Trial  lenses  are  excellent.  .Those  of  10  and  20  diopters  (100  and 
50  mm.,  4  to  2  in.  focus)  are  excellent  for  the  main  condenser  with 
a  focus  of  150  to  200  mm.  (6  to  8  in.).  The  microscope  must  be 
put  in  such  a  position  that  the  trial  lens  in  the  fork  before  the  sub- 
stage  condenser  shall  be  at  its  focal  distance  from  the  focus  of  the 
main  condenser.  The  diverging  cone  of  light  will  be  made  approxi- 


CH.  IX] 


HIGH  POWER  MICRO-PROJECTION 


275 


mately  parallel  (fig.  1536),  and  by  slight  adjustments  of  the  sub- 
stage  condenser  brilliant  images  are  produced. 


Condeni  Iser 


Radiant 


FIG.  152.     MICROSCOPE  FOR  PROJECTION  AND  FOR  DRAWING. 

W  —  i     The  negative  supply  wire  from  the  outlet  box  (fig.  3). 

W  +  i     The  positive  supply  wire  from  the  outlet  box. 

.S     Double-pole,  knife  switch. 

W  —  2    Wire  from  the  switch  to  the  binding  post  of  the  lower  carbon. 

W  +  2     Wire  from  the  knife  switch  to  the  rheostat. 

W  +  j     Wire  from  the  rheostat  to  the  upper  carbon  (+  H  —  C). 

ri,  r2     The  two  binding  posts  of  the  rheostat. 

Rheostat    The  controlling  device  for  the  current. 

ILC    Incandescent  lamp  cord. 

Inc.  Lamp     The  incandescent  lamp  with  a  wire  lamp  guard. 

This  lamp  is  for  use  in  working  about  the  projection  apparatus.  It  is  con- 
nected to  the  supply  wires  at  their  connection  with  the  switch  so  that  the 
incandescent  lamp  will  burn  whether  the  knife  switch  is  open  or  closed  (see  also 
fig.  2,  4). 

Radiant    The  arc  lamp. 

S+,  -5 —    The  set  screws  for  the  carbons. 

HC,  VC    The  horizontal  or  upper  and  the  vertical  or  lower  carbons. 

Condenser  The  triple-lens  condenser  with  water-cell  in  the  parallel  beam 
between  the  two  plano-convex  lenses. 

Axis,  Axis    The  optic  axis  of  the  condenser  and  the  microscope. 

Substage  Condenser  The  achromatic  condenser  under  the  stage  of  the 
microscope. 

P  L  The  concave  lens  for  making  parallel  the  converging  light  from  the 
large  condenser  before  it  enters  the  substage  condenser. 

St     Stage  of  the  microscope. 

Objective     The  projection  objective. 

Ocular    The  ocular  of  the  microscope  used  in  projection. 

M2  The  mirror  or  prism  placed  just  beyond  the  ocular  when  it  is  desired 
to  reflect  the  light  downward. 

Screen  Image  The  image  projected  upon  the  white  screen  by  the  projection 
microscope. 


276 


HIGH  POWER  MICRO-PROJECTION 


[CH.  IX 


Bl.  R     The  block  carrying  the  radiant  on  the  optical  bench. 

Bl  C    The  block  carrying  the  condenser  on  the  optical  bench. 

Bl  M    The  block  carrying  the  microscope  on  the  optical  bench. 

Base  Board  The  board  bearing  the  track  made  of  rods  and  serving  as  an 
optical  bench. 

Projection  Table  The  table  supporting  the  apparatus  and  holding  it  at  the 
proper  height  for  use. 

The  above  method  refers  especially  to  high  powers — objectives 
of  2  to  8  mm.  equivalent  focus.  For  powers  lower  than  those  just 
mentioned  one  can  get  better  results  by  the  use  of  a  main  condenser 
with  a  second  element  of  200  to  150  mm.  focus  and  no  substage 
condenser,  or  by  adopting  the  method  given  below  or  in  §  403 . 


Substage 
Condenser 


FIG.  153.     DIAGRAMS  TO  SHOW  METHODS  OF  PARALLELIZING  THE  CONE 
OF  LIGHT  FROM  THE  MAIN  CONDENSER. 

A  Method  of  parallelizing  the  converging  cone  of  light  from  the  main 
condenser  by  means  of  a  concave  lens  within  the  focus  (/). 

B  Method  of  parallelizing  the  diverging  cone  of  light  from  the  main  con- 
denser by  means  of  a  convex  lens  beyond  the  focus  (/). 

Arc  Supply     The  right-angled  carbons  of  the  arc  lamp. 

LT  L2     The  first  and  second  elements  of  the  triple,  main  condenser. 

Water  Cell    This  is  to  remove  the  radiant  heat. 

Axis    The  principal  axis  on  which  all  the  parts  are  centered. 

/    The  principal  focus  of  the  second  element  of  the  main  condenser. 

P.  L.     Parallelizing  lens.     Concave  in  A,  Convex  in  B. 

Substage  Condenser  This  is  the  first  or  lowest  element  of  the  substage  con- 
denser of  the  achromatic  form  (fig.  i5oA).  See  also  fig.  150  B.  for  the  Abbe 
form  of  substage  condenser. 


CH.  IX] 


HIGH  POWER  MICRO-PROJECTION 


277 


Finally  if  one  uses  a  main  condenser  with  a  focus  of  30  or  38  cm. 
(i 2  to  15  in.)  excellent  results  can  be  obtained  with  all  powers  (16 
to  2  mm.)  by  so  placing  the  microscope  that  the  converging  cone  of 
the  main  condenser  shall  enter  the  substage  condenser  at  a  point 
where  the  light  cone  is  of  about  the  diameter  of  the  substage  con- 
denser (fig.  I54A-B).  It  may  be  necessary  to  raise  or  lower  the 
substage  condenser  slightly  to  obtain  the  most  brilliant  screen 
image. 

Fair  results  can  also  be  obtained  in  this  way  by  using  main  con- 
densers of  15,  20  and  25  cm.  (6,  8,  10  in.)  focus,  but  much  more 


FIG.  154. 


DIAGRAMS  TO  SHOW  THE  POSITION  OF  THE  SUBSTAGE  CONDENSER 
WHEN  NO  PARALLELIZING  LENS  is  USED. 


A  The  substage  condenser  is  within  the  focus  (/)  at  a  point  where  the  long, 
light  cone  is  of  about  the  same  diameter  as  the  substage  condenser. 

B  The  substage  condenser  is  beyond  the  focus  (£)  of  the  long  focus  main 
condenser,  at  a  point  where  the  diverging  cone  is  of  about  the  same  diameter  as 
the  substage  condenser.  This  is  the  better  position  for  the  substage  condenser 
of  the  ordinary  microscope. 

Arc  Supply    The  right-angled  carbons  of  the  arc  lamp. 

Lj  L2     The  first  and  the  second  elements  of  the  main  condenser. 

Water  Cell     This  is  to  remove  the  radiant  heat. 

Axis    The  principal  axis  on  which  all  the  parts  are  centered. 

/  The  principal  focus  of  the  second  element  of  the  main  condenser.  In  both 
cases  the  focus  is  long  as  compared  with  fig.  153. 

Substage  Condenser  This  is  the  first  or  lowest  element  of  the  substage  con- 
denser. It  is  of  the  achromatic  type  (fig.  150  A).  See  figure  150  B  for 
the  Abbe  form  of  substage  condenser  with  parallel  and  with  converging  light. 


278  HIGH  POWER  MICRO-PROJECTION  [Cn.  IX 

brilliant  pictures  can  be  produced  by  using  also  a  parallelizing  lens 
as  indicated  in  §  402  A. 

If  one  has  an  optic  bench  apparatus  (fig.  121,  158,  159)  one  can 
get  good  results  with  the  condensers  of  all  foci  by  placing  the 
microscope  so  that  a  diverging  cone  of  light  enters  the  substage 
condenser  (fig.  1546).  It  will  then  be  necessary  to  lower  the 
substage  condenser  slightly  for  the  higher  powers. 

§  403.    Kohler  method  of  using  the  substage  condenser. — The 

general  principle  is  shown  in  fig.  170.  The  microscope  is  moved 
toward  the  main  condenser  until  the  focus  is  at  the  iris  diaphragm. 
One  can  tell  when  the  main  condenser  is  focused  on  the  iris  dia- 
phragm in  the  same  way  as  that  in  focusing  on  the  black  hood  of  the 
objective  (§  375)  viz.,  by  noting  when  the  image  of  the  crater  and 
the  tip  of  the  lower  carbon  appear  on  the  iris.  After  the  image  is 
focused  on  the  iris  diaphragm  the  iris  is  opened  to  admit  the  cone 
of  light,  and  the  substage  condenser  is  raised  or  lowered  slightly  to 
get  the  most  brilliant  light.  As  one  can  see  by  the  diagrams  of 
light  cones  and  the  plates  of  the  light  rays  and  the  light  cones,  the 
light  is  diverging  beyond  the  focus  so  that  diverging  and  not 
parallel  light  enters  the  substage  condenser.  As  the  condenser 
cannot  focus  diverging  light  at  the  same  level  that  it  would  focus 
parallel  light  it  may  be  necessary  to  lower  the  substage  condenser 
somewhat  to  get  the  most  brilliant  image  with  high  powers.  Fur- 
thermore, if  a  concave  lens  of  10  to  20  diopters  is  put  in  the  fork  as 
described  in  §  402  A  the  image  will  be  markedly  brighter  unless  a 
very  long  focus  main  condenser  is  used  (fig.  171).  (See  also  Ch. 
XIV,  §  864). 

§  404.  Aperture  of  the  substage  condenser. — The  purpose  of 
the  substage  condenser  in  projection,  as  in  direct  observation 
with  the  microscope,  is  to  increase  the  aperture  of  the  illuminating 
cone.  And  as  it  is  now  one  of  the  fundamental  doctrines,  that  the 
resolution  or  making  visible  of  minute  details  depends  directly 
upon  the  aperture  of  the  objective  used,  naturally  as  much  as 
possible  of  the  aperture  of  the  objective  is  employed.  For  this, 
the  substage  condenser  diaphragm  should  be  wide  open,  so  that  the 


CH.  TXi 


HIGH  POWER  MICRO-PROJECTION 


279 


FIG.  155. 


THE  EFFECT  OF  USING  AN  IRIS  DIAPHRAGM  IN  THE  CONE  OF 
LIGHT  FROM  THE  MAIN  CONDENSER. 


.  The  second  element  of  the  condenser  is  shown  at  the  top.  The  focus'of J;he 
cone  of  light  from  the  condenser  is  shown  at  F,  the  axis  by  A. 

B  At  the  right  are  shown  in  millimeters,  three  diameters  of  the  cone  of  light 
with  three  different  openings  of  the  iris  diaphragm  (22,  33,  44  mm.) 

C  At  the  left  are  shown  the  apertures  corresponding  with  these  openings 
in  the  iris  diaphragm  (23°,  34°,  45°).  The  aperture  of  these  openings  is  also 
shown  above  the  circles. 

One  can  see  by  this  diagram  what  an  enormous  amount  of  light  is  lost  by 
making  the  illuminating  cone  smaller. 


280  HIGH  POWER  MICRO-PROJECTION  [Cn.  IX 

entire  beam  of  light  from  the  lamp  condenser  may  enter.  Then, 
just  as  in  ordinary  observation,  one  can  often  make  the  contrast 
more  striking  by  cutting  down  the  aperture  somewhat  by  closing 
more  or  less  the  substage  condenser  diaphragm.  It  must  not  be 
cut  down  too  much,  for  that  will  render  the  image  dim  and  defeat 
the  very  purpose  of  the  substage  condenser. 

As  a  general  statement,  much  more  of  the  aperture  of  the 
objective  can  be  used  in  projection  than  in  ordinary  direct  observa- 
tion in  the  microscope.  Naturally,  objectives  of  relatively  large 
aperture  give  the  more  brilliant  images  (see  §  855). 

§  405.  Oculars  to  use  in  projection. — Generally  speaking,  only 
low  powers  are  used  (x2,  X4,  x8).  The  lower  the  power  the  more 
brilliant  the  image.  Compensation  oculars  have  been  found  better 
than  the  Huygenian.  A  compensation  ocular  as  high  as  xi2  gives 
brilliant  images  for  short  screen  distances. 

One  should  not  forget  that  the  ocular,  when  used  in  projection, 
is  really  a  second  projection  system,  and  hence  the  image  will  be 
erect  on  the  screen  (fig.  207). 


§  404a.  Centering  the  substage  condenser. — As  the  substage  condenser 
becomes  one  of  the  optical  elements  in  projection,  its  principal  optic  axis  must 
be  centered  on  the  common  axis  of  the  entire  apparatus. 

It  is  assumed  that  the  microscope  without  the  substage  condenser  has  been 
properly  centered  as  directed  in  §  374-375. 

To  center  the  substage  condenser,  use  the  ocular  and  objective  (x4  ocular, 
8,  10  or  1 6  mm.  objective),  remove  the  bellows  if  present  (fig.  133),  place  a  piece 
of  white  cardboard  at  about  45  degrees  as  shown  in  fig.  1 16,  between  the  large 
condenser  and  the  substage  condenser,  and  light  the  cardboard  well  with  a 
mazda  lamp.  This  will  give  the  light  for  the  microscope. 

Now  put  a  preparation  on  the  stage  and  focus  the  microscope  as  for  ordinary 
observation.  Remove  the  specimen  and  close  the  substage  iris  diaphragm 
nearly  up.  With  a  pocket  magnifier  examine  the  eye-point  or  Ramsden's  disc 
(fig.  127  E  P)  beyond  the  ocular.  This  disc  of  light  appears  as  if  on  the  back 
lens  of  the  objective.  If  the  iris  is  properly  made  and  the  substage  condenser 
is  centered  with  the  objective  and  ocular,  the  center  of  light  will  appear  to  be 
exactly  in  the  middle  of  the  back  lens  of  the  objective  (fig.  151).  If  the  sub- 
stage  is  not  in  the  optic  axis  then  the  disc  of  light  will  appear  eccentric;  and 
if  the  substage  condenser  is  markedly  off  the  center  the  spot  of  light  will  make 
a  break  in  the  black  ring  on  one  side  as  shown  in  fig.  30,  1-4.  If  it  is  only 
slightly  off  center,  the  disc  of  light  will  seem  to  be  surrounded  by  a  dark  ring 
of  unequal  width.  If  the  substage  condenser  is  not  found  to  be  correctly 
centered,  the  centering  screws  (fig.  150)  must  be  used  to  move  it  slightly  until 
the  disc  of  light  is  central  as  shown  in  fig.  151. 

The  Abbe  condenser  found  on  most  microscopes  has  no  centering  screws. 
The  makers  center  the  instrument  carefully  and  fix  it  in  position.  If  it  is 
found  badly  out  of  center  it  is  best  to  return  it  to  the  makers  for  adjustment. 


CH.  IX] 


HIGH  POWER  MICRO-PROJECTION 


281 


§  406.    Range  of  objectives  to  use  with  a  substage  condenser. — 

Objectives  of  16,  12,  10,  8,  6,  4,  3,  and  2  mm.  equivalent  focus 
are  used  with  the  substage  condenser.  For  objectives  of  longer 
focus  than  16  the  substage  condenser  of  the  ordinary  form  is 
rarely  used.  Either  a  special  long  focus  substage  condenser  is  used 
or  the  ordinary  one  is  turned  aside  and  the  cone  of  light  from  the 
large  condenser  used  as  directed  above  (§  376). 

§  407.     Change  in  position  of  the  substage  condenser  for  differ- 
ent objectives  and  thickness  of  slides. — For  the  highest  powers 


FIG.  156.     PROJECTION  MICROSCOPE  OF  ZEISS. 

(From  the  4th  edition  (1899)  of  Zeiss1  catalogue  of  instruments  and  appliances 
for  Photo- Micrography  and  Projection). 

This  projection  apparatus,  which  in  its  main  features  was  described  in  Zeiss 
microscope  catalogue  No.  28,  (1889),  and  No.  29  (1891),  consists  of  an 
optical  bench  on  which  all  of  the  parts  needed  move  separately  so  that  any 
desired  arrangement  can  be  made  for  projection  of  large  objects  with  low  power 
or  smaller  objects  with  high  powers. 

Commencing  at  the  right: 

1  Arc  lamp  with  inclined  carbons,  and  with  fine  adjustments  to  center  the 
source  of  light  (crater  of  the  positive  carbon). 

2  First  element  of  the  condenser  consisting  of  a  meniscus  and  a  plano- 
convex lens,  to  render  the  light  beam  parallel. 

3  Water-cell. 

4  Second  element  of  the  condenser  to  converge  the  light-beam. 

5  Iris  diaphragm  to  cut  down  the  light-cone  if  desirable. 

6  Stage  and  substage  condenser. 

7  Projection  objective  and  fine  focusing  device.     In  the  figure  no  ocular 
is  used. 

This  arrangement  of  the  parts  enables  the  user  to  employ  a  microscope  with 
oculars  or  amplifiers,  or  the  simple  apparatus  here  shown,  or  photographic 
objectives. 


282 


HIGH  POWER  MICRO-PROJECTION 


[Cn.  IX 


(2-3  mm.  oil  or  water  immersion)  and  for  the  3  and  4  mm.  dry 
objectives  the  condenser  is  usually  very  close  up  to  the  slide,  so 
that  the  object  is  practically  in  the  focus  of  the  beam  of  light. 

For  the  8,  10,  12,  and  16  mm.  objectives  the  substage  condenser 
must  be  separated  sufficiently  from  the  specimen  to  light  the  whole 
field. 

It  will  be  found  in  practice  that  one  must  be  more  precise  in 
keeping  the  substage  condenser  at  just  the  right  level  for  projec- 
tion than  for  ordinary  direct  microscopic  observation.  Hence,  it 
will  be  found  that  for  a  thin  slide  the  condenser,  even  for  high 
powers,  may  need  to  be  separated  slightly  from  the  object,  while  if 
the  slide  on  which  the  specimen  is  mounted  is  thick,  the  condenser 
may  need  to  be  as  close  to  it  as  possible. 

§  408.  Screen  distance  for  high  power  projection. — This 
should  not  be  excessive,  for  even  in  the  darkest  room  the  image  will 


FIG.  157.     LEWIS  WRIGHT'S  PROJECTION  MICROSCOPE. 

(From  Wright's  Optical  Projection). 
C    Condenser  of  three  plano-convex  lenses. 
A     Alum  cell  for  absorbing  radiant  heat. 

P     Plano-concave  lens  of  highly  dispersive  glass  to  aid  in  correcting  the 
aberrations  of  the  condenser  and  to  render  the  light  parallel. 
S  C    Substage  condenser.     For  low  powers  but  one  lens  is  used. 
S    Stage. 

O     Object  and  objective. 
A  M    Amplifier. 
F    Fine  focusing  adjustment. 
RZ     Rack  and  pinion,  coarse  focusing  adjustment. 
Rt     Coarse  adjustment  for  the  substage  condenser. 


CH.  IXl  HIGH  POWER  MICRO-PROJECTION  283 

be  too  dim  if  the  screen  distance  is  over  two  or  three  meters  (6  to  10 
feet). 

With  objectives  of  4,  6,  8,  10  mm.  and  lower  powers,  one  can 
use  a  greater  distance  with  satisfaction,  but  for  the  oil  and  water 
immersions,  a  distance  of  one  to  two  meters  (3  to  7  feet)  gives  the 
best  results.  This,  of  course,  refers  to  minute  details.  If  one 
simply  wants  size,  the  limit  is  much  greater;  but  that  is  not 
scientific  projection. 

§  409.  Kind  of  screen  for  high  power  projection. — The  prin- 
ciple enunciated  by  Goring  and  Pritchard  must  be  kept  in  mind. 
The  whiter  and  smoother  the  screen,  the  more  brilliant  the  image 
and  the  clearer  the  details.  Nothing  has  been  found  better  by 
the  writers  than  smooth,  white  bristolboard.  This  is  also  very 
easily  procured,  and  when  it  becomes  dirty  or  discolored,  it  can  be 
cheaply  replaced.  We  have  also  found  white  cardboard  in  sheets 
of  71x112  cm.  (28  x  44  in.)  good. 

§  410.  Specimens  to  project  with  high  powers. — These  must 
have  in  a  good  degree  the  qualities  of  specimens  giving  clear  images 
to  the  eye  in  direct,  microscopic  observation.  That  is,  they  should 
have  definite  outlines  and  contrasting  colors;  for  example,  well 
stained  preparations  of  red  and  white  blood  corpuscles  mounted  in 
balsam  and  projected  with  the  oil  immersion  objective. 

Preparations  of  bacteria,  well  stained  and  mounted  in  balsam, 
may  be  projected  with  the  oil  immersion. 

Thin  histologic  and  embryologic  sections,  if  well  stained  and 
mounted  in  balsam,  answer  well.  The  nuclei  of  cells  show  well, 
also  the  band  of  cilia  in  a  ciliated  epithelium,  and  the  cells  in 
mitotic  division.  Naturally,  well  prepared  plant  preparations 
have  the  advantage  of  very  sharp  outlines. 

§  411.  High  powers  with  the  vertical  microscope. — Any  prep- 
aration which  can  be  projected  well  with  high  powers  may  be  used 
on  the  vertical  microscope  (§  397).  Of  course,  there  is  some  loss 
of  light  in  the  double  reflection  required  (fig.  147,  176),  but  if  the 
screen  is  within  two  meters  (6  ft.)  distance  and  the  observers  few 
and  close,  results  are  fairly  satisfactory.  For  example,  if  one  has 


284  USE  OF  ALTERNATING  CURRENT  [Cn.  IX 

water  in  which  there  are  many  large  bacteria  and  infusoria,  a  most 
striking  picture  on  the  screen  is  made.  For  this  projection  a  water 
immersion  is  excellent.  An  oil  immersion  may  also  be  used  and 
also  a  dry  objective  of  4  to  6  mm. 

For  securing  a  large  field,  the  objective  and  amplifier  are  better 
than  an  objective  and  ocular  (§355). 

USE  OF  ALTERNATING  ELECTRIC  CURRENT  WITH  THE  PROJECTION 

MICROSCOPE 

§  412.  It  is  unfortunate  that  it  should  ever  be  necessary  to  use 
alternating  current  in  micro-projection ;  but  if  that  is  all  which  can 
be  obtained,  much  can  be  accomplished  with  it  by  skillful  handling. 

(For  a  discussion  of  the  difference  between  direct  and  alternating 
current  and  the  relative  amount  of  light  yielded  by  the  two,  also 
for  the  possibility  of  getting  direct  from  alternating  current  by 
means  of  a  motor-generator  set,  or  by  a  ' 'current  rectifier,"  see  Ch. 

xui,  §  681-683, 751-752). 

§  413.  Wiring  the  Arc  Lamp. — This  is  exactly  as  for  the  magic 
lantern,  (fig.  3).  And  as  with  all  arc  lamp  work  there  must  always 
be  present  some  form  of  regulating  device  like  a  rheostat  or  induc- 
tor (fig.  145,  197,  §  748). 

§  414.  Arrangement  of  the  carbons. — For  micro-projection  the 
carbons  should  always  be  at  right  angles,  and  the  light  will  then  be 
almost  wholly  from  the  upper  or  horizontal  carbon  (fig.  191).  As 
this  is  in  the  optic  axis  and  looks  directly  toward  the  condenser  it 
is  the  most  satisfactory  source  of  light  available  with  this  as  with 
the  direct  current  lamp  for  micro-projection.  This  is  because  the 
image  of  the  crater  of  one  carbon  is  as  large  as  can  be  received  by 
the  projection  objective. 

It  is  especially  necessary  for  micro-projection  that  the  lamp  have 
fine  adjustments  to  keep  the  crater  exactly  centered  (fig.  3,  146). 

§  415.  Amount  of  current  necessary. — As  the  alternating 
current  gives  less  than  one-third  as  much  available  light  as  the 
direct  current  one  cannot  project  with  such  high  powers  nor  pro- 
duce so  large  screen  images  as  with  the  direct  current  (fig.  302). 


CH.  IX]      MICRO-PROJECTION  WITH  HOUSE  CURRENT  285 

For  example,  with  direct  current  of  10  amperes  one  can  accom- 
plish a  great  deal  in  micro-projection  if  the  manipulation  is  skillful. 
To  get  equally  brilliant  results  with  alternating  current  would 
require  30  to  40  amperes  of  current.  The  heating  is  also  excessive 
with  the  high  amperages.  (See  Ch.  XIII,  §  768). 

If  alternating  current  must  be  used  for  projection  with  the  micro- 
scope, one  should  not  expect  too  much,  but  get  as  good  results  as 
possible  by  observing  carefully  the  conditions  giving  good  screen 
images,  viz.,  apparatus  in  perfect  order  and  alignment  on  one  axis; 
a  good  screen  and  a  dark  room. 

It  is  not  wise,  according  to  our  experience,  to  try  to  use  more  than 
25  amperes  alternating  current  for  micro-projection,  and  it  is  better 
as  regards  the  specimens  and  apparatus,  to  be  satisfied  with  the 
results  which  can  be  obtained  with  15  to  20  amperes.  An  arc 
lamp  with  carbons  at  right  angles  is  to  be  preferred. 

§  416.  Centering  the  apparatus  on  one  axis,  separating  the 
elements  properly  and  the  conduct  of  an  exhibition  are  precisely 
as  for  the  direct  current  light.  The  results,  however,  cannot  be 
made  as  satisfactory,  although,  as  stated  above  (§  412),  by  care  and 
skill  much  can  be  accomplished. 

THE  PROJECTION  MICROSCOPE  ON  THE  HOUSE  ELECTRIC  LIGHTING 

SYSTEM 

§  417.  As  with  the  magic  lantern  (§  127),  the  small  electric 
current  (4  to  6  amperes)  available  from  the  regular  house  lighting 
system  gives  very  gratifying  results. 

Small  carbons  (6-8  mm.  diam.)  are  employed  and  either  one 
of  the  small  arc  lamps  especially  designed  for  the  purpose  or  an 
ordinary  arc  lamp  with  adapters  or  bushings  can  be  used. 

Of  course  the  direct  current  is  much  more  effective,  but  even  with 
the  alternating  current,  which  is  now  so  common  in  lighting  sys- 
tems, successful  projection  with  the  microscope  can  be  done. 

The  small  carbons  form  a  minute  crater,  and  thus  approximate 
closely  to  a  point  source  of  light,  which  is  the  ideally  perfect  source 
from  the  optical  standpoint.  From  our  experience  this  is  a 


286  MICRO-PROJECTION  WITH  SUNLIGHT  [Cn.  IX 

better  source  of  light  for  the  microscope  than  the  lime  light,  and 
now  electric  lighting  is  so  common  that  one  can  use  almost  any 
room  in  a  house  or  laboratory  at  night  for  a  projection  room. 

Of  course  one  should  not  expect  too  much,  but  for  small  audiences 
— 50  to  100 —  and  with  a  moderate  sized  screen — 2-3  meters — 
(6-10  ft.)  astonishingly  satisfactory  micro-projection  can  be  done. 

§  418.    Hand-feed  and  automatic  lamps  for  small  currents.— 

Most  of  the  small  current  lamps  are  of  the  hand-feed  type  whatever 
the  form  of  the  electric  current  (a.  c.  or  d.  c.)  but  some  automatic 
ones  have  been  constructed  (fig.  44,  205).  Large  arc  lamps  may, 
by  special  arrangement,  be  so  adjusted  that  they  give  good  results 
automatically  from  5  to  25  amperes  (e.  g.  the  automatic  lamp  of 
A.  T.  Thompson  and  of  the  Bausch  and  Lomb  Optical  Co.,  fig. 
186,  187). 

As  for  the  usual  lantern  arc  lamps,  only  those  for  the  direct 
current  have  hitherto  been  constructed  of  the  automatic  form. 

For  a  full  discussion  of  the  wiring  and  setting  up  of  the  apparatus 
see  Ch.  Ill  and  XIII,  §  128  and  fig.  3,  40,  45. 

Do  not  forget  that  a  rheostat  or  ballast  of  some  kind  must  be 
used  on  every  outfit  where  an  arc  lamp  is  employed  (§  129,  748). 

Remember  the  precautions  for  turning  on  and  off  the  current 
when  using  the  house  circuit  (§  133).  For  a  further  use  of  these 
small  currents  in  drawing,  see  Ch.  X,  §  486. 

MICRO-PROJECTION  WITH  SUNLIGHT 

§  419.  This  was  the  first  light  used  for  micro-projection  and 
remains  the  best.  If  it  were  only  available  at  all  times  it  would 
be  universally  employed. 

§  420.    Arrangement  of  the  parts  of  the  apparatus. — For  the 

heliostat  to  keep  the  sunlight  in  a  constant  position  one  should 
consult  Chapter  VI. 

After  getting  parallel  light  from  the  sun  in  a  constant  position, 
then  one  should  use  the  proper  condenser  (fig.  74) .  The  remainder 
of  the  apparatus  is  precisely  as  for  the  projection  so  far  discussed 
and  all  the  requirements  of  centering  and  arranging  at  the  proper 


CH.  IX]  MICRO-PROJECTION  WITH  LIME  LIGHT  287 

distance  from  one  another  are  as  for  the  electric  light  described 
above. 

As  the  spot  of  light  must  remain  in  exactly  the  same  place  to  be 
received  by  the  small  lenses  of  the  projection  objective,  it  is  neces- 
sary to  regulate  the  hand  heliostats  oftener  than  for  the  magic 
lantern. 

It  may  also  be  necessary  to  make  slight  corrections  in  the  mirror 
of  the  clock-driven  heliostat  from  time  to  time.  The  law  is :  The 
axial  ray  must  correspond  with  the  optic  axis  of  the  apparatus. 

§  421.  Use  of  a  water-cell. — The  radiant  energy  of  the  sun  is 
so  great  that  a  water-cell  to  remove  as  much  of  it  as  possible  except 
the  luminous  part  (§  844)  is  as  desirable  as  with  the  electric  light. 
It  is  also  desirable  to  have  a  specimen  cooler  (fig.  121). 

PROJECTION  MICROSCOPE  WITH  THE  LIME  LIGHT 

§  422.  The  management  of  the  lime  light  for  the  projection 
microscope  is  exactly  as  for  the  magic  lantern  (see  §163,  164),  only 
more  attention  will  be  necessary  to  keep  the  best  possible  light  all 
the  time.  The  image  of  the  luminous  spot  should  be  focused  on 
the  hood  of  the  objective  as  for  the  electric  arc.  While  there  is 
not  so  much  danger  from  overheating  as  with  the  electric  light  or 
sunlight,  it  is  desirable  to  use  a  large  water-cell.  The  stage  cooler  is 
also  an  advantage.  For  the  correct  form  of  a  condenser  see  §  363. 

As  the  intrinsic  brilliancy  of  the  lime  light  is  less  than  that  of 
sunlight  or  the  electric  light  one  must  not  expect  so  much  of  it  as 
of  them. 

§  423.  Other  sources  of  light  are  insufficient  to  give  good 
micro-projection  except  in  a  very  limited  degree,  and  for  some 
special  purposes.  See  under  drawing,  Ch.  X,  §  463. 

HOME-MADE  PROJECTION  APPARATUS 

§  424.  Projection  table. — For  all  kinds  of  projection  the  table 
should  be  of  convenient  height,  so  that  the  operator  can  stand  dur- 
ing the  exhibition.  A  height  of  100  centimeters  (40  inches)  is 
suitable  for  most  persons.  The  size  of  the  top  varies  greatly  with 


288 


HOME-MADE  PROJECTION  APPARATUS 


[CH.  IX 


the  work  to  be  done.  For  the  work  of  micro-projection,  drawing, 
etc.,  contemplated  in  this  and  the  following  chapter  a  table  of  the 
following  dimensions  has  served  admirably:  Height,  100  centi- 
meters (40  in.).  Size  of  top  125  cm.  (50  in.)  long;  50  cm.  (20  in.) 
wide.  The  legs  are  about  5  cm.  (2  in.)  square,  and  have  large 
screw  eyes  in  the  lower  ends  for  leveling.  The  table  should  be 


FIG.  158.     HOME-MADE  OPTICAL  BENCH. 

1 1 1 1     The  track  of  rods  or  tubes  on  the  baseboard. 

Radiant    The  block  carrying  the  arc  lamp. 

as  Asbestos  paper  between  the  track  rods  at  the  arc  lamp  end  of  the  optical 
bench. 

Condenser     The  block  carrying  the  condenser. 

Stage  The  block  carrying  the  stage  of  the  projection  apparatus  or  the  lan- 
tern-slide holder. 

Microscope  The  block  carrying  the  projection  microscope  or  the  lantern 
slide  or  other  projection  objective. 

////    The  railing  flanges  holding  the  sockets. 

base     The  baseboard. 

rigidly  made  so  that  there  will  be  a  minimum  of  vibration.  If  the 
table  vibrates  there  is  a  disagreeable  trembling  of  the  screen  image. 
(For  pictures  of  such  a  table  see  fig.  133,  182). 

Carrying  out  the  precautions  against  reflections  from  light  sur- 
faces, the  table  is  made  dull  black  or  brown.  This  is  easily 
accomplished  by  using  some  dull  black  paint  like  "dead-black 
Japalac"  or  other  dull  black,  or  dull  brown  paint,  thinned  some- 
what with  turpentine. 

The  anilin  black  stain  used  for  laboratory  tables  is  also  most 
excellent  (§  42 4a). 

To  the  projection  table  should  be  fastened  the  rheostat,  and  the 
ammeter,  if  one  is  used ;  also  the  lamp  switch  and  the  incandescent 
lamp  (fig.  133).  Then  the  table  can  be  moved  from  one  place  to 


CH.  IX]  HOME-MADE  PROJECTION  APPARATUS  289 

another  and  be  ready  for  projection  by  connecting  the  supply  wires 
for  the  lamp  to  the  line  at  any  outlet  box  (fig.  3). 

§  425.    Lathe  bed  or  optical  bench  for  projection  apparatus. — 

For  the  projection  microscope,  and  for  general  experimental  pur- 
poses there  is  no  form  of  projection  outfit  so  suitable  and  flexible 
as  the  lathe-bed  type.  It  is  easily  and  cheaply  constructed.  Any 
teacher  with  a  little  ingenuity  and  the  aid  of  a  tin-smith,  black- 
smith, plumber,  and  carpenter  or  cabinet-maker,  can  construct  all 
except  the  optical  parts.  The  optical  parts  can  be  obtained  of 
dealers  or  manufacturers  of  microscopes  and  projection  apparatus. 
There  is  this  further  advantage  in  getting  up  a  projection  outfit, 
the  person  who  does  it  will  know  enough  to  use  it.  He  will  not 


§  424a.  Stain  for  laboratory  tables. — During  the  last  few  years  an  excellent 
method  of  dying  wood  with  anil  in  black  has  been  devised.  This  black  is 
lustreless,  and  it  is  indestructible.  It  can  be  removed  only  by  scraping  off  the 
wood  to  a  point  deeper  than  the  stain  has  penetrated. 

It  must  be  applied  to  unwaxed  or  unvarnished  wood.  If  wax,  paint  or  var- 
nish has  been  used  on  the  tables,  that  must  be  first  removed  by  the  use  of 
caustic  potash  or  soda  or  by  scraping  or  planing.  Two  solutions  are  needed : 

SOLUTION  A 

Copper  sulphate 125  grams 

Potassium  chlorate  or  permanganate 125  grams 

Water 1000  cc. 

Boil  these  ingredients  in  an  iron  kettle  until  they  are  dissolved.  Apply  two 
coats  of  the  hot  solution.  Let  the  first  coat  dry  before  applying  the  second. 

SOLUTION  B 

Anilin  Oil 120  cc. 

Hydrochloric  Acid 180  cc. 

Water 1000  cc. 

Mix  these  in  a  glass  vessel  putting  in  the  water  first.  Apply  two  coats  with- 
out heating,  but  allow  the  first  coat  to  dry  before  adding  the  second. 

When  the  second  coat  is  dry,  sandpaper  the  wood  and  dust  off  the  excess 
chemicals.  Then  wash  the  wood  well  with  water.  When  dry  sandpaper  the 
surface  and  then  rub  thoroughly  with  a  mixture  of  equal  parts  turpentine  and 
linseed  oil.  The  wood  may  appear  a  dirty  green  at  first  but  it  will  soon  become 
ebony  black.  If  the  excess  chemicals  are  not  removed  the  table  will  crock.  An 
occasional  rubbing  with  linseed  oil  and  turpentine  or  with  turpentine  alone  will 
clean  the  surface.  This  is  sometimes  called  the  Danish  method,  Denmark  black 
or  finish.  See  Jour.  Ap.  Micr.,  Vol.  I,  p.  145;  Bot.  Zeit.,  Vol.  54,  p.  326,  Bot. 
Gazette,  Vol.  24,  p.  66,  Dr.  P.  A.  Fish,  Jour.  Ap.  Micr.,  Vol.  VI.,  pp.  211-212. 
The  Anatomical  Record,  Vol.  V.  191 1,  pp.  145-146.  (Quoted  from  The  Micro- 
scope, by  Gage,  nth  ed.  1911,  pp.  282-283). 


2 go  HOME-MADE  PROJECTION  APPARATUS  [Ce.  IX 

expect  the  apparatus  to  do  the  work  of  a  machine,  and  also  to 
supply  all  of  the  intelligence  to  enable  it  to  do  so. 

§  426.  Baseboard  and  track. — For  the  lathe-bed  carrying  all 
the  apparatus  (fig.  121,  159)  a  flat  board  about  2  cm.  (J^in.)  thick 
is  used  for  the  base.  The  width  and  length  can  be  made  to  suit 
the  apparatus  designed.  The  dimensions  for  that  shown  in  fig. 
1 58-1 59  are:  Length  125  cm.  (4ft.);  width  22.5  cm.  (8^  in.). 

The  track  which  serves  as  a  guide  to  the  blocks  bearing  the  differ- 
ent pieces  of  apparatus  (fig.  121)  is  best  made  of  two  brass  tubes  or 
rods  12  mm.  (^  in.)  in  diameter  and  the  full  length  of  the  base- 
board (§  426a). 

§  427.  Fixing  the  track  to  the  baseboard. — For  this,  holes 
should  be  bored  through  the  tubes  or  rods,  being  careful  to  have 
the  holes  parallel  so  that  there  will  be  no  torsion  or  twist  when  the 
tubes  are  fastened  to  the  board.  If  rods  are  used  the  screw  holes 
must  be  countersunk.  If  tubes  are  used  then  the  upper  wall 
should  have  a  larger  hole  than  the  lower  and  a  slender  screw  driver 
used,  (fig.  159  ts),  then  the  screw  head  goes  through  the  upper  wall 
and  presses  against  the  lower  side  only. 

One  tube  or  rod  is  fixed  firmly  to  the  base,  thus :  With  a  straight 
edge  like  a  T-square  make  a  straight  line  on  the  baseboard  where 
the  track  is  to  be  laid  and  then  fasten  the  one  track  accurately  along 
this  line  so  that  it  will  be  perfectly  straight. 

Now  for  the  other  track  lay  it  as  follows :  Use  apparatus  blocks 
(§  428)  near  the  ends  of  the  baseboard  and  put  the  loose  rod  in 
place.  Press  the  block  down  firmly  so  that  the  loose  track  will  be 
forced  into  the  groove.  Put  screws  in  the  end  holes,  but  do  not 
screw  them  down  firmly.  If  there  are  intermediate  holes  as  in 
fig.  158-159  move  a  block  near  the  hole,  press  it  down  firmly  and 
then  put  in  a  screw,  but  do  not  screw  it  in  firmly. 

§  426a.  For  the  rods,  one  can  procure  the  thin,  polished  or  nickeled  brass 
tubing  used  for  railing,  or  the  thick  brass  tubes  used  instead  of  iron  tubing.  The 
measurement  given  means  the  total  diameter.  Of  course  one  can  use  any 
desired  diameter  by  varying  the  size  of  the  V-shaped  notches  in  the  apparatus 
blocks  (fig.  158  A)  or  the  position  of  the  cleats  (fig.  159).  If  brass  tubing  is 
employed  for  the  track,  the  size  known  to  the  plumber  is  that  of  the  bore,  not 
the  outside  diameter.  Tubing  with  %th  or  %th  inch  bore  answers  well.  _  The 
outside  diameters  will  be  10  and  13.5  mm.  (13/32  and  17/32  in.)  respectively. 


CH.  IX]  HOME-MADE  PROJECTION  APPARATUS  291 

This  will  make  a  track  along  which  the  blocks  will  move  freely. 
If  both  tracks  were  firmly  fixed  the  blocks  would  have  to  be  con- 
structed with  extreme  precision  or  the  blocks  would  bind.  They 
would  also  bind  if  the  tracks  were  not  perfectly  parallel  at  all 
points.  The  loose  track  gives  slightly  and  thus  compensates  for 
any  little  irregularity  of  the  track  or  apparatus  block. 

§  428.  Apparatus  blocks.— These  are  shown  in  figures  158, 159. 
They  must  be  sufficiently  heavy  so  that  the  various  pieces  of  appara- 
tus they  carry  will  be  steady ;  and  finally  the  sockets  for  receiving 
the  stems  of  the  apparatus  must  be  on  the  blocks  in  a  position  so 
that  the  parts  like  the  stage  and  the  microscope  can  be  brought 
sufficiently  close  together. 

Size  and  weight  of  the  different  blocks  for  the  apparatus  figured 
(fig.  158,  159): 

1.  Arc  lamp  block.     12^  x  12^"  cm.  (5x5  in.) ;  weight  i  kilo. 
(2  Ibs.),  (fig.  158). 

2.  Condenser  block.     12^x10  cm.  (5x4^.);  weight  2  kilos. 
(4^ Ibs.),  (fig.  1 58). 

3.  Stage  block,  i2>^  x  6  cm.  (5x2  in.) ;  weight,  i  kilo.  (2  Ibs.), 
(ng.  158). 

4.  Microscope  block,   12^x10  cm.   (5x4  in.);    weight,   2-3 
kilos.  (4-6  Ibs.),  (fig.  158). 

5.  Block  for  lantern-slide  carrier,    12^x6  cm.    (5x2   in.); 
weight  #  kilo,  (i  lb.),  (fig.  158). 

6.  Block  for  lantern  objective,  or  a  photographic  objective, 
(fig.  158),  12^2  x  10  cm.  (5x4  in.);   weight,  2  kilos.  (4^2  Ibs). 

7.  Block  for  horizontal  microscope,  17x12^  cm.  (7x5  in.), 
weight  2>£  kilos.  (5^  Ibs.),  (fig.  145). 

§  429.     Construction  of  the  apparatus  blocks. — If  one  has  the 

facilities  of  a  machine  shop  and  foundry  at  his  disposal  these 
apparatus  blocks  may  be  made  of  cast  iron,  smoothed  and  grooved 
on  a  planer.  In  like  manner  the  lathe  bed  with  V's  can  be  made 
(fig.  134).  Lacking  these  facilities  one  can  prepare  blocks  of  wood 
which  will  answer  almost  perfectly  as  follows:  Select  some  fine 
grained  board  2  cm.  to  2.5  cm.  thick  (%  to  i  in.),  and  cut  it  into 


2Q2 


HOME-MADE  PROJECTION  APPARATUS 


[Cn.  IX 


blocks  of  the  required  size  for  the  special  purpose.     The  blocks  can 
be  made  as  heavy  as  desired  by  adding  sheets  of  lead  (fig.  isSA). 

For  the  guides  to  follow  the  track,  one  can  make  V-shaped 
grooves,  or  more  easily,  strips  of  the  proper  thickness  can  be  screwed 
to  the  block  (fig.  159).  One  of  the  strips  should  be  screwed  tightly 
to  the  block,  and  the  other  should  have  screw  holes  through  the 
strip  considerably  larger  than  the  screws,  then  it  will  be  possible  to 
make  slight  changes  in  position  to  get  an  exact  fit.  When  in  the 
exact  place  desired  the  screws  can  be  set  firmly.  As  the  large  holes 
in  the  strips  will  be  larger  than  the  heads  of  the  screws,  metal 
washers  should  be  employed  (fig.  159). 

§  430.  Sockets  for  the  stems  of  the  apparatus. — There  is  first 
screwed  to  the  top  of  the  block,  a  railing  flange,  and  into  this  is 
screwed  a  short  tube  of  the  size  to  receive  the  stem  or  post  of  the 


bl 


& 


^>  -c^i.  -  ^ 


Jb 


FIG.  is8A.     SHIELD  FOR  A  PROJECTION  OBJECTIVE. 

This  shows  the  method  of  supporting  an  objective  in  a  shield.  The  shield  is 
supported  by  a  bolt  with  a  fan  shaped  end  (p). 

The  bolt  or  stem  enters  the  socket  and  is  held  in  place  by  a  screw  (s). 

bl  Apparatus  block  composed  of  lead  sheets  above  and  a  block  of  wood 
below.  The  block  of  wood  has  V-shaped  grooves  for  sliding  along  the  track. 

b     Baseboard  with  track,  end  view. 


CH.  IX] 


COMBINED  PROJECTION 


293 


apparatus.  This  tube  has  a  set  screw  in  the  side  to  hold  the  post 
at  any  desired  level  (fig.  is8F).  In  order  to  be  able  to  perfect  the 
centering  of  the  apparatus,  the  screw  holes  in  the  flanges  are  made 
larger  than  the  screws  so  that  by  loosening  the  screws  the  flange 
can  be  shifted  slightly  from  side  to  side.  If  necessary  one  can  use 
washers  to  increase  the  size  of  the  screw  heads,  so  that  the  holes 
in  the  flange  can  be  quite  large. 

§  431.     Wooden  shields  for  holding  objectives,  etc. — For  hold- 
ing projection  objectives  of  low  power,  shields  of  thin  board  (i  to 


FIGURE  158?.     SECTIONAL  VIEW  OF  A  RAILING  FLANGE  AND  SOCKET. 

/    The  flange  in  section.     The  screw  holes  are  made  large  for  centering. 
5     The  set  screw  to  hold  the  post  in  place. 

p     Post  extending  down  into  the  socket.     It  is  held  at  any  desired  height 
by  the  set  screw  (s). 

sc     Socket  for  receiving  the  post  or  stem  of  any  piece  of  apparatus. 

1^2  cm.,  y£  in.  thick)  can  be  used,  and  a  post  or  stem  of  iron  made 
from  a  bolt  by  hammering  out  the  end  in  the  form  of  a  fan  (fig. 
1 58 A).  To  aid  in  centering,  the  screw  holes  in  this  post  should 
also  be  larger  than  the  screws. 


MICROSCOPE  AND  LANTERN-SLIDE  PROJECTION  COMBINED 

§  432.  With  an  outfit  of  the  lathe-bed  type  (fig.  138),  it  is  very 
simple  to  change  from  lantern-slide  to  micro-projection  and  the 
reverse.  All  that  is  necessary  is  to  put  the  lantern-slide  carrier 
next  the  condenser,  and  the  lantern-slide  projection  objective  on  its 
block  in  position.  The  stage  and  the  microscope  must  be  set  off 
the  track  on  the  table.  The  only  difficulty  is  that  the  second  ele- 
ment of  the  condenser  for  the  micro-projection  is  of  too  short  a 
focus  for  most  lantern-slide  projection.  This  can  be  overcome  in 


294 


COMBINED  PROJECTION 


[Cn.  IX 


table 


b3 


o 

o 

JL  J- 

ffl 

b4l 

Base    Board 


Base 

Board 

t           t 

(8) 

*& 

b1 

0 

© 

e 

© 

© 

e 

b2 

© 

e 

© 

e 

b3 

© 

e 

© 

e 

b4 

© 

e 

t              t 

Base 

Board 

FIG.  159.     HOME-MADE  OPTICAL  BENCH  FOR  ALL  PURPOSES. 

Base  Board  This  is  drawn  at  the  right  as  if  transparent  to  show  the  track 
and  under  side  of  the  carrying  blocks,  and  the  various  ways  in  which  the  guid- 
ing cleats  can  be  applied. 

it  tt    The  tubular  tracks  on  which  the  carrying  blocks  ride. 


CH.  IX]  COMBINED  PROJECTION  295 

b  i  Block  with  four  guiding  cleats  of  wood.  The  screw  holes  in  the  inside 
cleat  at  the  left  and  the  outside  one  at  the  right  are  made  large,  and  washers 
are  used  on  the  screws.  This  is  to  make  accurate  centering  possible. 

b  2  Block  showing  two  guiding  cleats  between  the  tracks.  Only  one 
cleat  has  large  screw  holes  for  centering. 

b  3  The  third  carrying  block  with  the  guide  cleats  on  the  outside  of  the 
track  rods.  Only  one  has  large  screw  holes  for  centering. 

b  4  The  fourth  carrying  block  with  guide  cleats  at  only  one  end,  and  with 
centering  holes  in  one  cleat. 

At  the  left    Sectional  views  of  the  carrying  blocks. 

In  b  i  is  shown  how  to  make  a  table  for  carrying  apparatus  along  the  optical 
bench,  and  at  t  s,  the  method  of  screwing  the  track  tubes  to  the  baseboard. 

In  b  4  is  shown  how  to  attach  a  shield  with  an  opening  for  lantern  slides  (0). 

two  ways:  (i)  The  arc  lamp  can  be  put  closer  to  the  condenser, 
thus  making  the  beam  between  the  elements  diverging  instead  of 
parallel  (fig.  i),  or  (2)  a  condenser  lens  of  longer  focus  can  be  used 
for  the  lantern-slide  projection.  In  much  of  the  modern  projection 
apparatus  the  condenser  lenses  are  easily  changed  (see  fig.  166). 


FIG.  160.     UNIVERSAL  LEVEL. 
(Cut  loaned  by  the  L.  S.  Starr ett  Co.}. 

A  level  like  this  which  serves  for  vertical  and  horizontal  leveling  is  very 
convenient  and  essential  for  projection  work. 

The  second  method  of  combined  projection  is  to  have  two  com- 
plete lanterns  side  by  side,  one  for  micro-projection  and  one  for 
lantern-slide  work.  In  this  case  there  should  be  a  double- pole, 
double-throw  switch ;  then  one  can  turn  either  lantern  off  or  on  at 
will  (fig.  162,  164). 

Finally,  in  much  of  the  modern  apparatus  special  provision  is 
made  for  combined  projection  (see  fig.  164-176). 


296  PROJECTION  MICROSCOPES  ON  THE  MARKET       [Cn.  IX 


FIG.  161.     THUMB  SCREWS  AND  THUMB  NUTS. 

(From  the  Catalogue  of  the  Hartford  Screw  Company}. 

Thumb  screws  and  thumb  nuts  are  necessary  if  one  is  to  construct  home- 
made apparatus. 


PROJECTION  MICROSCOPES  OBTAINABLE  IN  THE  OPEN  MARKET 

§  433.  The  projection  microscope  so  far  considered  in  this 
chapter  was  designed  to  give  the  range  needed  for  modern  micro- 
projection  in  a  biologic  or  other  laboratory,  that  is,  for  use  with 
specimens  slightly  smaller  than  lantern  slides  (50  to  65  mm.  in 
diameter)  to  those  of  i  mm.  or  less  (fig.  121,  147). 


CH.  IX] 


COMBINED  PROJECTION 


297 


Screen     Imac? 


Screen          Image 


V  C 


W2 


FIG.    162.     COMBINED  LANTERN-SLIDE  AND  MICRO-PROJECTION  WITH 
Two  COMPLETE  OUTFITS  SIDE  BY  SIDE. 

Wi  W2     The  supply  wires  from  the  outlet  box  (fig.  3). 
D  S     Double-pole,  double-throw  knife  switch. 
/     Binding  post  of  supply  wire  Wz. 

2  Binding  post  for  supply  wire  W  i. 

3  Binding  post  for  the  wire  (W  j)  from  the  switch  to  the  rheostat  at  the 
right. 

4  Binding  post  for  the  wire  (W 5)  to  the  lower  carbon  of  the  arc  lamp  at  the 
right. 

5  Binding  post  on  the  switch  for  the  wire  (W  6}  to  the  rheostat  at  the  left. 

6  Binding  post  on  the  switch  for  the  wire  (W  8}  to  the  lower  carbon  of  the 
arc  lamp  at  the  left. 

H  i,  H  2     Hinges  for  the  switch  blades. 

j  i,  j  2,  j  3,  j  4  Jaws  for  receiving  the  switch  blades  when  the  switch  is 
closed. 

SH  Switch  handle  for  opening  and  closing  the  switch.  The  switch  is 
closed  at  the  right.  On  the  left  the  handle,  bar  and  switch  blades  are  shown 
with  dotted  lines. 


298  PROJECTION  MICROSCOPES  ON  THE  MARKET     [Cn.  IX 

W  8    The  wire  to  the  left  lamp,  lower  carbon. 

W  6,  W  7  Wire  including  the  rheostat,  passing  to  the  upper  carbon  of  the 
left  arc  lamp. 

L  Rheostat     Rheostat  for  the  left  lamp. 

r  3,  r  4     The  binding  posts  of  the  left  rheostat. 

L  Lamp    The  left  arc  lamp. 

F    Feeding  mechanism  for  the  carbons. 

cl     Clamp  for  fixing  the  arc  lamp  in  any  vertical  position  on  its  standard. 

s  s     Set  screws  for  the  carbons. 

H  C    Horizontal  or  upper  carbon. 

V  C    Vertical  or  lower  carbon. 

R  Rheostat    Rheostat  for  the  right  arc  lamp. 

r  i,  r  2     Binding  posts  for  the  rheostat. 

Wj  W  4.  Wire  from  the  switch  through  the  right  rheostat  to  the  upper 
carbon  of  the  right  arc  lamp. 

W  5     Wire  to  the  lower  carbon  of  the  right  lamp. 

R  Lamp     The  right  lamp.     It  is  exactly  like  the  left  one. 

L  Lamp,  R  Lamp     The  arc  lamps  for  the  two  projectors. 

Condensers  The  triple-lens  condensers  with  water-cells  for  the  two  pro- 
jectors. 

Axis,  Axis,  Axis,  Axis     Principal  optic  axis  in  the  two  projectors. 

P  Objective     The  projection  objective  at  the  left. 

Microscope    The  projection  microscope  at  the  right. 

Screen  Image,  Screen  Image  The  images  formed  on  the  screen  by  the  two- 
instruments. 

NOTE. — In  using  these  projectors  it  is  only  necessary  to  turn  the  switch 
handle  over  to  the  one  desired  and  that  lamp  can  be  lighted.  One  can  turn 
from  one  to  the  other  at  will. 

A  more  economical  arrangement  would  be  to  have  a  single  rheostat  inserted 
along  either  Wi  or  W2  before  reaching  the  knife  switch,  then  the  single  rheostat 
would  serve  for  both  lanterns. 

With  the  two  rheostats,  as  here  shown,  both  lanterns  could  be  run  at  the 
same  time  if  there  were  two  switch  handles  and  double  blades  hinged  at  the 
center  (Hi,  H  2). 

The  projection  microscopes  in  the  open  market  rarely  possess 
anything  like  this  range.  Very  few  will  project  an  object  as  great 
as  25  mm.  in  diameter. 

It  seems  to  the  writers  of  this  book  that  the  makers  have  unduly 
limited  the  range  of  their  apparatus  by  a  too  rigid  insistence  on  the 
use  of  substage  condensers  and  projection  oculars,  and  also  by  the 
effort  to  make  combined  apparatus.  Combination  always  means- 
compromise  and  more  or  less  loss  of  individual  efficiency. 

It  is  certain,  too,  that  most  of  them  have  not  fully  appreciated 
the  necessity  for  dull  black  surfaces.  The  bright  finish  is  probably 
to  please  the  eye  when  the  apparatus  is  not  in  operation.  It 
certainly  is  not  good  for  the  eyes  when  the  apparatus  is  in  opera- 
tion. 


CH.  IX]     PROJECTION  MICROSCOPES  ON  THE  MARKET  299 

However,  many  opticians  are  coming  to  finish  their  apparatus 
in  black,  and  all  of  them  are  ready  to  make  modifications  in  their 
instruments  which  they  are  convinced  will  make  them  more  effec- 
tive and  convenient  for  those  who  are  to  use  them.  But  as  many 
men  have  many  minds  it  is  not  possible  for  the  manufacturers  to 
please  every  one  in  all  particulars,  hence  the  apparatus  in  the  open 
market  must  represent  a  kind  of  average.  While  the  authors 
realize  the  limitations  mentioned  above,  it  is  a  pleasure  to  be  able 
to  assert  without  reserve  that  the  quality  and  design  of  the  appa- 
ratus obtainable  at  the  present  time  are  excellent. 

§  434.  As  the  projection  microscopes  most  common  in  America 
are  of  German,  English  and  home  manufacture  some  examples  are 
illustrated  below. 


FIG.  163.     LEITZ  PROJECTION  MICROSCOPE. 
(From  Leitz  Catalogue). 

1  Arc  lamp. 

2  Condenser  next  the  arc  lamp. 

3  Water-cell. 

4  The  lantern-slide  holder. 

5  Iris  diaphragm. 

6  Biconcave,  illuminating  lens  to  give  the  light  the  right  angle  before  it 
enters  the  substage  condenser. 

7  Stage  and  substage  condensers,  on  a  revolver  for  use  with  different  powers. 

8  Projection  objectives  on  a  revolving  nose-piece. 

9  Projection  oculars  on  a  revolver. 

The  enclosing  curtain  is  turned  over  the  top  to  uncover  the  parts.     (See  fig. 
96  for  the  entire  apparatus  in  its  latest  form,  1914). 


300 


PROJECTION  MICROSCOPES  ON  THE  MARKET      [Cn.  IX 


FIG.  164.     PROJECTION  MICROSCOPE,  MAGIC  LANTERN  AND  MEDIOSCOPE 

WITH  SINGLE  RADIANT. 
(Cut  loaned  by  Williams,  Brown  &  Earle}. 

The  arc  lamp  and  condenser  move  laterally  so  that  each  instrument  can  be 
illuminated  at  will. 

A  The  medioscope  is  an  achromatic  combination  of  large  aperture  for 
objects  of  large  size,  but  smaller  than  lantern  slides. 

B  Projection  microscope  with  large  projection  ocular.  It  has  a  water-cell 
(D)  in  the  path  of  the  light.  The  arc  lamp  and  condenser  (N  O  P)  are  in  place 
for  micro-projection. 

C    Projection  objective  for  lantern  slides. 


FIG.  165.     NEW  REFLECTING  LANTERN  WITH  THE  PROJECTION- 
MICROSCOPE. 

(Cut  loaned  by  Williams,  Brown  &  Earle). 

This  figure  shows  the  new  combined  lantern  of  Williams,  Brown  &  Earle  with 
the  projection  microscope  in  position,  the  magic  lantern  objective  having  been 


CH.  IX]  TROUBLES  WITH  MICRO-PROJECTION  301 

removed.  For  projection  the  mirror  must  be  in  place  to  reflect  the  light  along 
the  axis  as  for  lantern  slides.  The  change  to  the  projection  of  opaque  objects 
is  almost  instantaneous,  but  for  lantern-slide  projection  the  projection  micro- 
scope must  be  removed  and  the  lantern-slide  objective  put  in  place,  but_the 
apparatus  is  so  constructed  that  this  is  easily  accomplished. 


FIG.    1 66.     IMPROVED,    COLLEGE,    BENCH    LANTERN    ARRANGED    FOR 

MICRO-PROJECTION. 
(Cut  loaned  by  the  Mclntosh  Stereopticon  Co.). 

The  optical  bench  consists  of  a  long  baseboard  with  the  two  guide  rods 
supported  by  three  brackets. 

As  each  part  is  independent  it  can  be  changed  in  position  or  entirely  removed 
and  other  apparatus  put  in  its  place,  thus  giving  great  flexibility. 

TROUBLES  WITH  THE  PROJECTION  MICROSCOPE 

§  435.  The  source  of  troubles  with  the  projection  microscope 
are  mainly  the  same  as  with  the  magic  lantern.  These  have  been 
fully  discussed  at  the  end  of  Chapter  I  (§  62-98).  See  §  i28a  for 
the  blowing  of  fuses  with  the  arc  lamp  on  the  house  system. 

The  special  troubles  with  the  projection  microscope  are  almost 
wholly  due  to  the  smallness  of  the  lenses  necessary  for  micro-pro- 
jection; and  as  the  foci  of  these  lenses  are  relatively  short,  slight 
changes  in  the  position  of  one  of  the  elements  of  the  apparatus,  and 
slight  deviations  from  the  true  axis  produce  correspondingly  great 
effects.  It  is  necessary  to  be  more  exact  in  micro-projection,  but 
the  great  fundamental  principles  are  exactly  as  for  the  magic 
lantern. 

§  436.     Insufficient  illumination  on  the  screen. — Besides  those 
given  in  Chapter  I  the  following  may  be  causes : 
i.     Too  large  a  screen  image  may  be  attempted. 


302 


TROUBLES  WITH  MICRO-PROJECTION 


[Cn.  IX 


FIG.    167.     UNIVERSAL   PROJECTOSCOPE   SHOWING   THE   ARRANGEMENT 
FOR  HORIZONTAL  TRANSPARENCIES  AND  FOR  MICROSCOPIC  PROJECTION. 

(Cut  loaned  by  the  C.  H.  Stoelting  Co.}. 
Commencing  at  the  left : 

1  Feeding  screws  for  the  carbons. 

2  Fine  adjustment  for  moving  the  arc  lamp  back  and  forth  along  the  axis. 
3-4     Fine  adjustment  screws  for  moving  the  arc  vertically  and  laterally  to 

keep  the  crater  in  the  axis. 
L  H    Lamp  and  lamp-house. 
C    First  element  of  the  two-lens  condenser. 
F-F    Supports. 
T    Water-cell. 

M    Mirror  above  the  objective  to  reflect  the  light  to  the  vertical  screen. 
MI  My     Mirror  in  position  to  reflect  the  horizontal  beam  directly  upward. 
C2  C3     Second  element  of  the  two-lens  condenser. 
R     Projection  microscope. 

B     Optical  bench  on  which  slide  the  different  pieces  of  apparatus. 
Bx  B2     Supports  of  the  optical  bench.     (See  also  fig.  16,  102). 

2.  The  object  may  not  be  in  the  best  position  in  the  light  cone 

(fig.  132). 

3.  The  substage  condenser,  when  that  is  used,  may  be  a  little 

too  near  or  too  far  from  the  specimen.     Slight  changes  in 


CH.  IX] 


TROUBLES  WITH  MICRO-PROJECTION 


303 


FIG.  1 68.     THOMPSON'S  PROJECTION  MICROSCOPE. 
(Cut  loaned  by  the  A.  T.  Thompson  Co.). 

The  projection  microscope  with  the  substage  condenser  system  is  attached 
to  the  reflectoscope  (fig.  97)  in  the  position  where  vertical  opaque  objects  are 
placed ;  this  allows  the  direct  beam  of  light  to  be  utilized  in  micro-projection. 

The  stage  and  the  objective  holder  are  independent,  and  no  ocular  is  used. 
This  permits  the  projection  of  large  objects  with  low  powers  or  smaller  objects 
with  high  powers.  From  the  short  tube  employed,  the  field  is  not  restricted. 

its  position  often  work  wonders.  The  substage  condenser 
may  be  too  near  to  the  large  condenser  or  too  far  from  it 
so  that  the  light  cone  does  not  reach  it  in  its  most  favorable 
position. 

4.  The  room  may  not  be  dark  enough  or  external  light  may  fall 

directly  on  the  screen  from  some  window  or  open  door. 

5.  Never  forget  the  carbons.     A  slight  mal-position  or  decen- 

terins:  of  the  crater  mav  cause  all  the  trouble. 


304 


PROJECTION  MICROSCOPES  ON  THE  MARKET     [Cn.  IX 


(Balance  of  descriptive  matter  on  next  page} 


CH.  IX]     PROJECTION  MICROSCOPES  ON  THE  MARKET  305 

Commencing  at  the  left : 

Large,  well  ventilated,  light-tight  lamp-house.  As  shown  in  fig.  104,  105, 
the  lamp-house  with  the  lamp  and  first  element  of  the  condenser  can  be  inclined 
to  direct  the  light  downward  upon  an  opaque  object. 

Following  the  lamp-house  is  a  dark  box  for  opaque  projection.  The  large 
projection  objective  with  mirror  is  above  and  the  table  for  the  opaque  objects 
below.  Within  the  dark  box  is  a  mirror  so  inclined  that  it  reflects  part  of  the 
scattered  light  back  upon  the  object  (see  also  fig.  105).  Opaque  objects  up  to 
20  cm.  (8  in.)  square  can  be  projected. 

Following  the  large  objective  for  opaque  projection  is  an  objective  for  lantern- 
slide  or  other  projection  with  the  object  in  a  horizontal  position.  Following 
this  is  the  polarizing  apparatus  of  glass  plates  (see  §  880).  The  second  element 
of  the  condenser  serves  for  lantern-slide  and  for  low  power  micro-projection, 
but  for  high  powers  this  is  turned  out  as  here  shown  and  a  small  double  convex 
lens  in  the  dark  chamber  near  the  first  element  of  the  condenser  is  swung  into 
position  and  serves  to  project  an  image  of  the  crater  at  the  plane  of  the  dia- 
phragm of  the  substage  condenser  (fig.  170). 

Just  beyond  the  bellows  are  shown  the  projection  microscope  and  the 
lantern-slide  objective.  These  are  so  hinged  parallel  to  the  axis  that  the 
microscope  can  be  turned  laterally  and  thus  bring  the  lantern-slide  objective  in 
position.  In  the  picture  the  lantern-slide  objective  is  turned  aside  and  the 
projection  microscope  is  in  position. 

The  substage  condensers  for  different  objectives  are  shown  on  a  revolving 
carrier,  as  are  also  the  micro-projection  objectives  and  the  projection  ocular 
and  amplifier. 


le 


C 

FIG.  170.     DIAGRAM  OF  THE  ILLUMINATING  SYSTEM  FOR  HIGH  POWER 

PROJECTION. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 

This  is  a  modification  of  the  Kohler  system  (  §  401-403),  and  consists  of  the 
first  element  of  the  triple  condenser  (meniscus  and  convex  lens)  to  render  the 
beam  parallel.  The  small,  convex  lens  near  the  condenser  serves  to  project 
an  image  of  the  crater  upon  the  plane  of  the  diaphragm  of  the  substage  con- 
denser. This  is  designed  to  fill  the  aperture  of  the  substage  condenser  and, 
hence,  of  the  high  power  objectives. 

L     The  radiant. 

C  The  meniscus  and  convex  lens  of  the  condenser  and  the  small  special 
convex  lens  for  micro-projection. 

L'     Inverted  image  of  the  radiant. 

E     Substage  condenser. 

5    The  specimen. 

C'    Image  of  the  small  condensing  lens  in  the  plane  of  the  specimen  (5). 


306  PROJECTION  MICROSCOPES  ON  THE  MARKET     [Cn.  IX 


CH.  IX]      PROJECTION  MICROSCOPES  ON  THE  MARKET  307 

FIG.  171.     NEW  STYLE  CONVERTIBLE  BALOPTICON  FOR  MICROSCOPE,  LAN- 
TERN-SLIDE AND  OPAQUE  PROJECTION,  AND  FOR  THE  PROJECTION  OF 
LARGE  TRANSPARENCIES  IN  A  HORIZONTAL  POSITION. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

As  shown  in  the  picture,  this  instrument  is  designed  for  projecting  all  kinds 
of  objects  either  in  a  vertical  or  in  a  horizontal  position.  For  the  large  trans- 
parencies the  object  is  placed  on  the  broad  plate  beneath  the  objective. 
Immediately  under  the  object  is  the  condenser  lens  of  20  cm.  (8  in.)  diameter, 
thus  making  it  possible  to  project  X-Ray  plates,  brain  sections,  etc.,  20  cm. 
(8  in.)  in  diameter.  A  mirror  in  the  dark  chamber  directs  the  horizontal  beam 
from  the  first  element  of  the  condenser  vertically  as  in  all  projection  of  this 
kind. 

For  the  large  transparencies  the  projection  objective  is  in  a  vertical  position 
with  mirror  to  reflect  the  light  to  a  vertical  screen  and  to  overcome  the  left  to 
right  inversion. 


FIG.  172.     UNIVERSAL  BALOPTICON  FOR  OPAQUE  OBJECTS,  MICROSCOPIC 

OBJECTS  AND  FOR  LANTERN  SLIDES  OR  OTHER  TRANSPARENT  OBJECTS 

IN  A  VERTICAL  OR  A  HORIZONTAL  POSITION. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

For  the  opaque  projection  and  lantern  slides  in  a  vertical  position  see  fig.  106. 

For  lantern  slides  or  other  transparent  objects  in  a  horizontal  position  the 
arrangement  for  vertical  slides  is  pushed  back,  and  this  brings  the  condenser 
lens  and  plate  for  supporting  horizontal  objects  over  the  opening.  The  same 
mirror  is  used  for  directing  the  beam  of  light  upward  as  for  the  vertical  slides. 


308  TROUBLES  WITH  MICRO-PROJECTION  [Cn.  IX 

For  micro-projection  the  microscope  is  placed  in  line  with  the  objective  for 
opaque  objects,  the  objective  serving  as  a  condenser.  The  light  passes  directly 
from  the  radiant  through  the  first  element  of  the  condenser  and  the  objective 
for  opaque  objects  to  the  microscope.  The  microscope  is  so  hinged  that  it  can 
be  turned  aside  and  the  other  forms  of  projection  quickly  brought  into  use. 

6.  There  may  be  mist  on  some  of  the  glass  surfaces  as  the  water- 
cell,  or  some  glass  surface  like  the  objective  front  may  be 
dirty. 

§  437.  Unequal  illumination  of  the  screen. — This  is  often  due 
to  the  lack  of  centering  of  some  element. 

1 .  It  is  usually  the  crater  of  the  upper  carbon  that  gets  out  of 

the  axis.     It  is  easily  corrected  by  means  of  the  fine  adjust- 
ments of  the  lamp  (fig.  3). 

2 .  There  may  be  some  less  transparent  part  of  the  object  over 

part  of  the  field.     One  can  easily  determine  this  by  moving 
the  specimen  slightly. 

3.  Part  of  the  mask  (§  384,  fig.  143,  148)  may  be  in  the  field. 


FIG.  173.    BAUSCH  &  LOME'S  SIMPLEST  FORM  OF  PROJECTION  MICROSCOPE. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

This  is  designed  for  low  power  projection  and  consists  of  an  objective  holder, 
rack  and  pinion  focusing  adjustment,  stage  and  substage  condenser  for  low 
powers.  The  whole  is  put  in  place  of  the  projection  objective  for  lantern  slides. 
This  simple  outfit  added  to  the  magic  lantern  enables  one  to  do  very  successful 
micro-projection. 

§  438.  Hazy  images  may  be  due  to  direct  light  on  the  screen 
from  some  window,  etc.  Keep  especially  in  mind  also  that  internal 
reflections  in  the  objective,  the  microscope  tube  or  the  amplifier 
tube  will  cause  hazy  images  (§370,  371),  also  dirt  or  balsam  on  the 
front  lens  of  the  objective. 


Ce.IXJ 


TROUBLES  ^rmiUCRO-PROJECTIOX 


309 


i.    They  may  be  caused  by  air  bubbles  in  the  water-cell  or  in 
the  stage  coder. 


if"  .  - 


FIG.  174.    Smrif  ADomox  TO  THE  MACK  LAXTOLV  FOR 

MICKO-PKOJECIIDX. 
iCmt  fane*  Jnr  tkr  Spemar  Lou  C*.). 


which  the  objective  holder ; 

tfee  hut <*i  n-sfidc  objective  is 
is  used.    This 
at 


:  ,  •:•--..=•: 

«_        <     .  _  -m  -M       * 

He  oroagm  aanm  m 
No 


For 


wifli  it  a  gnat 


projection  tiae  nncxoscope  is  tamed  to  the  top  of  the  lamp 
and  the  fanicm-afade  objecting  g>  tmncd  oa  Ms  toaBe  bade  into  post* 

.:•-  ->.-:    -.-.:    -.- 


2.  They  may  be  caused  by  dark  spots  or  bubbles  in  the  slide  or 

specimen. 

3.  Dark  spots  on  the  condenser,  amplifier  or  ocular  may  cause 

them. 

{  440.  General  conditions  for  good  micro-projection. — With 
good  specimens,,  dean  glass  surfaces,  and  all  the  elements  on  one 
axis,  there  should  be  no  trouble  in  getting  a  good  screen  image  on  a 
suitable  screen  and  in  a  weE  darkened  room. 

It  would  be  of  very  great  advantage  for  any  man  who  aspires  to 
use  the  projection  microscope  effectively,  if  he  could  see  the  room, 
apparatus,  and  exact  method  of  work  of  some  one  who  had  mas- 
tered the  art.  Good  projection  win  not  do  itself. 


310  PROJECTION  MICROSCOPES  ON  THE  MARKET      [Cn.  IX 


FIG.  175.     MODEL  4-5  DELINEASCOPE  WITH  THE  MICROSCOPE  IN  A  VERTICAL 
POSITION  FOR  HORIZONTAL  OBJECTS. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

This  figure  is  to  show  the  course  of  the  rays  for  lantern  slides  in  a  vertical 
position  and  for  microscopic  objects  in  a  horizontal  position. 

A  mirror  M  reflects  the  light  vertically  through  the  horizontal  specimen,  and 
by  means  of  a  prism  (PR)  in  the  tube  of  the  microscope  the  vertical  light  is 
made  to  extend  out  horizontally  to  the  screen. 

A  joint  in  the  microscope  frame  makes  it  possible  to  turn  the  microscope 
down  in  front  of  the  instrument  after  turning  the  lantern-slide  objective  aside 
on  its  hinge.  Then  vertical  objects  can  be  projected  in  the  usual  manner,  or 
by  using  the  prism  (PR)  the  image  can  be  reflected  down  upon  a  horizontal 
drawing  surface. 


CH.  IX]     PROJECTION  MICROSCOPES  ON  THE  MARKET 


FIG.   176.     MODEL  8  DELINEASCOPE  SHOWING  THE  POSITION  OF  THE 
MICROSCOPIC  ATTACHMENT  FOR  VERTICAL   AND  FOR  HORIZONTAL 

OBJECTS. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

This  projection  microscopic  attachment  is  designed  to  use  with  or  without 
oculars  or  amplifiers,  and  for  microscopic  objectives  of  all  foci  from  125 
mm.  to  the  highest  available.  The  substage  condenser  consists  of  several 
lenses  which  are  easily  turned  in  place  or  out  of  position.  By  making  a  suitable 
combination  any  object  and  any  objective  can  be  used.  To  enable  the  opera- 
tor to  get  the  object  in  the  right  position  in  the  cone  of  light  there  is  a  rack  and 
pinion  movement  moving  microscope  and  stage  toward  or  from  the  condenser. 
This  is  done  by  the  lower  milled  head  shown.  The  upper  milled  head  is  for 
the  usual  coarse  adjustment  and  a  micrometer  screw  is  present  for  the  fine 
adjustment  (see  also  fig.  177). 


312 


PROJECTION  MICROSCOPES  ON  THE  MARKET     [Cn.  IX 


FIG.  177.     DIAGRAM  SHOWING  THE  COURSE  OF  THE  RAYS  FOR  LANTERN 
SLIDES  AND  FOR  MICROSCOPIC  OBJECTS  IN  A  VERTICAL  AND  IN  A 
HORIZONTAL  POSITION  WITH  MODEL  8  DELINEASCOPE. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

T    Table  for  opaque  objects. 

W    Wheel  by  which  the  table  is  raised  and  lowered. 

D     Diaphragm  which  may  be  used  above  the  table. 

B     Bulb  which  always  illuminates  the  interior  of  the  machine. 

C    Condensing  lenses  in  front  of  the  arc. 

O     Large  objective  for  opaque  work. 

OT     Smaller  objectives  for  vertical  attachment. 

M    Mirror  for  throwing  light  downward  to  the  lantern  slide. 

Mt  Mirror  for  throwing  a  perpendicular  beam  out  through  the  lantern- 
slide  compartment. 

M2  Mirror  used  in  connection  with  the  projection  of  the  vertical  side  of  an 
object. 

M3  Mirror  which  assumes  a  position  of  45°  when  the  microscope  is  used 
perpendicularly. 

P  Prism  which  is  used  in  the  prism  chamber  when  the  microscope  is  used 
perpendicularly  or  for  drawing  on  a  horizontal  surface  when  the  microscope 
is  horizontal. 

5  Shelf  upon  which  the  lantern  slide  is  placed  previous  to  throwing  it  up 
into  the  optical  axis  by  the  handle. 

H    Handle  of  the  lever  for  raising  lantern  slides  into  position. 


CH.  IX] 


DO  AND  DO  NOT  IN  MICRO-PROJECTION 


313 


§441.     Summary  of  Chapter  IX: 


Do 

1.  Use  actual  objects  in  lec- 
tures and  discussions  as  well  as 
diagrams  (§352). 

2.  Employ  a  projection  micro- 
scope with  equipment  for  speci- 
mens ranging  from  60  mm.  to 
less   than    i   mm.   in  diameter 

(§354). 

3 .  In  demonstrating  with  the 
projection  microscope  use  first  a 
low  power  and  show  the  rela- 
tions of  parts,  then  use  higher 
powers  to  show  details. 

4.  Use     objectives     without 
oculars  from  125  mm.  to  4  mm. 
focus  (§355)- 

5 .  Oculars  or  amplifiers  can  be 
used  with  all  the  objectives  on 
the  microscope   (fig.   138),  but 
preferably  with  those  not  higher 
than  8  mm.  focus. 

6.  Use  a  screen  distance  from 
5  to  10  meters  (16  to  33  feet). 

7.  It  is  better  to  use  a  micro- 
scope in  the  usual  manner  if 
very  high  powers,  like  the  oil 
immersion,  are  to  be  used  (§355). 

8.  If   possible   use   a   triple- 
lens  condenser  (§  363). 


Do  NOT 

1 .  Do  not  stop  with  diagrams 
where  actual  specimens  can  be 
shown.     Diagrams    alone    are 
liable  to  give  false  impressions. 

2.  Do    not    use    projection 
apparatus  with  a  narrow  range 
of  field  or  of  powers. 


3.  Do  not  show  minute  de- 
tails without  first  showing  the 
object  as  a  whole,  so  that  rela- 
tions can  be  clearly  recognized. 

4.  Do    not    use    oculars    for 
projecting  for  large,  class  demon- 
strations.    Oculars  restrict  the 
field  too  much. 

5.  Do    not    use    oculars    or 
amplifiers    unless    for    special 
reasons. 


6.  Do  not   have   the  screen 
distance  too  great. 

7 .  Do  not  try  to  make  out  the 
finest  details  by  projection,  but 
use  a  microscope  in  the  ordinary 
way. 

8.  Do  not  use  a  poor  con- 
denser for  micro-projection,  the 
triple  form,  meniscus  next  the 
radiant,  is  best. 


DO  AND  DO  NOT  IN  MICRO-PROJECTION          [Cn.  IX 


9.  Make  the  room  dark  and 
use  a  perfectly  white  image- 
screen  for  micro-projection 
(§  360)- 


9.  Do  not  try  to  project  with 
the  microscope  in  a  room  that 
cannot  be  properly  darkened  or 
with  a  dirty  screen. 


10.  Use  only  the  direct  current 
arc    light    for  micro-projection, 
unless  compelled  to  use  alter- 
nating current  (§  412). 

11.  Use  an  ammeter  and  a 
variable  rheostat  or  other  bal- 
ancing   device    and    be    very 
careful  about  the  wiring  (fig.  2-3 
188). 

12.  The  arc  lamp  must  have 
fine  adjustment  screws  to  enable 
one  to  keep  the  arc  centered  on 
the  objective  front  (§  362). 

13.  Always  use  a  water-cell 
in  micro-projection  with  the  arc 
lamp  radiant  (§  364). 

14.  Use  a  mechanical   stage 
for  serial  sections  (§  366). 


15.  Blacken  the  objective 
mounts,  and  all  metal  parts  of 
the  projection  apparatus  to 
avoid  glare;  make  sure  there 
are  no  shiny  surfaces  within  the 
projection  apparatus  (§  370— 


10.  Do  not  use  alternating 
current  if  it  is  possible  to  obtain 
direct  current. 


11.  Do  not  neglect  the  am- 
meter and  the  variable  rheostat 
when    installing    a    projection 
microscope. 

12.  Do  not  try  to  use  an  arc 
lamp  without  fine  adjustments, 
otherwise  the  crater  cannot  be 
kept  centered. 

13.  Do  not  project  with  the 
arc  lamp  without  using  a  water- 
cell  to  absorb  the  radiant  heat. 

14.  Do    not     try    to     show 
selected    sections    of    a    series 
without  the  help  of  a  mechani- 
cal stage. 

15.  Do  not  leave  the  objec- 
tive mounts  with  brilliant  re- 
flecting surfaces  to  dazzle  the 
eyes  of  the  operator,  and  do  not 
leave  shiny  surfaces  within  the 
apparatus  to  give  cross  lights 
and  make  the  image  dim. 


CH.  IX]          DO  AND  DO  NOT  IN  MICRO-PROJECTION 


315 


1 6.  Use  a  hood  on  the  objec- 
tive to  aid  in  centering  the  light 
and  in  placing  the  objective  the 
right  distance  from  the  conden- 
ser (§  372);  a  light  shield 
beyond  the  objective  to  stop 
stray  light  is  also  an  advantage 
(§  373)- 


1 6.  Do  not  forget  the  advan- 
tages of  an  objective  hood  for 
centering  the  light  and  prevent- 
ing glare;  and  do  not  omit  the 
light  shield  to  cut  off  stray  light. 


17.  It  is  of  the  utmost  im- 
portance that  every  part  be 
accurately  centered  for  micro- 
projection  (§375),  and  that  the 
parts  should  be  separated  from 
one  another  the  right  distance 
(§  376,  382). 


17.  Do  not  fail  to  have  all 
parts  accurately  centered,  and 
the  correct  distance  apart. 


1 8.  Remember  that  it  is  a 
pure  waste  to  use  too  great  an 
amperage  (§  378). 


1 8.  Do  not  use  a  greater  cur- 
rent than  necessary. 


19.  As  the  same  object  is  to 
be  shown  entire  and  with  magni- 
fied details  and  different  objects 
require  different  magnifications, 
it  is  convenient  to  have  two, 
three  or  four  objectives  of 
different  powers  in  a  revolving 
nose-piece  (§  379). 


19.  Do  not  show  all  objects 
with  the  same  objective,  but 
have  two  or  three  on  a  revolving 
nose-piece  so  that  different 
powers  can  be  used  with  the 
minimum  of  trouble. 


20.  For  exhibition  purposes 
it  is  a  great  advantage  to  use 
carbons  whose  ends  have  been 
shaped  by  previous  burning  in 
the  lamp  (§380). 


20.  Do  not  forget  to  shape 
the  ends  of  the  carbons  by  burn- 
ing them  awhile  in  the  arc  lamp 
before  any  formal  exhibition. 


DO  AND  DO  NOT  IN  MICRO-PROJECTION 


[Cn.  IX 


21.  Be  sure  that  the  carbons 
are  in  the  correct  mutual  posi- 
tion to  give  a  good  light.  A 
screen  image  of  the  burning 
carbons  often  is  of  real  help 

(§381). 


21.  Do  not  omit  the  correct 
setting  of  the  carbons.  A  good 
light  cannot  be  produced  with 
the  carbons  in  the  wrong  mutual 
relation. 


2  2 .  Mask  the  preparations  for 
exhibition  (§  384). 


23.  Remember  the  advan- 
tages of  a  large  field  for  seeing 
the  relation  of  parts  (§  387). 


22.  Do  not  exhibit  specimens 
which  are  not  properly  masked. 
It  is  necessary  to  be  able  to 
work  with  certainty  and  rapid- 
ity in  an  exhibition. 

23.  Do  not  forget  the  impor- 
tance of  a  large  field  so  that  the 
relations  of  parts  can  be  seen. 


24.  Remember  that  one  can 
do  good  projection  work  with 
an  ordinary  microscope  (§  393). 

25.  For  objects  which  must 
remain  in  a  horizontal  position, 
a  vertical  microscope  must  be 
used;    this  involves  the  use  of 
two  mirrors  or  of  a  mirror  and  a 
prism  to  reflect  the  light  upward 
and   then   horizontally   to   the 
screen  (§397). 


24.  Do  not  forget  that  one 
can  do  very  good  work  by  using 
an  ordinary  microscope  in  pro- 
jection. 

25.  Do  not  try  to  use  a  hori- 
zontal microscope  when  one  in  a 
vertical  position  is  called  for. 


26.  Have  everything  in  per- 
fect order  and  adjustment  when- 
ever an  exhibition  of  micro- 
scopic objects  is  to  be  made. 
Haphazard  work  will  give  only 
haphazard  results  (§  400). 


26.  Do    not 
projection. 


do    haphazard 


CH.  IX] 


DO  AND  DO  NOT  IN  MICRO-PROJECTION 


317 


27.  For  high  powers  like  oil 
immersions,  the  screen  distance 
must  be  short,  the  screen  and 
light    perfect,    the   room   very 
dark  and  the  spectators  close 
to    the    screen    (§  401-410). 

28.  Remember    the    advan- 
tages of  the  small-carbon  arc 
lamp  for  use  on  the  house  light- 
ing system  for  drawing  and  for 
demonstrating  to  a  few  (§  417). 

29.  Use  sunlight  when  it  is 
available  (§419). 

30.  One     can     do     excellent 
micro-projection      by      home- 
assembled  apparatus  (§  424). 


31.  For  passing  from  micro- 
projection  to  lantern-slide  pro- 
jection it  must  be  remembered 
that  the  lantern-slide  picture  is 
much  brighter  with  the  same  arc 
light.     To  avoid  the  great  con- 
trast, one  would  do  well  to  use 
a    tinted    glass    in    the    magic 
lantern  to  soften  the  light  as 
for   opaque    and    lantern-slide 
projection  (§  282). 

32.  Study       faithfully       the 
"troubles"  with  the  magic  lan- 
tern in  Ch.  I,  and  in  this  chap- 
ter (§  435-439). 


27.  Do  not  try  high  power 
projection  for  a  long  screen  dis- 
tance, a  light  room  or  a  poor 
screen,  or  anything  else  not  in 
accordance  with  the  most  exact- 
ing work. 

28.  Do  not  forget  the  advan- 
tages of  the  small-carbon  arc 
lamp    on    the    house    lighting 
system  for  drawing  and  demon- 
strations for  a  few  persons. 

29.  Do  not  neglect  the  most 
brilliant   light,    i.    e.,    sunlight, 
when  it  is  available. 

3  o .  Do  not  refrain  from  micro- 
projection  because  you  do  not 
have  an  expensive  special  out- 
fit. Home-made  apparatus  is 
often  more  effective  and  can  be 
assembled  by  any  one. 

31.  Do  not  forget  the  phy- 
siology of  vision  in  passing  from 
a  dim  to  a  brilliant  light  or  the 
reverse. 


3  2 .  Do  not  expect  the  appra- 
atus  to  supply  the  brains. 


THE  METRIC  SYSTEM 


[Cn.  IX 


THE  METER  FOR 
LENGTH 


THE  GRAM  FOR 
WEIGHT 

THE  LITER  FOR 
CAPACITY 


10  CENTIMETER  RULE 

THE  UPPER  EDGE  IN  MILLIMETERS,  THE  LOWER  IN  CENTIMETERS,  AND  HALF 

CENTIMETERS 

THE   METRIC   SYSTEM 
UNITS  THE  MOST  COMMONLY  USED  DIVISIONS  AND  MULTIPLES 

f  Centimeter  (cm.),  o.oi    Meter;    Millimeter  (mm.),  o.ooi 
Meter;    Micron  (AC),  o.ooi  Millimeter;     the  Micron  is 
<       the  unit  in  Micrometry. 

Kilometer,    1000  Meters;    used  in  measuring  roads  and 
I      other  long  distances. 

{Milligram  (mg.),  o.ooi  Gram. 
Kilogram,  1000  Grams,  used  for  ordinary  masses,    like 
groceries,  etc. 

Cubic  Centimeter  (cc.),  o.ooi  Liter.      This  is  more  com- 
mon than  the  correct  form,  Milliliter. 

Divisions  of  the  Units  are  indicated  by  the  Latin  prefixes:  deci,  o.i ;  centi, 
o.oi;  milli,  o.ooi;  micro,  one  millionth  (o.oooooi)  of  any  unit. 

Multiples  are  designated  by  the  Greek  prefixes;  deka,  10  times;  hecto,  100 
times;  kilo,  1000  times;  myria,  10,000  times;  mega,  one  million  (1,000,000) 
times  any  unit. 

TABLE  OF  METRIC  AND  ENGLISH  MEASURES 

Meter   (M.)    (unit  of  length)  =  iop  centimeters; 

1,000  millimeters,  1,000,000  microns  (/*) 39-38  inches;   3.28  feet; 

1.094  yard. 

Centimeter    (cm.)=.oi    meter;     10    millimeters, 

10,000 microns  (/*).  -3937  (!)  inches 

Millimeter   (mm.)=.ooi    meter,    .1    centimeter, 

i  ,000  microns  (/x) .  .03937  (I )  inches 

Micron  (M)  =.ooi  millimeter  (unit  of  measure  in 

micrometry) 000,039,37  inch 

1 725000  inch 

Liter  (L.)  (unit  of  capacity)  =  1,000  cubic  centi- 
meters     (i  quart  approx.) 

Cubic  Centimeter  (cc.)  =  .001  liter (TV  cubic  inch  approx.) 

Gram  (g.)  (unit  of  weight) 15-43  grains. 

Kilogram  (Kg.)  =  1,000  grams 2.2046  (2^)  pounds. 

Yard  =3  feet,  36  inches 91-44  centimeters. 

Foot  =  one- third  yard,  12  inches     3°-47  centimeters. 

Inch  =^g  yard,  TV  foot 2.54  centimeters. 

.001  (T(jW)  inch 0254  millimeters  =25.4  M. 

Fluid  ounce  =  8  fluidrachms 29.57  (3°)  cubic  centimeters. 

Pound  (Lb.)  (avoirdupois)  =  16  ounces 453-6  grams. 

Ounce  (oz.)  (avoirdupois)  =437^2  grains 28.35  (3°)  grams. 

Ounce  (oz.)  (Troy  or  apothecaries)  =480  grains  .  .31.10  (30)  grams. 


CHAPTER  X 

DRAWING  AND  PHOTOGRAPHY  BY  THE  AID  OF 
PROJECTION  APPARATUS. 

§  450.    Apparatus  and  Material  for  Chapter  X : 

Room  with  electric  current  supply  (§  453);  Arc  lamp  and 
rheostat  or  other  regulating  device  (§  462,  493) ;  Water-cell  (§  504) ; 
Carbons  of  various  sizes  for  small  and  large  currents  (§  486,  488-9) ; 
Condenser  suitable  for  the  objects  to  be  projected  (§467,  533) ;  Mi- 
croscope with  objectives  and  oculars  and  with  a  45  degree  mirror 
or  a  prism  (§  458-459,  493) ;  Photographic  objectives  for  projecting 
the  images  of  large  objects  for  use  with  negatives  or  lantern  slides 
(§  534) ;  Movable  drawing  surface  (§  459-460) ;  An  opaque  lantern 
(§  469) ;  A  photographic  camera  with  ground-glass  focusing  screen 
(§  471);  Metric  measures;  Transparent  micrometer  (§  508+); 
Letters  on  tissue  paper  for  drawings  (§  528+);  Photographic 
paper,  negatives  and  chemicals  (§  532,  547);  See  also  the  needs 
in  Ch.  I,  (§  i). 

§  451.  For  the  history  of  drawing  with  projection  apparatus 
see  the  Appendix,  with  its  references  to  literature. 

It  will  also  be  advantageous  to  consult  the  works  given  in  Ch.  I, 
§  2  and  the  catalogues  of  the  manufacturers  of  projection  and 
photographic  apparatus.  The  Eastman  Kodak  Co.  has  published 
a  very  useful  booklet  on  Enlarging.  This  deals,  not  with  micro- 
scopic, but  with  the  moderate  enlargements  up  to  20  diameters  with 
photographic  objectives. 

DRAWING  WITH  PROJECTION  APPARATUS 

§  452.  The  aid  which  projection  apparatus  could  give  for 
getting  accurate  drawings  was  recognized  from  the  beginning;  and, 
indeed,  this  was  considered  one  of  its  most  important  uses. 

By  the  aid  of  projection  apparatus  accurate  drawings  can  be 
made  by  any  careful  worker,  although  artistic  perfection  can  be 
added  only  by  those  gifted  of  nature.  Even  for  born  artists  it  is 
helpful  in  getting  the  details  of  complex  objects  in  due  position  and 
in  correct  proportion. 

319 


320  ROOM  FOR  DRAWING  [Cn.  X 

The  range  of  possibility  is  great,  for,  by  the  aid  of  projection 
apparatus,  one  can  draw  the  images  produced  by  the  objectives 
used  with  the  magic  lantern,  photographic  objectives,  and  micro- 
scopic objectives  of  all  powers.  The  microscopic  objectives  may 
also  be  combined  with  amplifiers  or  with  oculars  for  projecting  the 
images  to  be  drawn. 

Drawing  with  projection  apparatus  has  the  advantage  over 
drawing  with  the  camera  lucida  that  one  can  see  the  entire  specimen 
in  one  field.  More  important  still,  the  artist  can  use  both  eyes. 
There  is  entire  freedom  of  head  and  eyes,  the  image  remaining 
constantly  in  one  place,  regardless  of  the  position  of  the  draughts- 
man. 

ROOM  FOR  DRAWING  WITH  PROJECTION  APPARATUS 

§  453.  Any  room  suitable  for  projection  is  also  suitable  for 
drawing  with  projection  apparatus. 

Any  laboratory  which  can  be  made  moderately  dark  in  the 
day  time  is  suitable  for  day  work;  and,  of  course,  any  room  is 
suitable  in  the  evening. 

§  454.  Special  photographic  and  drawing  room. — Many  labora- 
tories have  one  or  more  photographic  rooms  which  are  also  used  for 
drawing.  These  are  mostly  separate  rooms.  Sometimes  they  are 
adjoining  a  laboratory,  and  sometimes  they  are  like  a  ticket  booth 
in  a  large  railroad  station,  i.  e.,  a  room  within  a  larger  room  (fig. 
179).  This  is  the  plan  adopted  in  the  Wistar  Institute  (Anat. 
Record,  1907)  and  in  the  author's  laboratory  (Proc.  Amer.  Micr. 
Soc.,  1906,  p.  44-45).  If  these  rooms  are  painted  dull  black 
within,  stray  light  is  absorbed,  and  it  is  much  easier  to  get  sharp 
pictures.  In  a  black  room  the  door  can  be  left  partly  open  and 
thus  secure  better  ventilation. 

As  the  radiant  gives  off  much  heat  it  is  an  advantage  to  have  an 
electric  fan  in  the  room  if  one  works  several  hours  at  a  time.  It 
is  especially  necessary  in  hot  weather, — summer  vacations  when 
teachers  have  time  for  research. 

§  455.     A  drawing  room  made  with  screens  or  curtains. — If  one 

has  not  a  permanent  room  or  booth  in  the  laboratory,  a  fairly  good 


CH.  X]  PROJECTION  APPARATUS  FOR  DRAWING 


321 


substitute  can  be  made  by  means  of  opaque  curtains  enclosing  one 
corner  of  a  room.  This  would  be  something  like  the  early  drawing 
rooms  or  tents  used  by  Kepler  and  others  for  sketching  landscapes 
(fig.  88,  89).  It  is  advantageous  to  have  the  cloth  curtains 
rendered  fire -proof  by  saturating  them  with  a  solution  of  sodium 
tungstate,  or  some  other  fire-proofing  solution  (see  Popular  Science 
Monthly,  Vol.  LXXXI,  1912,  p.  397). 

(Proceedings  of  the  Amer.  Assoc.  Adv.  Science,   Vol.    XLIII, 
1894,  p.  119.) 


FIG.   179.     PHOTOGRAPHIC  AND  DRAWING  BOOTH  (P  D)  IN  A  LARGE 

LABORATORY. 

This  booth  contains  water  and  electric  supply  for  photography  and  for 
projection  work  including  drawing,  printing  and  photo-micrography. 

PROJECTION  APPARATUS  FOR  DRAWING 

§  456.  The  apparatus  used  for  drawing  may  be  the  ordinary 
magic  lantern  (fig.  1-2),  the  projection  microscope  (fig.  121),  the 
opaque  lantern  (fig.  92-111),  or  a  photographic  camera  (fig.  117, 
217). 

§  457.  Drawing  on  a  vertical  surface. — For  this,  the  only  addi- 
tion to  any  of  the  forms  of  projection  apparatus  is  a  vertical  draw- 
ing-board, mounted  so  that  it  may  be  moved  to  a  greater  or  less 
distance  from  the  apparatus  to  get  the  desired  size  of  image.  Or 
one  may  use  a  fixed  wall  for  the  drawing  surface  and  move  the 


322  PROJECTION  APPARATUS  FOR  DRAWING  [Cn.  X 

apparatus  back  and  forth  to  get  the  different  sizes  required.     (For 
getting  the  picture  like  the  object  see  §  512). 

§  458.  Drawing  on  a  horizontal  surface. — From  the  earliest  use 
of  projection  apparatus  for  drawing,  it  was  the  custom  to  draw  the 
image  on  a  vertical  surface,  or  by  means  of  a  plane  mirror  to  change 
the  direction  of  the  rays  of  light  so  that  the  image  would  fall  on  a 
horizontal  surface.  It  was  found  also  that  when  a  plane  mirror 


FIG.  1 80.     PROJECTION  MICROSCOPE  FROM  CHEVALIER  (Planche  2). 

M  Mirror  reflecting  the  sun's  rays  (RR1,  rr1)  to  the  condenser  (C) ;  from 
the  condenser  they  pass  to  the  substage  condenser  (c)  and  are  condensed  upon 
the  object  (o). 

L     Achromatic  objective. 

A  Amplifier  composed  of  a  plano-convex  and  a  double  concave  lens;  this 
amplifier  makes  the  rays  much  more  divergent,  i.  e.,  BB1  instead  of  bb1. 

P  Right-angled  prism  acting  as  a  45  degree  mirror  to  project  the  image 
down  upon  a  horizontal  surface  for  drawing. 

was  used,  the  image  on  the  horizontal  surface  appeared  erect. 
Sometimes  the  mirror  was  placed  before  the  objective  and  changed 
the  direction  of  the  rays  90  degrees  (fig.  89),  and  sometimes  it  was 
used  to  bend  the  rays  downward  after  passing  through  the  objec- 
tive. With  the  microscope  and  magic  lantern  the  mirror  is  usually 
beyond  the  objective  (fig.  182,  193). 

Reflecting  prisms  have  been  much  employed  with  the  microscope 
instead  of  mirrors  (fig.  180,  192).  They  have  the  advantage  of 
giving  more  perfect  reflection  and  of  avoiding  doubling  of  the 
image,  as  occurs  with  a  plane  mirror  silvered  on  the  back. 


CH.  X]  PROJECTION  APPARATUS  FOR  DRAWING  323 

§  459.    Drawing  table  with  attached  45  degree  mirror. — One  of 

the  simplest  and  most  convenient  arrangements  for  the  magic 
lantern  and  the  microscope  is  to  have  a  large  mirror  attached  to 
the  drawing  table.  The  table  and  mirror  can  then  be  moved 
toward  or  from  the  projection  apparatus  to  aid  in  getting  the 
desired  magnification  (fig.  182). 


FIG.  181.     KORISTKA'S  SIMPLE  DRAWING  OUTFIT. 
(From  Koristka's  Microscope  Catalogue). 

This  drawing  outfit  can  be  connected  with  the  house  lighting  system. 

1  Nernst  lamp  for  illumination. 

2  Condenser  connected  with  the  lamp-house. 

3  Stage  of  the  microscope. 

4  Projection  objective. 

5  45°  mirror  for  reflecting  the  rays  down  upon  the  horizontal  drawing 
surface. 

6  Horizontal  drawing  surface.     The  drawing-board  slides  along  the  axis, 
thus  making  it  possible  to  vary  the  distance  and  hence  to  increase  or  diminish 
the  size  of  the  drawing  at  pleasure. 

When  sitting  down  to  draw,  a  convenient  height  for  the  table  is 
76  cm.  (2^2  ft.).  The  one  shown  in  fig.  182  has  a  top  100  cm.  long 
and  75  cm.  wide  (39  x  30  inches). 

The  plate  glass  mirror  is  75  cm.  long  and  60  cm.  wide  (2^2  x  2  ft.). 
It  is  permanently  fixed  at  45  degrees  inclination;  and  to  avoid  the 
sharp  angle  at  the  base  of  the  mirror  it  is  raised  from  the  table  10 
to  15  cm.  (4  to  6  in.). 

The  mirror  itself  is  in  a  strong  wooden  frame,  and  it  is  supported 
by  vertical  and  horizontal  pieces,  as  shown  in  figure  182. 


324 


PROJECTION  APPARATUS  FOR  DRAWING 


[CH.  X 


FIG.   182.     DRAWING  WITH  PROJECTION  APPARATUS  AND  A  MOVABLE 
TABLE  WITH  45°  MIRROR. 

Commencing  at  the  left: 

Supply  wires  to  the  table  switch. 

From  one  pole  of  the  table  switch  a  wire  extends  to  the  binding  post  of  the 
upper  carbon  of  the  arc  lamp. 

From  the  other  pole  of  the  switch  a  wire  extends  to  the  rheostat  (R)  and  from 
the  rheostat  to  the  binding  post  of  the  lower  carbon. 

Arc  lamp  within  the  lamp-house. 

The  metal  lamp-house  is  shown  as  if  transparent,  as  it  was  left  in  position 
during  only  a  part  of  the  time  while  the  photograph  was  exposed. 

Condenser  and  water-cell  (fig.  121). 

Stage  of  the  microscope  with  stage  water-cell. 

Projection  microscope  with  objectives  in  the  revolving  nose-piece,  a  shield 
to  stop  stray  light  and  an  amplifier  in  the  end  of  the  large  tube. 

The  lamp,  condenser,  stage  and  microscope  are  on  independent  blocks  and 
can  be  moved  freely  on  the  optical  bench.  The  picture  of  the  10  centimeter 
rule  under  the  door  of  the  lamp-house  gives  the  scale  of  the  picture. 

R     Adjustable  rheostat. 

20-10  These  numerals  show  the  range  of  current  which  the  rheostat  per- 
mits. The  arrow  indicates  the  way  to  turn  the  knob  to  increase  the  current 
(see  fig.  281,  Ch.  XIII). 

On  the  legs  at  the  left  is  a  shelf  for  the  rheostat. 

The  adjustable  drawing  shelf  has  an  arrangement  for  moving  up  and  down 
on  metal  ways  which  can  be  attached  to  any  table,  whatever  the  form  of  the 


CH.  X]  PROJECTION  APPARATUS  FOR  DRAWING  325 

legs.  The  supporting  brackets  are  so  jointed  that  the  shelf  can  be  let  down 
when  the  large  drawing  table  needs  to  be  brought  up  close  to  the  projection 
table.  This  method  of  moving  the  drawing  shelf  and  lowering  it  is  due  to 
Dr.  B.  F.  Kingsbury. 

As  one  must  sit  close  to  the  table,  there  should  be  no  vertical  rail 
under  the  front  edge  to  interfere  with  the  knees  of  the  artist.  At 
this  edge  there  is  a  strengthening  piece  flat  against  the  top.  On 
the  other  edge  and  at  the  ends  are  the  usual  vertical  rails.  To 
ensure  the  rigidity  of  the  table,  there  are  pieces  passing  across  the 
ends  between  the  legs  and  near  the  bottom,  and  a  middle  piece 
extending  lengthwise  between  these  end  pieces,  thus  holding  the 
table  legs  at  the  two  ends,  so  that  they  cannot  spread  either  side- 
wise  or  endwise  (fig.  182). 

The  legs  are  6  cm.  (2^  in.)  square,  and  smooth  on  the  lower  end 
so  that  the  table  can  be  moved  easily,  or  casters  may  be  used.  The 
entire  table  is  finished  in  dull  black  and  all  the  corners  rounded. 

§  460.  Projection  table  with  drawing  shelf. — The  simplest  of 
all  arrangements  for  drawing  with  the  projection  microscope  and 
the  magic  lantern  is  a  projection  table  with  an  adjustable  shelf 
attached  to  the  end  (fig.  183,  187).  For  this  arrangement  the 
mirror  or  prism  for  reflecting  the  light  downward  must  be  close  to 
the  objective  or  to  the  end  of  the  microscope. 

As  the  shelf  can  be  raised  to  the  level  of  the  table  top  or  depressed 
about  50  cm.  (20  in.),  it  is  possible  to  get  quite  a  range  of  magnifica- 
tion from  the  different  image  distances  alone,  using  the  same  objec- 
tive; but,  of  course,  the  upper  range  is  not  so  great  as  with  a 
separate  drawing  table.  With  the  drawing  shelf,  however,  one 
can  get  lower  powers,  as  the  image  can  be  closer  to  the  end  of  the 
objective.  By  using  different  objectives  one  can  get  all  the  range 
desired  with  either  arrangement.  The  single  table  and  adjustable 
shelf  is,  of  course,  much  the  cheaper. 

If  one  uses  the  table  and  drawing  shelf  it  is  necessary  that  the 
apparatus  be  movable  on  the  optical  bench,  so  that  the  objective 
may  be  beyond  the  end  of  the  table  over  the  drawing  shelf.  This 
is  easily  accomplished  with  an  optical  bench  like  that  shown  in  fig. 
158-159.  In  case  one  desires  a  larger  drawing  surface  than  the 


326 


PROJECTION  APPARATUS  FOR  DRAWING  [Cn.  X 


FIG.  183. 


ARRANGEMENT  FOR  DRAWING  OBJECTS  THE  SIZE  OF  LANTERN 
SLIDES. 


The  illumination  can  be  by  the  ordinary  heavy  lantern-slide  current,  or  by 
the  small  current  of  the  house  lighting  supply.  The  5  ampere  current  is 
sufficient  for  drawing.  If  one  wishes  to  draw  on  a  horizontal  surface,  then  a 
mirror  is  put  beyond  the  objective.  If  the  drawing  is  on  a  vertical  surface,  as 
for  wall  diagrams,  then  the  mirror  is  removed. 

w     Supply  wire  cable  from  the  outlet  box  (fig.  3). 

t  w    Wires  to  the  arc  lamp. 

5     Table  switch. 

r     Rheostat  of  the  theater-dimmer  type  with  a  range  of  5  to  35  amperes. 

/     Arc  lamp. 

a  a  a     Axis. 

c     Condenser  and  water-cell. 

Is     Lantern-slide  support. 

o     Projection  objective  for  large  objects. 

m     45°  mirror  to  reflect  the  light  down  upon  the  horizontal  drawing  shelf. 

as     Adjustable  drawing  shelf. 

b  Baseboard  with  track  along  which  the  carrying  blocks  can  be  moved 
independently. 

attached  shelf,  a  small  drawing-board  may  be  clamped  to  the  shelf 
as  shown  in  fig.  183. 


CH.  X] 


PROJECTION  APPARATUS  FOR  DRAWING 


327 


The  size  of  the  projection  table  is  the  same  as  given  above  (§  424) . 
A  convenient  size  for  the  drawing  shelf  is  50  cm.  (20  in.)  long,  and 
25  cm.  (10  in.)  wide. 

In  fig.  183  the  legs  of  the  table  are  square  and  straight  and  the 
shelf  slides  up  and  down  on  the  legs,  being  clamped  in  any  desired 
position  by  the  thumb  nuts. 


FIG.  184.     DR.  RILEY'S  ATTACHMENT  TO  AN  ORDINARY  MAGIC  LANTERN 

FOR  DRAWING. 

(Science,  Vol.  XXIX,  1909,  p.  37-38). 

AB     Mirror  support. 
CD     Mirror  and  mirror  frame. 

F    Clamp  for  fastening  the  mirror  support  in  position  in  front  of  the  magic 
lantern  objective. 

E     Drawing  paper  under  the  mirror. 


In  figure  182  is  shown  a  neat  and  efficient  arrangement  designed 
by  Dr.  B.  F.  Kingsbury,  in  which  the  shelf  is  hinged  so  that  it  can 
be  lowered  out  of  the  way  when  using  the  drawing  table  with 
attached  45  degree  mirror.  The  guides  for  sliding  the  shelf  up  or 
down  and  clamping  it  in  any  desired  position,  are  of  metal  and  can 
be  attached  to  any  table  whether  the  legs  are  square,  tapering  or 
of  any  other  form. 


328  RADIANTS  FOR  DRAWING  [Cn.  X 

RADIANTS  FOR  DRAWING  APPARATUS 

§  461.  General. — The  best  light  for  projection  is  naturally  the 
best  light  for  drawing  with  projection  apparatus.  One  must 
always  keep  in  mind  that  a  rather  dim  light  in  a  perfectly  dark 
room,  after  one  has  been  long  enough  in  it  to  acquire  twilight 
vision,  may  seem  quite  brilliant.  The  old  observers  with  their 
very  dim  artificial  lights  understood  this  well,  and  did  much  with 
projection  apparatus  which  at  first  sight  would  seem  impossible  to 
us. 

The  electric  arc  and  other  brilliant  artificial  lights  are  so  common 
at  the  present  that  many  have  come  to  feel  that  they  cannot  see  at 
all  unless  the  object  is  flooded  with  light.  But,  excepting  those 
who  are  night-blind,  that  is,  have  poor  twilight  vision,  much  can  be 
done  with  the  Welsbach  mantle  light,  the  alco-radiant  mantle 
light,  etc.  Even  a  kerosene  lamp  of  good  quality  is  very  service- 
able, but  one  must  always  keep  in  mind  that  the  dimmer  the  light- 
source,  the  darker  must  be  the  work-room,  and  the  more  care  must 
be  taken  to  avoid  stray  light.  Too  high  powers  should  not  be  used 
with  weak  lights.  For  high  power  drawing  very  brilliant  light  is 
necessary. 

§  462.  Arc  lamp  with  direct  current. — This  is,  of  all  the 
artificial  sources,  the  most  satisfactory  for  drawing,  as  for  projec- 
tion (fig.  3).  With  it  the  drawing  room  need  not  be  very  dark,  and 
one  can  obtain  sufficient  light  for  the  highest  powers  with  which  it 
is  desirable  to  draw.  Ordinarily  a  5-10  ampere  current  is  sufficient 
(see  also  §  485) .  If  low  amperages  are  used  the  apparatus  is  not  so 
greatly  heated  as  with  higher  amperages,  and  furthermore  the 
specimens  are  less  liable  to  injury  from  overheating. 

The  same  lamp  that  is  used  for  projection  is  suitable  for  drawing. 
There  is  some  advantage  in  having  an  automatic  arc  lamp,  then  the 
artist  will  not  have  to  bother  about  the  lamp  except  to  supply  it 
with  proper  carbons,  and  to  see  that  they  are  in  proper  position. 
With  the  hand-feed  arc  lamps  the  carbons  must  be  brought  closer 
together  about  every  3-5  minutes.  It  is  a  convenience  if  the  artist 


CH.  X]  DRAWING  WITH  THE  MAGIC  LANTERN  329 

has  some  sort  of  device,  like  a  Hooke's  jointed  rod,  so  that  the  lamp 
may  be  adjusted  without  getting  up  (see  fig.  43). 

For  the  arc  lamp  on  the  house  circuit  see  Ch.  Ill  and  §  486  below. 

§  463.  Other  radiants  for  drawing. — Any  of  the  sources  of  light 
discussed  in  the  first  six  chapters  can  be  used  for  drawing.  One 
must  use  the  precautions  given  in  those  chapters  for  getting  a  good 
screen  image  by  a  proper  alignment  and  separation  of  the  elements 
of  the  apparatus,  and  by  suiting  the  darkness  of  the  room  to  the 
light. 

DRAWING  WITH  THE  MAGIC  LANTERN 

§  464.  Drawing  wall  diagrams. — The  simplest  form  of  projec- 
tion for  drawing  is  with  the  magic  lantern.  With  it  the  preparation 
of  wall  diagrams  is  very  easy  (fig.  185). 

If  one  has  a  lantern  slide  of  the  picture  or  object  to  be  drawn  it  is 
put  into  the  lantern  as  for  ordinary  projection.  The  drawing- 
board  is  then  arranged  at  a  distance  to  give  the  desired  size,  and 
then  all  the  lines  traced  with  a  crayon,  a  brush  or  a  coarse  pen.  One 
can  use  water  colors  or  paints.  For  the  black  nothing  is  better 
than  India  ink. 

If  one  has  a  smooth  wall  to  which  the  drawing  paper  or  cloth  can 
be  fastened,  then  the  lantern  can  be  moved  closer  or  farther  away 
to  get  the  desired  size. 

If  one  has  no  lantern  slide,  then  a  negative  may  be  made  of  the 
subject  to  be  drawn,  and  the  negative  used  in  the  lantern  instead  of 
the  lantern  slide.  The  negative  should  not  be  too  dense  or  the 
lines  will  not  come  out  clearly. 

For  making  negatives  to  draw  from,  it  is  advantageous  to  use 
lantern-slide  dry  plates.  These  will  be  of  the  right  size  for  the 
lantern  and  are  more  transparent  than  ordinary  negatives. 

For  lettering  diagrams  nothing  is  more  convenient  than  the  large 
rubber  type  found  in  sets  used  in  advertising  and  sign  making. 

§  465.  Getting  the  desired  size. — Any  desired  size  may  be 
obtained  by  varying  the  distance  between  the  drawing  surface  and 
the  projection  objective.  Either  the  lantern  or  the  drawing  surface 
or  both  must  be  movable. 


330 


DRAWING  WITH  THE  MAGIC  LANTERN 


[Cn.  X 


The  size  of  the  drawing  can  be  varied  without  moving  the  lantern 
or  the  drawing  surface  by  using  an  objective  of  longer  focus  for  a 
smaller  diagram,  or  of  a  shorter  focus  for  a  larger  diagram  (see  also 
§  507). 

§  466.  Use  of  the  magic  lantern  for  small  drawings. — It  fre- 
quently happens  that  a  small  drawing  of  some  large  object  is 
needed  for  publication.  This  may  be  some  natural  object  or  a 
piece  of  apparatus.  The  object  or  piece  of  apparatus  is  placed  in  a 
good  light  and  a  small  negative  made  on  a  lantern-slide  plate,  being 
careful  not  to  make  the  negative  too  dense.  After  this  is  dry,  it 
can  be  put  into  the  lantern-slide  carrier  and  projected  upon  the 
drawing  paper,  and  the  outlines  accurately  traced.  Then  with  a 
pen  and  India  ink  one  can  ink  in  the  lines  and  add  any  necessary 
shading  free-hand,  having  the  object  or  piece  of  apparatus  in  view 
so  that  it  can  be  accurately  done.  The  exact  magnification  or 


Condenser 


KS 


FIG.  185.     SIMPLEST  FORM  OF  MAGIC  LANTERN  WITH  ARC  LIGHT  FOR  USE 

IN  DRAWING. 

SW    Supply  wires. 

So  K     Socket  with  its  key  switch. 

5 P     Separable  attachment  plug. 

LW    Wires  extending  from  the  cap  of  the  plug  to  the  knife  switch. 

KS    Knife  switch  for  turning  the  current  on  and  off. 

Rheostat     The  balancing  device  for  regulating  the  current. 

Arc  Lamp     The  arc  lamp  with  right-angled  carbons. 

Condenser  The  two-lens  condenser  with  the  first  (i)  and  the  second  (2) 
elements. 

LS    Position  of  the  lantern  slide  or  other  large  object. 

Objective  with  c,  its  center. 

Axis  Axis  The  principal  optic  axis  of  the  condenser  and  the  objective. 
The  radiant  must  be  centered  on  this  axis. 

Image  Screen     The  drawing  surface  on  which  the  image  is  projected. 


CH.  X]  DRAWING  WITH  THE  MAGIC  LANTERN  331 

reduction  of  the  picture  can  be  determined  by  photographing  a 
metric  or  other  measure  (fig.  178)  on  the  same  plate  with  the 
object  or  piece  of  apparatus. 

§  467.     Size  of  condenser  necessary  for  making  drawings.— 

When  lantern  slides,  or  negatives  made  on  lantern-slide  plates  or 
other  plates  of  that  size  are  used,  the  condenser  of  any  magic  lan- 
tern will  answer.  Sometimes,  however,  it  is  desired  to  make  dia- 
grams or  drawings  from  negatives  of  larger  size.  There  are  two 
ways  of  accomplishing  this: 

(1)  A  lantern  slide  can  be  made  from  the  large  negative  by  the 
aid  of  a  photographic  objective  as  described  in  Ch.  VIII,  §  329. 
This  can  then  be  used  in  the  ordinary  lantern. 

(2)  If  the  large  negative  is  to  be  used  direct,  then  the  condenser 
of  the  magic  lantern  must  be  of  sufficient  size  to  illuminate  the 
negative.     That  is,  the  condenser  must  have  a  diameter  a  little 
greater  than  the  diagonal  of  the  negative  to  be  illuminated  and 
drawn  (see  fig.  114). 

§  468.  Drawing  on  a  horizontal  surface  by  the  aid  of  the  magic 
lantern. — This  is  easily  accomplished  by  using  a  45  degree  mirror 
or  a  prism  beyond  the  objective  (fig.  192). 

One  must  be  careful  to  put  the  negative  or  lantern  slide  in  the 
carrier  in  such  a  way  as  to  give  an  erect  image  (§  512). 

If  the  negative  or  lantern  slide  or  other  object  is  too  dense,  so 
that  the  light  is  relatively  dim,  the  image  will  be  duplicated  when  a 
mirror  silvered  on  the  back  is  used,  therefore,  one  must  use  a  prism 
or  a  mirror  silvered  on  the  face  for  these  dark  objects.  For  very 
transparent  objects  the  image  appears  single  even  with  a  mirror 
silvered  on  the  back,  the  silver  image  being  so  much  brighter  than 
the  glass  image  that  the  latter  does  not  show. 

One  can  use  the  magic  lantern  and  separate  table  with  a  45  degree 
mirror  (fig.  182)  or  the  mirror  can  be  fastened  to  the  projection 
table  as  in  Dr.  Riley's  device  (fig.  184)  or  the  mirror  may  be  close 
to  the  objective,  and  the  adjustable  drawing  shelf  used  (fig. 
183). 


332  DRAWING  WITH  THE  EPISCOPE  [Cn.  X 

DRAWING  WITH  THE  EPISCOPE  OR  REFLECTING  LANTERN 

§  469.  If  one  has  access  to  a  lantern  for  opaque  objects  (Chap. 
VII),  diagrams  may  be  made  from  pictures  in  books  and  from  suit- 
able objects  without  the  trouble  of  making  a  negative  or  a  lantern 
slide.  The  object  is  put  in  position  in  the  reflecting  lantern  and  its 
image  thrown  upon  the  drawing  surface.  It  can  then  be  traced 
as  for  a  lantern-slide  image,  and  the  details,  shading  and  lettering 
added  as  described  for  diagrams  made  from  lantern  slides  or  from 
negatives  (§464)- 

§  470.  Drawing  on  a  horizontal  surface  by  the  aid  of  the  opaque 
lantern. — If  the  apparatus  is  suitably  arranged,  the  mirror  will 
throw  the  image  downward  upon  a  horizontal  surface  instead  of  out 
horizontally.  Then  the  tracing  can  be  made  as  for  a  lantern  slide 
(§  468).  There  is  one  difficulty  with  the  reflecting  lantern  in  mak- 
ing drawings.  If  the  object  to  be  drawn  is  of  some  thickness,  only 
a  part  of  it  will  be  in  focus  at  any  one  time,  hence  it  is  not  easy  to 
get  the  parts  in  true  perspective.  (For  erect  images  see  §  514). 

If  one  makes  a  small  negative  with  a  good  objective,  the  perspec- 
tive will  be  good  and  all  the  parts  will  be  in  focus. 

When  this  negative  is  projected  upon  the  drawing  surface  with  an 
ordinary  lantern,  all  the  parts  of  the  image  will  be  in  focus. 

If  one  wishes  drawings  of  flat  objects,  pictures  in  books,  etc.,  the 
opaque  lantern  answers  admirably,  but  heavy  currents  are  re- 
quired, and  it  is  not  so  safe  for  inexperienced  persons  as  the  magic 
lantern  with  a  small  current  and  a  negative  or  a  lantern  slide  (see 
further  in  Ch.  VII,  §  290). 

DRAWING  WITH  A  PHOTOGRAPHIC  CAMERA 

§  471.  The  drawing  of  enlargements  or  reductions  of  opaque 
objects  with  the  photographic  camera  has  been  much  practised. 
The  object  is  put  in  a  good  light  and  arranged  to  show  the  desired 
aspect,  then  a  photographic  camera  is  directed  toward  it,  and  the 
bellows  lengthened  or  shortened  until  the  picture  on  the  ground- 
glass  focusing  screen  is  of  the  desired  size.  Then  the  plate  holder 
with  a  clear  glass  or  a  focusing  screen  of  clear  glass  is  used  and  over 


CH.  X]       DRAWING  WITH  CAMERA  AND  MICROSCOPE  333 

it  some  tracing  paper.  By  covering  the  head  with  a  focusing  cloth 
to  shut  out  the  surrounding  light,  one  can  trace  the  outlines  of  the 
object  on  the  tracing  paper,  and  transfer  these  to  ordinary  drawing 
paper,  and  proceed  to  ink  them  in  and  give  the  shading  necessary 
free-hand. 

With  the  magic  lantern  or  with  the  opaque  lantern  the  image  is 
projected  upon  the  drawing  surface  and  regular  drawing  paper  can 
be  used  to  make  the  original  pencil  tracing  upon,  but  with  the 
camera  one  must  use  translucent  paper  for  the  tracing  and  then 
transfer  it  to  the  drawing  paper.  (To  get  an  erect  image  with 
translucent  paper  see  §  519). 

DRAWING  WITH  THE  PROJECTION  MICROSCOPE 

§  472.  Range  of  objects. — For  drawing  as  for  projection  it  is 
exceedingly  desirable  that  the  projection  microscope  should  enable 
the  investigator  to  commence  where  the  magic  lantern  leaves  off, 
and  to  carry  the  work  to  its  utmost  possibilities;  that  is,  begin- 
ning with  large  specimens  of  50  to  60  mm.  (2  in.)  in  diameter  re- 
quiring low  objectives,  and  going  on  from  this  to  the  smallest 
objects  visible  and  using  the  oil  immersion  objective  at  the  other 
extreme. 

To  realize  this  ideal  possibility  one  must  have  available  for 
drawing  some  such  outfit  as  that  described  in  Ch.  IX  for  projec- 
tion ;  and  in  addition  suitable  arrangements  for  reflecting  the  image 
down  upon  a  horizontal  drawing  surface.  Fortunately,  the  addi- 
tions are  relatively  simple  and  inexpensive. 

Finally,  for  the  widest  usefulness  in  drawing  there  must  be  the 
possibility  of  using  the  ordinary  house  electric  lighting  system  for 
an  electric  lamp  with  small  carbons  (see  §  486). 

§  473.  Drawing  large  objects  with  low  powers. — For  this  it  is 
necessary  to  have  a  stage  with  a  large  opening  (fig.  134),  and  the 
objective  must  be  mounted  in  a  shield  with  no  tube  at  all  (fig.  138), 
or  the  tube  must  be  short  and  of  large  diameter,  so  that  the  field  is 
not  restricted  (fig.  137).  Finally,  there  must  be  some  means  of 
increasing  or  diminishing  the  distance  between  the  objective  and 
the  drawing  surface  to  get  the  desired  magnification. 


334  DRAWING  WITH  PROJECTION  MICROSCOPE  [Cn.  X 


•   h 


FIG.  1 86. 


APPARATUS  FOR  DRAWING  WITH  THE  MICROSCOPE  WITHOUT 
AN  OCULAR  OR  SUBSTAGE  CONDENSER. 


The  arc  lamp  is  Mr.  Albert  T.  Thompson's  automatic  lamp  for  direct  current, 
5-25  amperes.  This  is  the  first  automatic  arc  lamp  for  right-angled  carbons. 

By  means  of  the  optical  bench  carrying  all  the  apparatus,  the  different 
parts  are  pulled  forward  so  that  the  microscope  tube  and  mirror  project  over 
the  drawing  shelf.  This  is  adjustable  up  and  down  for  varying  the  magnifi- 
cation. 

The  stage  of  the  microscope  (st)  is  independent  and  contains  a  large  glass 
water-cell  against  which  the  specimen  rests.  It  conducts  away  the  heat  from 
the  specimen. 

a  a  a     Optic  axis. 

b     Optical  bench  with  track. 

c     Triple  condenser  with  water-cell. 

/     Thompson's  automatic  arc  lamp  for  5-25  amperes  direct  current. 

m  Microscope  without  ocular.  The  45°  mirror  reflects  the  light  down  upon 
the  drawing  surface. 

r     Adjustable  rheostat. 

5     Double-pole  knife  switch  (table  switch). 

st  Stage  with  the  stage  water-cell  for  cooling  the  specimen.  The  stage  is 
entirely  separate  from  the  microscope. 

sh  Shield  25  cm.  in  diameter  to  stop  any  stray  light  from  the  stage  of  the 
microscope. 

wi     Double  cable  supplying  the  electric  current  to  the  apparatus. 

iv 2     Flexible  cables  from  the  switch  to  the  lamp. 


CH.  X]  DRAWING  WITH  PROJECTION  MICROSCOPE  335 

§  474.  Varying  the  drawing  distance. — The  drawing  distance 
is  easily  varied  by  means  of  a  movable  table  like  that  figured  (fig. 
182),  or  by  an  adjustable  shelf  attached  to  the  projection  table  (fig. 

183). 

Another  way  of  varying  the  size  of  the  drawing  is  to  use  higher  or 
lower  objectives,  the  drawing  distance  remaining  the  same  (see 

§  HO?)- 

§  475.  Lighting  the  object. — For  large  objects  and  low  powers 
the  best  way  to  illuminate  the  object  is  to  use  the  main  condenser 
only  and  to  put  the  object  in  the  cone  of  light  where  it  is  fully 
illuminated  (fig.  132).  If  the  drawing  shelf  is  used  this  will  involve 
moving  the  lamp  and  condenser  toward  the  drawing-board;  for  the 
microscope  must  be  beyond  the  end  of  the  table,  so  that  the  image 
can  be  thrown  down  on  the  shelf,  (fig.  186).  The  change  in  posi- 
tion of  any  part  or  parts  is,  of  co.urse,  very  easy  with  an  optical 
bench  (fig.  158-159). 

§  476.     Drawings  with  objectives  of  16,  12,  10,  and  8  mm.— 

With  objectives  of  this  range  without  an  ocular,  one  can  draw 
objects  varying  from  5  to  2  mm.  in  diameter.  For  lighting,  use  the 
large  condenser  and  focus  the  image  of  the  crater  on  the  hood  of  the 
objective  (fig.  140),  and  then  push  the  stage  up  toward  the  objec- 
tive until  the  object  is  in  focus,  finishing  the  fine  focusing  with  the 
micrometer  screw  of  the  microscope. 

DRAWING  WITH  THE  PROJECTION  MICROSCOPE,  INCLUDING    AN 
OCULAR  AND  A  SUBSTAGE  CONDENSER. 

§  477.    Drawing  fine  details  with  high  powers  (8  to  2  mm.  focus) . 

— As  pointed  out  for  the  projection  of  images  showing  fine  details 
(§  401),  it  is  necessary  to  use  a  substage  condenser  to  get  the  neces- 
sary aperture  of  the  lighting  beam,  and  to  use  an  ocular  to  com- 
pensate for  objective  defects.  If  one  uses  a  water  or  an  oil  immer- 
sion objective  the  proper  immersion  fluid  must  be  used  between  the 
cover-glass  and  the  objective,  as  in  ordinary  microscopic  work. 

§  478.  Parallelizing  the  converging  beam  of  light. — The  sub- 
stage  condenser  used  for  ordinary  observation  is  designed  for  ap- 


336 


DRAWING  WITH  HIGH  POWERS 


[Cn.  X 


FIG.    187.     AN   ORDINARY   MICROSCOPE   USED   WITH   THE   LAMP   AND 
CONDENSER  OF  A  MAGIC  LANTERN  FOR  DRAWING  OR  PROJECTION. 

W    The  supply  cable  from  the  outlet  box  (fig.  3). 

5     The  table,  knife  switch. 

r     Rheostat  of  the  theater-dimmer  type. 

/  The  automatic  arc  lamp.  This  is  the  three- wire  automatic  arc  lamp  of 
the  Bausch  &  Lomb  Optical  Company  for  5-25  amperes.  (For  the  method 
of  connecting  the  wires  see  §  704). 

c    Triple  condenser  with  water-cell. 

a  a     Principal  optic  axis. 

p  The  concave  parallelizing  lens  to  render  the  converging  cone  from  the 
condenser  parallel  or  nearly  so  before  entering  the  sub  stage  condenser  of  the 
microscope. 

m  The  microscope  in  a  horizontal  position.  If  it  is  to  be  used  for  drawing 
there  must  be  a  prism  or  mirror  beyond  the  ocular  to  reflect  the  light  down  on 
the  drawing  shelf. 

b     Baseboard  with  track  serving  as  an  optical  bench. 

a  s     Adjustable  drawing  shelf  on  the  front  of  the  projection  table. 

proximately  parallel  light  (fig.  150  A.  B.),  hence  it  is  necessary  to 
render  the  converging  cone  of  light  from  the  main  condenser 
approximately  parallel.  This  is  most  easily  accomplished  by  using 


CH.  X]  DRAWING  WITH  HIGH  POWERS  337 

a  plano-concave  or  double-concave  lens.  This  is  mounted  in  a 
fork-like  holder  and  is  set  in  the  socket  for  the  mirror  stem  of  the 
microscope  (fig.  152,  187).  Then  the  microscope  is  pushed  up 
toward  the  condenser  until  the  parallel  beam  is  of  sufficient  diam- 
eter to  fill  the  substage  condenser.  The  substage  condenser 
diaphragm  is  opened  to  its  full  extent. 

§  479.  Concave  lens  to  be  employed. — This  depends  upon  the 
focus  of  the  main  condenser.  If  the  focus  is  about  15  cm.  (6  in.),  use 
a  concave  lens  of  -i  6  to  -20  diopters  (§356).  If  the  main  condenser 
has  a  focus  of  20  to  40  cm.  (8  to  16  in.),  use  a  concave  lens  of  -8  to 
-12  diopters.  The  longer  the  focus  of  the  main  condenser  the 
shallower  can  be  the  concavity  of  the  parallelizing  lens.  Indeed, 
for  objectives  of  16,  12,  10,  and  8  mm.  focus  a  condenser  lens  of  25 
to  38  cm.  (10  to  15  inches)  focus  gives  very  good  results,  when 
the  substage  condenser  is  used  without  any  parallelizing  lens 
(%•  154). 

§  480.  Position  of  the  substage  condenser;  opening  of  the 
condenser  diaphragm. — As  pointed  out  in  Ch.  IX  (§  407),  the  posi- 
tion of  the  substage  condenser  must  be  very  precisely  determined 
for  different  objectives  and  for  different  thickness  of  slides. 

To  begin  with,  the  substage  condenser  diaphragm  is  opened  to 
its  full  extent.  Then  in  each  case  one  must  get  the  sharpest  possi- 
ble image  by  getting  the  best  position  of  the  substage  condenser, 
and  closing  the  diaphragm  more  or  less.  As  a  general  statement, 
the  diaphragm  should  be  considerably  wider  open  for  drawing 
than  for  ordinary  observation. 

§  481.     Oculars  to  employ  for  drawing. — Those  of  X2,  X3,  X4,  x6, 

x8,  and  xi2  may  be  used.  Naturally,  the  lower  and  medium 
powers  give  the  more  brilliant  images  as  for  direct  observation. 
One  will  rarely  need  to  use  an  ocular  higher  than  x8. 

§  482.  Mirror  or  prism  for  reflecting  the  image-forming  rays 
down  upon  the  drawing  surface. — For  high  power  drawing  it  is 
better  to  have  the  reflecting  mirror  or  prism  close  to  the  ocular 
(fig.  192)  rather  than  to  have  it  distant,  as  with  the  drawing  table 
in  figure  182. 


,338  DRAWING  WITH  HIGH  POWERS  [Cn.  X 

If  a  mirror  is  used  it  must  be  a  perfect  one  and  preferably  slivered 
on  the  face  to  avoid  duplicating  the  images.  If  it  is  silvered  on  the 
back  the  glass  must  be  thin.  A  totally  reflecting  prism  is  best, 
but  it  is  somewhat  expensive,  costing  about  twice  as  much  as  the 
mirror. 

§  483.  Avoidance  of  distortion. — Whichever  is  used  for  reflect- 
ing, it  should  be  fitted  with  a  stop  so  that  it  will  be  at  45  degrees 
with  the  main  axis,  then  the  image-forming  rays  will  be  reflected 
directly  downward  and  the  image  will  not  be  distorted,  provided 
of  course,  that  the  mirror  or  prism  is  directly  above  the  drawing 
surface.  If  it  were  turned  over  to  one  side  more  or  less,  the  image 
would  be  correspondingly  distorted. 

It  is  a  good  plan  for  one  to  become  familiar  with  the  distortions 
possible  in  drawing.  For  example,  if  the  mirror  or  prism  is  not  at 
45  degrees  with  a  horizontal  microscope  (fig.  182,  193),  the  spot  of 
light  on  the  drawing  surface  will  not  be  circular  but  elliptic,  the  axis 
of  the  ellipse  being  parallel  with  the  optic  axis  of  the  microscope. 
If  the  prism  or  mirror  is  not  directly  above,  but  turned  to  one  side, 
then  the  spot  of  light  will  be  elliptic  and  projected  to  one  side  of  the 
axis  of  the  microscope.  If  one  is  familiar  with  the  possible  dis- 
tortions it  will  be  easy  to  detect  them ;  then  they  can  be  corrected. 
Naturally,  a  drawing  should  be  accurate  when  finished. 

§  484.     Specimens  suitable  for  drawing  with  high  powers. — Any 

object  suitable  for  projection  can  have  its  image  projected  upon  a 
drawing  surface  (see  also  §  410). 

§  485.    Amount  of  electric  current  required  for  drawing. — If  one 

has  a  direct  current,  5  to  10  amperes  will  be  sufficient  for  all  draw- 
ing purposes.  The  specimens  must  usually  be  left  for  a  consider- 
able time  in  the  focus  or  near  the  focus  of  the  light  beam,  and  hence 
are  liable  to  overheating.  The  lower  the  amperage  the  less  the 
danger  from  the  overheating.  Then  it  is  not  good  for  the  eyes 
of  the  artist  to  have  the  light  on  the  drawing  surface  too  dazzling. 

With  alternating  current,  6  to  15  amperes  usually  suffice. 

Here,  as  in  all  other  projection,  skill  is  of  more  account  than 
overwhelming  electric  currents. 


CH.  X] 


DRAWING  WITH  HOUSE  CURRENT 


339 


PROJECTION  DRAWING  APPARATUS  WITH  THE  RADIANT 
CONNECTED  WITH  THE  HOUSE  LIGHTING  SYSTEM. 

§  486.  General  Statement. — As  shown  in  Chapter  III  (fig. 
41-43),  the  arc  lamps  using  small,  cored  carbons  (6  to  8  mm.  in 
diameter)  and  drawing  from  three  to  six  amperes  may  be  connected 
with  any  socket  for  an  incandescent  bulb  of  the  house  lighting 
system.  The  light  so  obtained  is  more  powerful  than  the  usual 
lime  light.  The  carbons  being  small,  the  light  approaches  closely 
to  the  ideal  point  source.  Consequently  for  all  projection  pur- 
poses, including  drawing,  this  form  of  arc  light  is  of  the  greatest 
importance  and  utility.  Of  course,  for  projection  in  a  large  hall 
it  is  insufficient,  but  for  the  relatively  small  screen  pictures  needed 
in  drawing  and  for  small  classes,  the  results  are  very  satisfactory. 

§  487.  Wiring,  rheostat  and  connections  for  the  arc  lamp 
attached  to  the  house  lighting  system. — This  is  shown  in  fig.  188- 
189  and  described  in  §  128-135.  Remember  and  practice  the 
advice  given  about  turning  the  current  on  and  off  (§  133),  and  the 
possibility  of  short  circuiting  and  burning  out  the  incandescent 
bulb  socket.  Never  use  an  arc  lamp  without  a  suitable  rheostat 
or  inductor.  (See  §  129,  also  §  i28a  for  fuses  on  the  house  system). 


Lamp  Socket 


FlG.    1 88. 


WIRING  AND  CONNECTIONS  OF  THE  ARC  LAMP  USED  ON  THE 
HOUSE  LIGHTING  SYSTEM  (See  fig.  45). 


340 


DRAWING  WITH  HOUSE  CURRENT 


[Cn.  X 


FIG.  189.    SMALL  ARC  LAMP  FOR  DRAWING. 

Commencing  at  the  left: 

Wi     Supply  wires 

So     Lamp  socket. 

K     Key  switch  in  the  socket. 

Sp     Separable  attachment  plug. 

W2     Wires  to  the  arc  lamp. 

W3     Wire  to  the  binding  post  of  the  upper  carbon. 

W4  Wire  to  the  rheostat  (R)  and  from  the  rheostat  to  the  binding  post 
of  the  lower  carbon. 

A  Support  of  the  arc  lamp;  the  lamp  can  be  raised  or  lowered  on  this 
support. 

F    Feeding  screws  for  the  carbons. 

H  C,  V  C    The  horizontal  and  the  vertical  carbons. 

In  In  Insulation  between  the  carbon  holders  and  the  remainder  of  the 
lamp.  This  prevents  the  current  from  taking  any  path  away  from  the  carbons. 

Ch     Chimney  over  the  arc. 

T  C  The  tube  and  the  condenser  in  the  movable  inner  tube.  The  con- 
denser is  at  its  focal  distance  from  the  crater,  and  therefore  the  rays  are  made 
parallel. 


CH.  X]  DRAWING  WITH  HOUSE  CURRENT  341 

Sh     Shield  to  stop  stray  light  and  to  aid  in  centering. 

C    Carbons  with  alternating  current.     They  are  of  the  same  size. 

D  Carbons  with  direct  current.  The  upper  one  is  8  mm.  and  the  lower  one 
6  mm.  in  diameter. 

E  Shield  or  disc  at  the  end  of  the  condenser  tube  showing  the  opening  of 
the  condenser  (C)  and  the  spot  of  light  at  the  right. 

§  488.  Arc  lamp  and  small  carbons. — The  form  of  arc  lamp  to 
use  on  the  house  circuit  is  not  of  particular  importance.  It  may 
be  very  conveniently  one  of  the  small  lamps  shown  in  fig.  41—44, 
201,  205,  or  it  can  be  an  ordinary  arc  lamp  for  greater  currents, 
but  supplied  with  long  clamping  screws,  bushings  or  adapters  for 
the  small  carbons  (§  127).  The  small  lamps  are  generally  of  the 
hand-feed  type  and  move  the  upper  and  the  lower  carbons  equally. 

§  489.  Size  of  carbons  for  direct  current. — A. — The  carbons 
found  useful  for  direct  current  are  as  follows,  all  being  of  the  soft- 
cored  variety: 

(1)  Upper  or  positive  carbon  7  mm.  in  diameter,  lower  or  nega- 
tive carbon  5  mm. 

(2)  Upper  carbon  8  mm.,  lower  6  mm. 

(3)  Upper  carbon  u  mm.,  lower  8  mm. 

B. — The  carbons  for  alternating  current  with  an  equal  feed  for 
the  upper  and  the  lower  carbon,  should  be  of  the  same  size,  and  this 
size  should  not  exceed  8  mm.  in  diameter  for  5  to  6  amperes.  If 
only  three  or  four  amperes  are  used,  then  it  is  better  to  have  carbons 
not  greater  than  6  mm.  in  diameter. 

§  490.  Reason  for  using  small  carbons. — In  order  to  have  the 
light  steady  and  thus  have  the  field  continuously  bright,  the  entire 
end  of  the  upper  carbon  should  be  white  hot. 

If  the  carbon  is  so  large  that  the  crater  covers  only  a  part  of  the 
tip,  the  crater  will  wander  about  on  the  end  of  the  carbon.  Every 
change  in  the  position  of  the  crater  changes  the  direction  of  the 
light,  beam.  While  the  crater  is  in  one  position  the  entire  field  of  a 
high  power  objective  may  be  brilliantly  illuminated;  if  the  crater 
wanders  to  a  new  position,  the  field  will  be  only  partly  or  not  at  all 
illuminated.  In  such  a  case,  one  must  constantly  change  the  posi- 
tion of  the  mirror  of  the  microscope  to  keep  the  field  bright.  If, 
however,  the  crater  is  nearlv  as  large  as  the  end  of  the  carbon,  it 


342 


DRAWING  WITH  HOUSE  CURRENT 


[Cn.  X 


will  wander  but  little,  if  at  all,  and  the  light  will  be  more  con- 
stant. 

§  491.  Feeding  the  carbons  together. — If  one  has  an  alternat- 
ing current  to  work  with,  the  small  arc  lamp  will  burn  about  10 
minutes  with  8  mm.  carbons  before  going  out.  With  the  right- 
angle  position  the  carbon  giving  the  light  remains  constantly  in 
the  axis.  With  inclined  carbons,  it  rises  constantly  above  the  axis. 
The  carbons  with  the  right-angle  arc  should  be  fed  about  every 
five  to  seven  minutes  to  insure  the  best  light. 


FIG.  190.     SIDE  AND  FRONT  VIEW  OF  SMALL  CARBONS  WITH  FIVE 
AMPERES  OF  DIRECT  CURRENT  (Natural  Size).    Compare  fig.  191. 


FIG.  191.     SIDE  AND  FRONT  VIEW  OF  SMALL  CARBONS  WITH  FIVE  AMPERES 
OF  ALTERNATING  CURRENT  (Natural  Size). 

The  crater  is  much  smaller  than  with  direct  current  (fig.  190). 


CH.  X]  DRAWING  WITH  HOUSE  CURRENT  343 

If  direct  current  is  used,  the  lamp  will  burn  for  about  six 
minutes  and  the  carbons  should  be  fed  together  every  three  to  five 
minutes.  (See  fig.  205). 


CONDENSER,  STAGE  AND  MICROSCOPE  FOR  DRAWING  WITH  THE 
HOUSE  LIGHTING  SYSTEM 

§  492.  Drawing  outfit. — If  one  has  a  drawing  outfit  consisting 
of  the  projection  apparatus  shown  in  figure  182,  all  that  is  necessary 
to  do  is  to  place  the  arc  lamp  with  its  small  carbons  in  the  lamp- 
house  and  arrange  it  exactly  as  for  projection. 

The  procedure  is  precisely  as  described  above  for  the  ordinary 
arc  lamp  on  the  usual  special  lantern  lighting  system  (Ch.  IX). 

§  493.  Small  Current  Outfit. — This  consists  of  an  arc  lamp 
using  small  carbons  (6  to  8  mm.  in  diameter)  and  a  rheostat  or  an 
inductor  (fig.  197)  not  allowing  over  5  to  6  amperes  of  current  to 
flow.  Instead  of  the  usual  large  condenser  (fig.  121),  a  small,  single, 
convex  lens  is  used.  This  is  of  70  to  100  mm.  (3  to  4  in.)  focus, 
and  37  to  50  mm.  (i>£  to  2  in.)  in  diameter,  and  is  placed  in  a  tube 
extending  straight  out  from  the  upper  carbon.  Usually,  also,  the 
lens  is  in  a  sliding  tube,  so  that  it  may  be  varied  in  distance  from 
the  source  of  light.  If  it  is  at  its  focal  distance  from  the  light,  the 
beam  will  be  approximately  parallel  (fig.  189);  if  farther  from  the 
light,  the  beam  will  be  converging. 

§  494.     Method  of  using  the  lamp  with  a  special  condenser. — 

There  are  three  methods  of  using  this  arc  lamp  and  special  con- 
denser : 

(1)  The  lamp  can  be  put  in  line  with  the  drawing  microscope 
and  a  converging  beam  thrown  directly  on  the  specimen  as  for  the 
large  apparatus  (fig.  132),  the  mirror  and  sometimes  the  substage 
condenser  having  been  removed  or  turned  aside. 

(2)  The  mirror  is  removed  from  the  microscope,  but  the  sub- 
stage  condenser  is  left  in  position,  and  a  parallel  beam  of  light 
thrown  directly  into  the  substage  condenser  along  the  optic  axis 

(fig.  20lA). 


344 

Substage 
Condenser 


DRAWING  WITH  HOUSE  CURRENT 


[Cn.  X 


FIG.  192. 


DIAGRAM  OF  THE  MICROSCOPE  ARRANGED  FOR  DRAWING  ON  A 
HORIZONTAL  SURFACE. 


The  light  is  from  an  arc  lamp  supplied  by  the  house  current  (fig.  188,  189). 

A  right-angled  prism  is  used  to  reflect  the  rays  down  upon  the  drawing  sur- 
face. 

The  designations  are  self  explanatory  except  in  the  ocular  r  i  means  the  real 
image  formed  by  the  objective  and  field  lens  (see  fig.  207). 

The  adjustment  screw  heads  at  the  side  of  the  microscope  are: 

/  a     Fine  adjustment. 

c  a     Coarse  adjustment. 

H    The  handle  in  the  pillar  for  carrying  the  microscope. 


(3)  From  the  difficulty  of  getting  the  small  lamp  and  condenser 
in  the  optic  axis  without  the  use  of  an  optical  bench  it  has  been 
found  much  easier  to  get  the  light  upon  the  specimen  and  through 
the  microscope  by  placing  the  arc  lamp  and  its  condenser  at  right 
angles  to  the  microscope,  and  to  use  the  regular  microscope  mirror 
for  reflecting  the  beam  through  the  substage  condenser  (fig.  193). 
If  the  substage  condenser  is  not  used,  the  mirror  reflects  the  beam 
directly  on  the  specimen,  as  for  low  power  projection. 


CH.  X]  DRAWING  WITH  HOUSE  CURRENT  345 

§  495.  Microscope. — Any  modern  microscope  with  a  good  sub- 
stage  condenser  can  be  used,  provided  it  is  supplied  with  a  flexible 
pillar,  so  that  the  tube  can  be  made  horizontal;  and  provided  also, 
that  the  fine  adjusting  mechanism  will  work  when  the  tube  is 
horizontal. 

There  must  be  a  prism  or  mirror  beyond  the  ocular  to  reflect  the 
image-forming  rays  downward  upon  the  drawing  surface  (fig.  192). 

The  discussion  of  avoidance  of  distortion,  the  proper  objectives, 
oculars,  etc.,  to  use,  which  was  given  in  the  earlier  part  of  this 
chapter  apply  here  (§  452,  483). 

§  496.  Position  of  the  microscope  for  drawing. — In  the  drawing 
outfits  thus  far  devised,  the  microscope  is  placed  in  one  of  the 
following  positions : 

(1 )  In  an  inverted  position  with  the  objective  pointing  directly 
upward  (as  in  the  large  Edinger  apparatus,  fig.  202). 

(2)  Inclined  at  45  degrees  (as  in  the  small  Edinger  apparatus, 
fig.  204). 

(3)  In  a  horizontal  position  (fig.  192). 

With  the  microscope  in  an  inverted,  vertical  position,  there 
should  be  no  distortion  of  the  image  if  the  drawing  surface  is 
horizontal. 

With  the  inclined  microscope,  the  mirror  used  must  be  so  inclined 
that  it  throws  the  image  directly  down  upon  the  horizontal  drawing 
surface,  or  the  image  will  be  distorted.  It  is  not  easy  to  tell  just 
the  inclination  of  the  microscope,  and  therefore,  the  exact  inclina- 
tion to  give  the  mirror,  to  make  the  axial  ray  perpendicular  to  the 
drawing  surface.  In  the  small  Edinger  apparatus  (fig.  204),  the 
directions  are  to  make  the  inclination  of  the  microscope  45  degrees 
and  the  inclination  of  the  mirror  22^"  degrees.  This  arrangement 
will  give  a  correct  image.  One  may  need  to  use  a  protractor  to 
make  sure  that  the  inclination  of  the  microscope  and  mirror  are 
exactly  correct. 

With  the  horizontal  microscope,  the  mirror  or  prism  is  so 
arranged  that  it  is  fixed  at  45  degrees  and  therefore  if  put  directly 
over  the  ocular  of  the  horizontal  microscope,  will  reflect  the  light 
perpendicularly  upon  the  drawing  surface,  thus  avoiding  distortion 
(see  §  483). 


346 


DRAWING  WITH  HOUSE  CURRENT 


[CH.  X 


With  the  horizontal  microscope,  unless  one  uses  a  table  with  a 
drawing  shelf  (fig.  187),  the  microscope  must  be  raised  on  a  block 
or  support  of  some  kind  and  clamped  to  the  block  so  that  it  will  be 
rigid  (fig.  193-194).  A  convenient  height  is  250  mm.  (10  inches). 

To  vary  the  magnification  slightly,  the  distance  can  be  made 
greater  by  using  an  additional  block,  or  it  may  be  made  less  by 
raising  the  drawing  surface.  For  a  very  convenient  arrangement 
for  changing  the  elevation  of  the  microscope  see  fig.  198,  20 iC. 

For  obtaining  the  scale  or  magnification  of  the  drawing  see 
§  508-510. 

§  497.  Getting  the  light  through  the  horizontal  microscope 
with  the  plane  mirror. — The  simplest  method  is  to  place  the  lamp 


FIG.  193.     DRAWING  WITH  THE  MICROSCOPE  IN  A  DARK  ROOM. 

In  the  arrangement  here  shown  the  light  is  from  a  small  arc  lamp  drawing 
current  from  the  house  lighting  system. 

The  supply  cable  and  the  lamp  socket  are  shown,  then  the  separable  attach- 
ment plug  and  the  supply  wires  with  the  rheostat  inserted  along  one  wire  (fig. 
188). 

The  arc  lamp  is  at  the  level  of  the  microscope  mirror  and  at  right  angles  with 
the  microscope  axis.  The  light  from  the  arc  lamp  is  reflected  up  to  the  sub- 
stage  condenser  by  the  mirror  and  passes  on  through  the  specimen  and  micro- 
scope as  shown  in  fig.  192. 

The  shield  between  the  lamp  and  drawing  surface  is  to  keep  stray  light  from 
reaching  the  drawing  surface.  The  shield  is  represented  as  transparent.  It 
was  left  in  place  during  only  a  part  of  the  time  of  the  exposure  in  making  the 
photographic  negative. 


CH.  X]  DRAWING  WITH  HOUSE  CURRENT  347 

and  the  microscope  at  right  angles.  Use  a  level  (fig.  1 60)  and  make 
sure  that  the  condenser  tube  is  horizontal,  and  the  axis  of  the  con- 
denser at  the  same  level  as  the  center  of  the  mirror.  Place  a  disc 
of  blackened  asbestos  or  tin  of  about  12.5  cm.  (5  in.)  in  diameter 
just  behind  the  condenser  as  shown  in  fig.  193—198.  This  is  easily 
done  by  making  a  hole  of  the  proper  size  in  the  disc  to  go  over  the 
condenser  tube  (fig.  195).  If  now  the  current  is  turned  on  and  the 
arc  established  the  light  will  extend  from  the  condenser  to  the  plane 
mirror  and  be  reflected  by  it,  if  it  is  set  at  45  degrees,  up  into  the 
substage  condenser.  From  the  lower  face  of  the  substage  con- 
denser a  part  of  the  light  is  reflected  back  to  the  mirror  and  from 
the  mirror  back  toward  the  lamp,  and  is  received  by  the  black  disc 
over  the  condenser  tube.  The  mirror  should  then  be  turned  until 
the  spot  of  light  enters  the  lamp  condenser.  The  mirror  will  then 
be  in  position  to  reflect  the  light  along  the  optic  axis  of  the  micro- 
scope. 

If  the  microscope  is  in  focus  on  the  object,  the  light  will  traverse 
the  objective  and  ocular  and  be  reflected  down  upon  the  drawing 
surface  by  the  mirror  or  prism  beyond  the  ocular. 

By  changing  the  mirror  slightly  while  watching  the  circle  of  light 
on  the  drawing  surface,  the  best  illumination  can  be  obtained. 

§  498.  Getting  the  light  through  the  microscope  with  the  con- 
cave mirror. — One  proceeds  exactly  as  described  above,  only  the 
light  reflected  back  to  the  black  disc  on  the  lamp  condenser  tube 
will  be  a  crescent  instead  of  a  circle.  The  middle  part  of  the  cres- 
cent can  be  reflected  into  the  lamp  condenser  and  then  the  light  will 
pass  through  the  microscope  and  be  reflected  down  upon  the  draw- 
ing paper,  provided  the  microscope  and  the  arc  lamp  are  at  right 
angles  and  at  the  proper  level. 

§  499.  Substage  condenser. — Use  the  substage  condenser  with 
objectives  of  16,  12,  10,  8,  6,  4,  and  2  mm.  focus.  For  objectives 
lower  than  the  16  mm.  the  substage  condenser  is  turned  aside. 

With  different  objectives  and  slides  of  different  thickness  the 
substage  condenser  is  changed  somewhat  in  position  to  get  the  best 
light  on  the  object  and  to  light  the  entire  field. 


348  DRAWING  WITH  HOUSE  CURRENT  [Cn.  X 

To  start  with,  the  substage  condenser  diaphragm  should  be 
opened  widely.  In  some  cases  the  picture  can  be  made  sharper  by 
afterward  closing  the  diaphragm  somewhat. 

For  drawing,  a  skillful  use  of  the  substage  condenser  is  very 
important.  One  must  be  more  precise  in  its  use  than  in  ordinary 
microscopic  observation. 


FIG.  194.     DRAWING  WITH  A  MICROSCOPE  WITH  THE  ARC  LAMP  AT  RIGHT 

ANGLES. 

In  this  picture  a  prism  is  placed  beyond  the  ocular  to  reflect  the  light  down- 
ward (fig.  192).  The  arc  lamp  is  on  the  back  side  of  the  microscope  with  the 
condenser  facing  the  mirror.  The  spot  of  light  on  the  shield  or  disc  above  the 
lamp  shows  that  the  light  is  not  centered  along  the  axis  of  the  microscope. 
The  mirror  must  be  turned  slightly  until  the  light  reflected  back  from  the 
substage  condenser  and  microscope  mirror  enters  the  condenser  tube  of  the 
arc  lamp  (see  fig.  195). 

§  500.  Plane  mirror  and  substage  condenser. — Use  the  plane 
mirror  and  substage  condenser  for  all  objectives  of  12,  10,  8,  6,  4, 
and  2  mm.  equivalent  focus. 

§  501.     Concave  mirror  and  substage  condenser. — For  the  16  to 

1 8  mm.  focus  objectives  use  the  substage  condenser  with  the  con- 
cave mirror.  It  may  also  be  necessary  to  separate  the  condenser 
somewhat  from  the  preparation  to  light  the  entire  field. 


CH.  X]  DRAWING  WITH  HOUSE  CURRENT  349 

§  502.     Concave  mirror  without  a  substage  condenser. — For 

objectives  of  20,  25,  30,  35,  40  and  50  mm.  focus  use  the  concave 
mirror  without  a  substage  condenser. 

§  503.  Immersion  objective. — For  immersion  objectives  used 
in  drawing  do  not  forget  to  use  the  proper  immersion  liquid 
between  the  cover-glass  and  the  objective ;  cedar  oil  for  the  oil 
immersions,  and  distilled  water  for  the  water  immersions. 


FIG.  195.     SHIELD  AT  THE  END  OF  THE  ARC  LAMP  CONDENSER  TUBE  TO 
AID  IN  CENTERING  THE  LIGHT. 

This  disc  is  of  blackened  sheet  iron,  asbestos  or  cardboard  and  is  125  to  150 
mm.  in  diameter.  It  is  placed  at  the  end  of  the  lamp  condenser  tube.  If  the 
light  is  centered,  then  that  reflected  back  from  the  substage  condenser  and 
microscope  mirror  will  enter  the  lamp  condenser  (C).  If  the  light  is  not  cen- 
tered there  will  be  a  round  spot  of  light  somewhere  outside  the  lamp  condenser. 
In  that  case  the  mirror  must  be  turned  slightly  until  the  reflected  light  enters 
the  lamp  condenser. 

If  the  plane  mirror  is  used  the  spot  of  light  will  be  nearly  circular;  with  the 
concave  mirror  it  will  be  crescentic. 

C     The  lamp  condenser. 

/     Spot  of  light  outside  the  condenser  showing  that  the  light  is  off  the  center. 


350 


DRAWING  WITH  HOUSE  CURRENT 
AVOIDANCE  OF  HEAT 


[CH.  X 


§  504.  When  the  small  currents  from  the  house  circuit  are  used 
the  heat  is  not  great  enough  to  injure  most  specimens  mounted  in 
balsam.  For  live  objects  and  objects  mounted  in  glycerin  or 
glycerin  jelly,  etc.,  it  would  be  wise  to  place  a  water-cell  in  the  beam 
before  it  reaches  the  microscope  (see  §  364,  394a,  fig.  206). 


FIG.    196. 


DRAWING  OUTFIT  FOR  THE  HOUSE  LIGHTING  SYSTEM  WITH  A 
BLACK  CLOTH  TENT  OVER  THE  MICROSCOPE. 


This  arrangement  answers  well  for  a  moderately  lighted  room.  Naturally 
the  opening  for  drawing  should  face  toward  some  dark  furniture  or  the  dark 
side  of  the  room,  not  toward  a  window. 

5  Separable  cap  to  attach  to  the  separable  plug  in  a  socket  of  the  house 
lighting  system. 

w  r  One  of  the  supply  wires  cut  and  inserted  into  the  binding  posts  of  the 
rheostat  (see  fig.  188). 

/  Small  arc  lamp  for  supplying  the  illumination.  It  is  at  the  level  of  the 
mirror  and  at  right  angles  to  the  microscope. 

m     Mirror  of  the  microscope. 

t  Cloth  tent  over  the  microscope.  It  appears  semi-transparent  as  it  was 
left  in  position  during  but  part  of  the  time  when  the  photograph  was 
taken. 


CH.  X]  LIGHT  SHIELD  FOR  DRAWING  351 

SHIELDING  THE  DRAWING  SURFACE  FROM  STRAY  LIGHT 

§  505.  Shield  for  working  in  a  dark  room. — If  one  works  in  a 
dark  room  all  that  is  necessary  to  screen  the  drawing  from  stray 
light  from  the  arc  lamp,  when  the  lamp  is  at  right  angles  to  the 
microscope,  is  a  blackened  cardboard  shield  (fig.  193). 

If  the  lamp  is  in  line  with  the  microscope,  it  will  be  necessary  to 
put  a  shield  with  a  perforation  for  the  light  beam  either  before  the 
beam  reaches  the  microscope,  or  it  may  be  put  over  the  tube  of  the 
microscope  so  that  it  will  shield  the  drawing  surface. 

§  506.  Drawing  in  a  light  room. — If  this  is  necessary  one  should 
get  in  as  shaded  a  part  of  the  room  as  possible.  To  screen  the 
drawing  surface  there  are  two  ways : 

(1)  There  may  be  a  cloth  for  enclosing  the  drawing  surface  and 
the  head  of  the  artist.     This  is  like  the  plan  used  in  focusing  a 
photographic  camera  (fig.  204). 

(2)  By  means  of  cardboard,  or  of  a  wire  frame  and  cloth  cur- 
tains, a  box  or  tent  is  built  around  the  drawing  surface  enclosing 
also  the  microscope  tube.     The  end  of  the  box  next  the  draughts- 
man is  open  sufficiently  for  him  to  see  the  image  (fig.  196,  198). 
The  drawing  surface  should  look  toward  some   dark  furniture 
or  a  dark  or  shaded  part  of  the  room,  and  except  for  the  most 
exacting  work  the  surface  will  be  sufficiently  shaded.     For  the  most 
exacting  work,  and  for  the  greatest  freedom  from  accessories,  the 
evening  or  a  dark  room  in  the  daytime  offers  the  best  facilities 
(§  453). 

How  TO  GET  ANY  DESIRED  MAGNIFICATION  IN  A  DRAWING 

§  507.  The  magnification  can  be  varied  by  any  of  the  following 
ways,  or  two  or  more  of  the  ways  may  be  combined. 

(a)  By  using  a  higher  or  lower  objective. 

(b)  By  using  an  amplifier,  of  greater  or  less  power,  with  the 
objective. 

(c)  By  using  a  higher  or  lower  ocular  with  the  objective. 

(d)  By  changing  the  distance  of  the  drawing  surface;    the 
farther  it  is  away  in  any  given  case  the  larger  will  be  the  image,  and 
the  nearer  it  is  the  smaller  the  image  (§  5ioa). 


352 


MAGNIFICATION  OF  DRAWINGS 


[Cn.  X 


FIG.  197. 


DRAWING  OUTFIT  FOR  THE  HOUSE  LIGHTING  SYSTEM,  USING 
AN  INDUCTOR  INSTEAD  OF  A  RHEOSTAT  (fig.  193). 


Commencing  at  the  left : 

The  supply  wires  to  the  lamp  socket,  and  the  supply  wires  from  the  separable 
attachment  plug  to  the  arc  lamp. 

One  of  the  supply  wires  is  connected  directly  with  the  arc  lamp  and  one  is 
cut  and  the  two  cut  ends  connected  with  the  two  poles  of  the  inductor  exactly 
as  with  a  rheostat  (fig.  188),  and  from  the  inductor  the  wire  is  continued  to  one 
of  the  binding  posts  of  the  arc  lamp. 

The  inductor  is  only  for  alternating  current  (§  736).  The  amperage  can  be 
varied  by  sliding  the  soft  iron  core  in  and  out  of  the  coil.  The  more  the  core 
is  inserted  the  greater  the  inductance  and  hence  the  less  the  amount  of  current 
that  is  allowed  to  flow. 

As  shown  in  the  picture,  the  core  is  only  partly  inserted  and  a  medium 
current  is  allowed  to  flow. 

If  one  uses  alternating  current  this  is  a  much  more  economical  method  of 
controlling  the  current  than  a  rheostat  (see  §  736+)  and  a  steadier  light  is 
produced. 

The  two  most  common  changes  are:  (i)  Using  a  higher  or 
lower  objective;  and  (2)  Changing  the  distance  of  the  drawing 
surface. 


CH.  X]  MAGNIFICATION  OF  DRAWINGS  353 

For  slight  variations  in  size  the  change  in  distance  is  by  far  the 
best  and  easiest  change  to  make. 

If  one  has  a  drawing  table  (fig.  182)  it  is  very  simple  to  push  it 
farther  from  or  closer  to  the  projection  apparatus. 

If  the  drawing  shelf  is  used  it  can  be  raised  or  lowered  (fig.  183). 

If  the  simple  apparatus  is  used  on  an  ordinary  table  the  entire 
microscope  can  be  raised  for  higher  magnifications  or  for  lower 
magnifications  the  drawing  surface  can  be  raised  to  bring  it  closer 
to  the  microscope  tube  (fig.  193-204),  or  the  microscope  can  be 
lowered  on  its  adjustable  support  (fig.  198). 

DETERMINATION  OF  THE  MAGNIFICATION  OF  A  DRAWING 

§  508.  For  getting  the  magnification  it  is  necessary  to  use  for 
an  object  a  transparent  micrometer  with  known  divisions  upon  it. 
For  most  of  the  work  done  a  micrometer  with  heavy  lines  every 
half  millimeter  is  satisfactory.  These  lines  may  be  ruled  on  glass 
and  filled  with  graphite,  or  they  may  be  made  by  photography 
(see  §  5o8a). 

For  example,  if  the  micrometer  used  has  half  millimeter  spaces, 
the  image  projected  on  the  screen  will  be  magnified  more  or  less 
according  to  the  distance  of  the  screen  and  the  objective  used. 
The  exact  size  of  the  image  is  easily  measured  on  the  drawing  sur- 
face with  a  millimeter  scale.  Suppose  that  two  of  the  half  milli- 
meter spaces  were  used  as  object,  the  object  would  be  one  milli- 
meter in  actual  size.  If  the  image  of  two  spaces  projected  on  the 
screen  or  drawing  surface  measured  2  5  millimeters  then  the  magni- 
fication would  be  25. 


§  508a.  One  can  make  a  satisfactory  micrometer  for  determining  the 
magnification  of  drawings  as  follows:  Make  a  negative  of  the  millimeter  scale 
(fig.  178,  21 1)  making  the  picture  exactly  half  the  size  of  the  original  scale,  then 
the  spaces  will  be  half  millimeters.  As  the  scale  is  black  with  light  lines  the 
negative  will  show  dark  lines  with  intervening  clear  spaces  exactly  as  is  a  glass 
micrometer. 

If  so  desired  the  micrometer  lines  may  be  covered  with  Canada  balsam  and 
a  cover-glass  applied  as  for  microscopic  specimens.  (See  The  Microscope, 
§  354~5>  P-  257)-  This  would  protect  the  lines  and  make  the  specimen  more 
transparent.  A  lantern-slide  plate  is  the  best  for  making  the  negative,  as  it 
gives  transparent  lines. 


354 


MAGNIFICATION  OF  DRAWINGS 


[Cn.  X 


Briefly  stated,  if  one  has  an  object  of  known  size,  then  the  size  of 
the  image  divided  by  the  size  of  the  object  will  give  the  magnifica- 
tion in  every  case.  In  the  example  given  above  the  object  is  i  mm. 
and  the  image  is  25  mm. :  25  -K  i  =  25.  Or,  if  one  took  a  single 
space  as  object,  that  is,  half  a  millimeter,  then  the  image  would 
measure  12.5  mm.,  and  12.5  -f-  .5  =  25  as  before  (see  §  5ioa). 


) 


FIG.  198.     SPENCER  LENS  COMPANY'S  APPARATUS  FOR  DRAWING  WITH 

THE  MICROSCOPE. 
(Cut  loaned  by  the  Spencer  Lens  Co.). 

This  consists  of  a  small  arc  lamp  with  the  proper  wiring,  rheostat  and  con- 
nections for  the  house  electric  supply.  The  lamp  has  all  the  adjustments,  and 
the  condenser  tube  is  telescoping  so  that  the  beam  of  light  may  be  parallel  or 
converging. 

The  microscope  is  supported  on  an  adjustable  shelf  which  can  be  raised  or 
lowered  on  the  vertical  rods,  thus  enabling  one  to  get  any  desired  magnification. 

The  vertical  supports  for  the  microscope  shelf  serve  to  carry  a  curved  metal 
band  to  support  the  cloth  curtains  to  shade  the  drawing  surface.  There  are 
two  curtains  and  they  hang  freely,  thus  avoiding  all  interference  with  the 
hands  in  drawing.  If  one  desires,  the  arc  lamp  can  be  put  in  line  with  the 
microscope  and  the  mirror  turned  aside. 

For  a  reflector  beyond  the  ocular  a  prism  is  used,  thus  avoiding  any  of  the 
defects  of  a  mirror. 


CH.  X] 


MAGNIFICATION  OF  DRAWINGS 


355 


MAGNIFICATION  OF  WALL  DIAGRAMS,  AND  DRAWINGS  MADE  WITH 
THE  OPAQUE  LANTERN 

§  509.  Scale  of  wall  diagram. — Make  the  diagram  any  desired 
size  as  directed  in  §  457.  Then  remove  the  negative  or  lantern 
slide  from  the  holder  and  insert  a  lantern  slide  or  negative  of  the 
metric  rule  (fig.  178,  211).  The  image  of  the  metric  rule  on  the 
drawing  surface  will,  of  course,  be  magnified  exactly  as  much  as  the 
diagram.  By  using  a  metric  measure,  one  can  find  the  magnifica- 
tion of  the  screen  image  exactly  as  described  in  §  508. 

For  wall  diagrams,  it  is  not  usually  very  important  to  know  the 
magnification.  All  that  is  necessary  is  to  get  it  large  enough  to  be 
seen  well.  But  if  one  wishes  to  show  the  relative  size  of  objects 
such  as  blood  corpuscles  of  different  animals,  then  the  magnifica- 
tion must  be  known. 


FIG.  199.     MODEL  4-5  DELINK ASCOPE  WITH  THE  PROJECTION  MICRO- 
SCOPE IN  A  HORIZONTAL  POSITION. 

(Cut  loaned  by  the  Spencer  Lens  Co.}. 

With  the  microscope  in  this  position  the  image  may  be  thrown  down  on  a 
horizontal  surface  for  drawing  by  pushing  the  totally  reflecting  prism  up  in 
the  microscope  tube  to  intercept  the  light  (see  fig.  109,  175). 


356 


MAGNIFICATION  OF  DRAWINGS 


[Cn.  X 


FIG.  200.     COMBINED  DRAWING  AND  PHOTO-MICROGRAPHIC  APPARATUS 

OF  THE  BAUSCH  &  LOME  OPTICAL  COMPANY  FOR  USE  ON  THE 

HOUSE  LIGHTING  SYSTEM. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

This  is  a  kind  of  universal  apparatus  serving  for  drawing  with  the  microscope, 
projection  with  a  microscope  and  with  a  magic  lantern;  opaque  projection, 
and  finally  for  photographing  with  all  objectives  and  with  the  microscope. 
It  can  be  used  in  a  horizontal,  an  inclined  or  a  vertical  position.  For 
drawing  with  the  microscope  in  a  horizontal  position  there  is  an  adjustable 
drawing  shelf  with  a  cloth  tent  for  shutting  out  daylight  in  a  light  room. 

The  large  condenser  enables  one  to  use  the  apparatus  on  specimens  of  all 
sizes  up  to  lantern  slides. 

§  510.  Scale  of  diagrams  or  drawings  made  with  the  opaque 
lantern. — If  one  uses  the  episcope  or  opaque  lantern,  or  a  photo- 
graphic camera  for  drawing,  it  is  very  easy  to  get  the  exact  magnifi- 
cation of  the  drawing  by  putting  a  metric  rule  upon  some  part  of 


CH.  X]  DRAWINGS  FOR  MODELS  357 

the  object,  or  beside  it.     It  will  be  at  the  same  scale  of  magnifica- 
tion or  reduction  as  the  drawing. 

In  practice  some  lines  of  the  image  of  the  scale  are  made  beside 
the  drawing.  For  example,  suppose  the  image  of  one  centimeter 
measured  on  the  drawing  was  10  centimeters  long,  one  would  know 
that  the  drawing  is  10  times  larger  than  the  object.  If  the  length 
of  the  centimeter  on  the  drawing  was  only  one-half  centimeter  long, 
then  one  would  know  that  the  drawing  is  only  half  as  large  as  the 
object  and  so  on  (§  5o8a,  Sioa). 

DRAWINGS  FOR  MODELS 

§  511.  Drawings  for  models. — These  are  made  much  more 
easily  with  projection  apparatus  than  with  the  camera  lucida  or  in 
any  other  way.  The  simple  drawing  outfit  for  use  on  the  house 
circuit  described  above  makes  it  possible  for  every  laboratory  and 
indeed  every  private  worker  to  use  this  effective  method,  even  if 
complete  projection  apparatus  and  heavy  lantern  currents  are  not 
available. 

In  making  drawings  for  models  several  steps  must  be  taken  in 
order  that  the  resulting  model  shall  be  anything  like  a  true  repre- 
sentation of  the  actual  object. 

(1)  The  object  (embryo,  etc.)  should  be  photographed  at  a 
known  magnification  before  it  is  sectioned. 

(2)  The  sections  should  be  made  of  a  known  thickness  (IOJJL, 
1551,  etc.). 


§  510a.  The  general  law  for  magnification  and  reduction. — With  a  given 
object  the  size  of  the  image  depends  directly  upon  the  relative  distance  of  the 
object  and  of  the  image  from  the  center  of  the  lens  (fig.  185,  209,  210).  If  the 
image  is  farther  from  the  center  of  the  lens  than  the  object  then  the  image  will 
be  larger  than  the  object;  conversely  if  the  image  is  nearer  the  center  of  the 
lens  than  the  object  then  it  will  be  smaller  than  the  object. 

For  example,  if  the  image  is  to  be  ten  times  as  large  as  the  object  the  image 
must  be  ten  times  as  far  from  the  center  of  the  lens  as  the  object. 

Conversely,  if  the  image  is  to  be  one-tenth  as  large  as  the  object  it  must  be 
formed  only  one- tenth  as  far  from  the  lens  as  the  object. 

In  lantern-slide  and  micro-projection,  and  in  photo-micrography  the  image 
is  much  larger  than  the  object  and  correspondingly  more  distant  from  the  cen- 
ter of  the  lens.  In  ordinary  portrait  photography  and  in  landscape  photo- 
graphy the  image  is  much  smaller  than  the  object,  and  consequently  the  image 
is  much  nearer  the  lens  than  the  object  (see  also  §  392a). 


DRAWING  WITH  THE  MICROSCOPE 


ICn.  X 


FIG.  201,  A,  B,  C.     SIMPLE  DRAWING  OUTFIT  FOR  THE  MICROSCOPE. 
(Cuts  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 


CH.  X]  DRAWING  FOR  MODELS  359 

There  is  a  hand-feed,  right-angled  arc  lamp  for  small  carbons,  wiring  and 
connections  for  the  house  circuit  and  a  rheostat  which  will  not  permit  over  6 
amperes  of  current  to  flow.  The  lamp  condenser  is  in  a  telescoping  tube  so 
that  either  a  parallel  or  a  converging  beam  of  light  can  be  obtained.  To  avoid 
stray  light  the  drawing  surface  is  enclosed  by  a  metal  box  with  one  side  removed. 

A     Drawing  outfit  with  the  lamp  and  microscope  in  line. 

The  microscope  is  supported  on  a  block  to  give  a  drawing  distance  of  254  mm. 
( 10  inches). 

B     Drawing  outfit  with  the  arc  lamp  at  right  angles  to  the  microscope. 

C  Drawing  outfit  with  the  microscope  on  an  adjustable  platform  and  the 
arc  lamp  at  right  angles  with  the  microscope. 

(3)  It  must  be  decided  in  the  beginning  how  much  larger  the 
model  is  to  be  than  the  original  object. 

(4)  The  objective  and  the  drawing  surface  must  be  chosen  and 
mutually  arranged  so  that  the  desired  magnification  is  attained 

(§  509). 

(5)  The  object  must  be  placed  on  the  stage  of  the  microscope  so 
that  the  image  reflected  down  upon  the  drawing  surface  will  be 
erect,  that  is,  exactly  like  the  object  and  not  inverted  in  any  way 
(see  below  §  512). 

(6)  Each  drawing  as  it  is  made  must  be  numbered  to  correspond 
with  the  number  of  the  section :     This  is  very  important. 

(7)  It  is  desirable  to  make  a  duplicate  set  of  drawings,  for  one 
set  is  used  up  in  making  the  model,  and  one  needs  a  set  for  reference. 

The  duplicate  drawings  are  easily  made  by  using  thin  carbon 
paper  as  in  duplicating  writing,  or  in  typewriting. 

(8)  Marking  the  position  of  the  apparatus.     If  all  the  draw- 
ings cannot  be  made  at  one  time,  then  the  objective,  the  ocular, 
if  one  is  used,  and  the  distance  of  the  drawing  surface  from  the 
tube  of  the  microscope  should  be  carefully  measured  or  indicated 
by  chalk  marks,  so  that  when  working  again  exactly  the  same 
magnification  can  be  used.     It  is  well  also  to  check  up  by  using  the 
stage  micrometer  again  (§  508).     Pictures  for  models  may  also  be 
made  by  photography,  (see  §  542). 

ERECT  IMAGES 

§  512.  It  has  been  known  from  the  first  use  of  projection  appar- 
atus that  the  projected  image  was  inverted,  and  that  this  is  true 
whether  a  simple  aperture,  a  simple  lens,  or  an  objective  of  several 


36° 


ERECT  IMAGES  IN  DRAWINGS 


[Cn.  X 


The  arc  lamp  is  of  the  Liliput 
form  with  small  right-angled  car- 
bons. 

The  lamp  condenser  is  large, 
such  as  is  used -for  lantern-slide 
projection,  hence  large  as  well 
as  small  objects  can  be  illumi- 
nated by  it. 

For  convenience  in  feeding  the 
carbons  there  is  a  rod  extending 
down  within  reach  of  the  artist. 

The  microscope  and  stage  are 
separate  and  independently 
movable  on  the  vertical  optical 
bench.  In  addition  to  the  lamp 
condenser  there  are  two  or  more 
substage  condensers  of  different 
foci. 

The  object  is  put  on  the  upper 
side  of  the  stage. 

The  microscope  can  be  used 
with  an  ocular,  or  the  draw  tube 
and  ocular  can  be  removed  from 
the  large  microscope  tube,  and 
then  objectives  alone"used,  thus 
giving  very  large  fields. 


FIG.  202.     EDINGER'S  VERTICAL  DRAWING  AND  PHOTOGRAPHIC 
^  APPARATUS  FOR  USE  ON  THE 
HOUSE  CIRCUIT. 

(Cut  loaned  by  Ernst  Leitz). 


CH.  X]  ERECT  IMAGES  IN  DRAWINGS  361 

lenses  is  used.  The  earliest  workers  also  saw  that  an  easy  way  to 
correct  for  this  was  to  invert  the  object,  then  its  image  would 
appear  in  the  natural  position.  But  some  objects  do  not  admit  of 
inversion,  hence  the  effort  to  obtain  erect  images  by  some  optical 
means. 

The  first  and  still  the  simplest  method  is  by  the  use  of  a  plane 
mirror  with  a  horizontal  screen  (fig.  88,  89,  181,  204).  The  mirror 
might  be  put  in  the  course  of  the  beam  before  or  after  it  has  passed 
the  objective.  Figure  89  shows  it  before  and  figure  182  after 
traversing  the  objective. 

It  was  demonstrated  by  Kepler  (1611)  and  practically  worked 
out  by  Scheiner  (1619)  that  erect  images  could  be  produced  by  the 
use  of  two  objectives  in  line.  The  first  objective  gives  a  real 
inverted  image  of  the  object,  and  the  second  gives  a  real,  erect 
image  of  the  inverted  image  (fig.  208).  This  is  what  occurs  when- 
ever an  ocular  is  used  with  an  objective  in  projecting  with  the 
microscope  (fig.  207). 

The  principles  for  getting  erect  images  with  projection  apparatus 
are  very  simple,  but  in  practice  it  is  a  little  puzzling  to  decide  off- 
hand just  how  to  arrange  the  object  so  that  the  screen  image  shall 
be  erect  and  not  show  any  of  the  inversions  (fig.  212-214).  This 
difficulty  arises  from  the  fact  that  in  the  different  kinds  of  projec- 
tion sometimes  an  opaque  object  is  used,  and  sometimes  a  trans- 
parent object;  sometimes  an  opaque  and  sometimes  a  translucent 
screen  is  employed;  sometimes  an  objective  only,  and  sometimes 
both  an  objective  and  an  ocular  are  used  for  projecting  the  image ; 
and  finally,  sometimes  it  is  necessary  to  use  a  mirror  or  prism  as  well 
as  an  objective  to  get  the  image  on  the  vertical  or  horizontal  surface 
where  it  is  to  be  seen  or  drawn. 

The  simplest  and  surest  way  to  get  the  microscopic  specimen  on 
the  stage  of  the  projection  microscope  in  a  position  which  will  give 
a  correct  image  for  drawing  is  the  following: 

i.  The  prepared  microscopic  specimen  is  placed  on  a  piece  of 
white  paper  so  that  it  appears  exactly  as  it  should  in  the  drawing, 
and  the  letters  a  and  k  are  written  on  the  cover-glass  between 
the  specimens  (fig.  220). 


362 


ERECT  IMAGES  IN  DRAWINGS 


[Cn.  X 


2. 


The  slide  is  then  placed  on  the  stage  of  the  projection  appara- 
tus and  its  image  thrown  on  the  drawing  surface.  In  case  the 
specimen  is  wrongly  placed  to  give  an  erect  image  the  letters  will 
show  it,  and  the  specimen  can  be  rearranged  until  the  images  of  the 
letters  are  correct  in  every  way,  then  of  course  the  images  of  the 
microscopic  specimens  will  be  erect  in  every  way  (see  also  §  517). 

§  513.  Erect  images  with  opaque  objects  in  a  photographic 
camera  with  translucent  screen. — Place  the  object  upside  down  in 
the  holder.  On  the  translucent  screen  it  will  be  erect  (fig.  211). 
If  the  object  cannot  be  put  upside  down,  the  image  will  appear 
wrong  side  up  on  the  translucent  screen  (fig.  212).  It  can  be  drawn 


FIG.  203.     LARGE  EDINGER  APPARATUS  IN  A  HORIZONTAL  POSITION  FOR 
PROJECTION  ON  A  VERTICAL  SCREEN. 

(Cut  loaned  by  Ernst  Leitz). 


CH.  X] 


ERECT  IMAGES  IN  DRAWINGS 


FIG.  204.     EDINGER'S  OUTFIT  FOR  DRAWING  WITH  AX  ORDINARY  MICRO- 
SCOPE AND  SMALL  ARC  LAMP  ON  THE  HOUSE  LIGHTING  SYSTEM. 

(Cut  loaned  by  Ernst  Leitz). 

This  is  the  first  form  of  the  drawing  outfits  using  the  ordinary  microscope 
and  the  small  arc  lamp  on  the  house  lighting  circuit.  It  was  demonstrated  at 
the  meeting  of  the  Anatomische  Gesellschaft  at  its  Leipzig  meeting,  April,  191 1. 

The  microscope  is  inclined  to  45°  and  the  mirror  at  an  angle  of  22.5°,  thus 
directing  the  light  vertically  down  upon  the  horizontal  drawing  surface. 

For  drawing  in  a  light  room  a  cloth  tent  is  provided  and  is  supported  above 
and  on  the  sides  by  metal  arches.  If  it  is  very  light  one  can  pull  the  cloth  over 
the  head  as  in  focusing  a  camera.  In  the  evening  or  in  a  dark  room  the  cloth . 
can  be  opened  widely  to  expose  the  drawing  surface. 

or  traced  in  this  position  and  the  drawing  turned  right  side  up, 
when  it  will  appear  like  fig.  211,  that  is,  correct  in  every  way. 

§  514.     Erect  images  with  the  opaque  lantern  or  episcope.— 

(A)     The  objective  horizontal,  the  object  and  the  drawing  surface 


364  ERECT  IMAGES  IN  DRAWINGS  [Cn.  X 

vertical.  The  object  is  placed  upside  down  in  its  vertical  holder. 
The  mirror  reflecting  the  image  upon  the  vertical  drawing  surface 
will  give  an  erect  image  (fig.  211). 

(B)  The  objective  and  the  drawing  surface  horizontal,   the 
object  vertical.     The  artist  with  his  back  toward  the  apparatus : 
Place  the  object  right  side  up  in  the  vertical  holder. 

(C)  Same  as  above,  but  with  the  artist  facing  the  apparatus  as 
with  the  drawing  shelf  in  fig.  183.     Place  the  object  wrong  side  up 
in  the  vertical  holder. 

(D)  Same,  except  that  a  vertical  translucent  screen  is  used. 
Place  the  object  wrong  side  up  in  the  vertical  holder;   do  not  use  a 


FIG.  205.   SMALL  ARC  LAMP  WITH  CLOCK-WORK  FOR  FEEDING  THE  CARBONS. 

(Cut  loaned  by  Ernst  Leitz). 

This  arc  lamp  for  the  house  circuit  has  a  clock-work  which  moves  the  carbons 
continuously.  The  arc  must  be  started  by  hand  as  for  a  hand-feed  lamp,  but 
when  once  started  the  lamp  will  burn  continuously  provided  the  carbons  burn 
off  as  fast  as  they  are  fed.  If  the  carbons  are  too  large  the  clock-work  will  feed 
them  together  faster  than  they  burn  away,  and  if  too  small  the  clock-work  feeds 
the  carbons  too  slowly  and  the  lamp  will  go  out. 

The  clock-work  has  a  regulating  device  for  speed  and  the  lamp  has  the  usual 
feed  wheel  for  hand  regulation. 

This  form  of  feeding  mechanism  is  equally  good  for  direct  and  for  alternating 
current  as  the  movement  is  entirely  controlled  by  the  clock-work.  Such  a 
lamp  is  especially  useful  for  drawing  and  for  photography. 


CH.  X] 


ERECT  IMAGES  IN  DRAWINGS 


365 


mirror  or  prism  with  the  objective,  but  point  the  objective  directly 
toward  the  screen. 

§  515.    Erect  images  of  horizontal  objects  with  the  episcope. — 

Vertical  drawing  surface  and  vertical  objective,  horizontal  object. 
The  object  is  placed  with  its  upper  edge  away  from  the  drawing 
surface  and  the  mirror  reflecting  the  image  to  the  vertical  screen 
will  make  it -erect  (fig.  211). 

§  516.  Erect  images  on  the  drawing  surface  with  the  magic 
lantern. — (A)  With  an  opaque,  vertical  drawing  surface.  Place 
the  transparency  in  the  slide-carrier  as  described  for  lantern  slides 
(fig.  7—8),  i.  e.,  with  the  object  facing  the  light  and  wrong  side  up. 

(B)  For  a  translucent,  vertical  drawing  surface.     Place  the 
object  facing  the  objective  and  wrong  side  up. 

(C)  For  an  opaque  horizontal  screen.    Place  the  object  so  that 
it  faces  the  objective  and  the  mirror  or  prism  reflecting  the  rays 
downward  will  give  an  erect  image  (see  B  and  C  in  §  514). 


ERECT  IMAGES  WITH  THE  PROJECTION  MICROSCOPE 

§  517.  Demonstration  of  the  position  of  objects  for  erect 
images. — The  simplest  way  to  determine  how  a  specimen  should  be 
placed  on  the  stage  of  the  microscope  to  give  an  erect  image  on  any 


FIG.  206.     MAGIC  LANTERN  ARC  LAMP  AND  TWO-LENS  CONDENSER  USED 
IN  MICRO-PROJECTION  AND  FOR  DRAWING. 

(See  fig.  146  for  full  explanation). 


366 


ERECT  IMAGES  IN  DRAWINGS 


ICn.  X 


kind  or  position  of  a  screen  is  to  use  a  specimen  prepared  as  follows : 
An  ordinary  microscopic  slide  is  varnished  as  directed  for  lantern- 
slide  glasses  (Ch.  VIII,  §317)  and  then  the  small  letters  a  and  k  are 
written  in  the  middle  with  a  fine  pen.  These  letters  are  selected 
because  both  in  script  and  in  printing  they  indicate  clearly  which 
side  up  they  are  and  which  way  they  face.  With  some  letters  it  is 
not  so  easy  to  determine  whether  they  have  suffered  an  inversion 
or  not. 

A  low  power,  50  to  100  mm.  focus  objective,  is  good  for  projecting 
the  image. 

One  could  use  a  lantern  slide  with  print  upon  it,  or  even  a  nega- 
tive. For  our  experiments  we  used  a  lantern  slide  or  negative  of 
the  metric  measure  (fig.  178,  211)  so  that  cuts  could  be  made  for 
this  book  which  were  exactly  like  the  images  obtained  on  the  screen 
with  the  transparency  in  different  positions. 


Ocular 


I 


Objective 


FIG.  207.     DIAGRAM  OF  THE  COURSE  OF  THE  RAYS  AND  THE  POSITION  OF 
THE  IMAGES  WHEN  AN  OCULAR  is  USED. 

Object     The  object  whose  image  is  to  be  projected. 

Objective    The  projection  objective. 

/  /  Field  lens  of  the  ocular.  It  acts  with  the  objective  to  give  a  real, 
inverted  image  r  i. 

r  i  The  real,  inverted  image  of  the  object  formed  by  the  objective  and  the 
field  lens  of  the  ocular. 

r1  i1  The  inverted  image  of  the  object  which  would  be  formed  by  the  objec- 
tive if  the  ocular  were  absent. 

e  I  Eye  lens  of  the  ocular.  It  acts  like  a  second  projection  objective  and 
gives  a  screen  image  of  the  real  image  (r  i}. 

Axis    The  optic  axis  of  all  the  lenses. 

Screen  Image  The  image  projected  by  the  eye  lens.  This  image  will  be 
right  side  up,  but  the  rights  and  lefts  will  be  reversed  on  the  ordinary  opaque 
screen.  If  a  translucent  screen  is  used  and  the  observer  is  behind  it,  the  image 
will  appear  erect,  and  the  rights  and  lefts  will  not  be  reversed. 


CH.  X]  ERECT  IMAGES  IN  DRAWINGS  367 

It  is  a  good  plan  to  have  a  specially  prepared  microscopic  slide  or 
a  lantern  slide  with  print  at  hand  whenever  one  is  going  to  draw, 
then  one  can  determine  quickly  and  exactly  how  the  specimen 
should  be  placed  to  give  an  erect  image.  A  simpler  method  still 
is  to  write  the  letters,  a,  k,  on  the  cover  of  the  specimen  to  be 
drawn  (§512,  fig.  220). 


POSITION  OF  THE  OBJECT  FOR  ERECT  IMAGES  WITH  THE  PROJECTION 

MICROSCOPE  AND  AN  OBJECTIVE  ONLY,  OR  WITH  AN  OBJECTIVE 

AND  AN  AMPLIFIER 

§  518.  For  an  opaque  vertical  screen. — Place  the  object  on  the 
stage  as  a  lantern  slide  is  placed  in  its  carrier  (§35),  that  is,  with  the 
specimen  facing  the  light  and  the  lower  edge  up.  With  a  micro- 
scopic specimen  this  would  bring  the  cover-glass  next  the  stage  and 
facing  away  from  the  objective  instead  of  toward  it,  as  in  ordinary 
microscopic  observation.  In  this  case  one  must  focus  through  the 
slide  instead  of  through  the  cover-glass.  This  can,  of  course,  be 
done  with  low,  but  not  with  high  powers.  (See  drawing  on  a  hori- 
zontal surface  §  524). 

With  the  specimen  placed  as  directed,  the  image  on  the  vertical 
opaque  screen  will  appear  erect  in  every  way  (fig.  211). 

If  one  faces  the  light  and  looks  at  the  specimen  on  the  stage  it  will 
look  like  fig.  214  that  is,  like  print  seen  through  a  sheet  of  paper 
wrong  side  up. 

§  519.  For  a  translucent  vertical  screen. — If  the  screen  is  of 
ground-glass  like  that  of  a  photographic  camera,  or  if  it  is  of  tracing 
paper  or  other  translucent  substance  supported  by  clear  glass,  the 
object  should  be  placed  on  the  stage  so  that  it  faces  the  objective, 
and  is  lower  edge  up. 

When  the  observer  looks  at  the  image  on  the  translucent  screen, 
i.  e.,  facing  the  light,  the  image  will  be  erect  like  fig.  211. 

When  he  faces  the  light  and  looks  at  the  object  on  the  stage  it  will 
appear  like  fig.  212,  i.  e.,  it  is  simply  upside  down. 


368 


ERECT  IMAGES  IN  DRAWINGS 


[Cn.'X 


POSITION  OF  THE  OBJECT  FOR  ERECT  IMAGES  ON  A  HORIZONTAL 

SURFACE  WITH  AN  OBJECTIVE  OR  WITH  AN  OBJECTIVE  AND  AN 

AMPLIFIER  AND  A  45  DEGREE  MIRROR  OR  PRISM 

§  520.  For  an  opaque  horizontal  screen. — (A)  If  for  a  drawing 
table  and  mirror  (fig.  182),  place  the  object  on  the  stage  so  that 
it  faces  the  objective  and  is  right  edge  up.  The  image  on  the 
horizontal  surface  will  appear  erect  when  the  observer  looks  at  it 
facing  away  from  the  light. 

The  object  on  the  stage  will  appear  erect  when  the  observer  looks 
at  it  facing  toward  the  light. 


FIG.  208.     KEPLER'S  METHOD  OF  PRODUCING  ERECT  IMAGES  BY  MEANS 
OF  Two  PROJECTION  LENSES. 

(From  Schemer's  "Oculus",  i6ig). 


CH.  X]  ERECT  IMAGES  IN  DRAWINGS  369 

(B)  If  the  mirror  is  very  close  to  the  objective  (fig.  183)  the 
natural  position  for  drawing  is  to  sit  facing  the  light.  The  object 
then  is  put  in  position  facing  the  objective  as  before,  but  upside 
down.  The  image  will  appear  erect  on  the  drawing  surface  when 
the  observer  faces  the  light. 

§  521.  For  a  translucent,  horizontal  screen. — In  some  of  the 
old  forms  of  sketching  apparatus  the  image  was  reflected  upward 
by  a  mirror  or  prism,  and  the  artist  drew  on  the  upper  surface. 


FIG.  209.     DIAGRAM  TO  SHOW  THAT  THE  SIZE  OF  THE  IMAGE  OF  AN  OBJECT 

DEPENDS  UPON  THE  RELATIVE  DISTANCE  OF  THE  OBJECT  AND  IMAGE 

FROM  THE  CENTER  OF  THE  PROJECTION  LENS. 

(From  The  Microscope). 

In  this  figure  the  image  is  four  times  as  far  from  the  center  of  the  lens  (cl) 
as  the  object,  hence,  from  the  law  of  similar  triangles,  the  image  must  be  four 
times  as  long  as  the  object. 

For  such  an  arrangement,  the  object  is  put  on  the  stage  facing  the 
light,  but  right  edge  up.  The  image  will  appear  erect  on  the 
translucent  screen  when  the  observer  faces  the  light  and  looks  down 
upon  the  screen.  For  this  experiment  the  mirror  or  prism  must  be 
on  the  lower  side  of  the  ocular  (fig.  215). 

POSITION  OF  THE   OBJECT  FOR  AN  ERECT    IMAGE    WITH    AN 
OBJECTIVE  AND  AN  OCULAR 

§  522.  For  an  opaque  vertical  screen. — The  object  should  face 
the  light  as  with  a  lantern  slide,  but  it  must  be  right  edge  up. 
With  a  microscopic  specimen  the  cover-glass  will  be  next  the  stage 
as  in  §  518.  On  the  screen  the  image  will  appear  erect  (fig.  211). 
The  object  on  the  stage  will  appear  reversed  like  print  seen  in  a 
mirror  (fig.  213).  . 


370 


ERECT  IMAGES  IN  DRAWINGS 


[CH.  X 


FIG.  210.     DIAGRAM  TO  SHOW  THAT  THE  SIZE  OF  THE  IMAGE  DEPENDS 

UPON  THE  DISTANCE  OF  THE  OBJECT  FROM  THE  CENTER  OF  THE  LENS. 

(From  The  Microscope). 

The  object  at  Object-a  necessitates  an  image  at  Image-a;  while  if  the  same 
object  is  moved  closer  to  the  lens,  as  at  Object-b,  the  image  will  move  farther 
from  the  lens  (Image-to)  and  be  correspondingly  larger. 

//    The  principal  foci  of  the  lens  (objective). 

axis     The  principal  axis  of  the  lens. 

Secondary  axis  a,  Secondary  axis  b  Represent  the  secondary  axes  which 
mark  the  limit  of  the  object  and  the  two  images. 

With  the  object  farther  from  the  lens  the  secondary  axes  are  in  full  lines, 
while  for  the  object  nearer  the  lens  the  secondary  axes  and  the  image  are  shown 
by  broken  lines. 

§  523.  For  a  translucent  vertical  screen. — The  object  is  put  on 
the  stage  facing  the  objective  and  right  edge  up.  The  image  will 


CH.  X]  ERECT  IMAGES  IN  DRAWINGS 

1O  CENTIMETER  RULE 


371 


The  upper  edge  is  in  millimeters,  the  lower  in  centimeters. 
FIG.  211.    CORRECT  IMAGE. 


01 

FIG.  212.    INVERTED  IMAGE. 

H3T3MITK3O  OI 


nr  lawof  ariJ  .aiaJamillim  ni  ai  sgba  isqqu 

FIG.  213.     MIRROR  IMAGE. 
nbbGL  cq&e  13  ID  njjjjiraGfGtg'  fpc  JOMGL  ID 


TO  CEMXI7IEXEH 

FIG.  214.     INVERTED  MIRROR  IMAGE. 

FIG.  211-214.     FIGURES  OF  A  METRIC  RULE,  FULL  SIZE,  TO  SHOW  CORRECT, 
INVERTED,  MIRROR  AND  INVERTED  MIRROR  IMAGES. 

These  representations  of  screen  images  show  the  result  of  placing  the  object 
in  different  positions  or  of  using  different  means  in  projection.  The  determin- 
ing factors  for  the  position  of  the  object  for  a  correct  screen  image  are: 

"(i)  Projection  by  an  objective  or  bv  an  objective  and  an  amplifier  (fig.  121, 
126). 

(2)  Projection  by  means  of  two  lenses  or  of  an  objective  and  an  ocular 
(fig.  207,  208). 


372 


ERECT  IMAGES  IN  DRAWINGS 


(3)  •  The  use  of  a  prism  or  of  a  mirror  to  change  the  direction  of  the  rays  on 
their  way  to  the  screen  (fig.  192). 

(4)  The  use  of  an  opaque  screen. 

(5)  The  use  of  a  translucent  screen. 

appear  erect  like  fig.  211  when  seen  through  the  translucent  screen 
and  facing  the  light. 

Facing  the  light,  the  object  on  the  stage  will  also  appear  erect. 


POSITION  OF  THE  OBJECT  FOR  AN  ERECT  IMAGE  WITH  AN  OBJECTIVE 

AND  OCULAR,  AND  A  45   DEGREE   MIRROR  OR  A  TOTALLY 

REFLECTING  PRISM 

§  524.  For  an  opaque  horizontal  screen. — (A)  For  the  draw- 
ing table  and  mirror  (fig.  182),  place  the  object  on  the  stage  so  that 
it  faces  the  objective  and  is  with  the  lower  edge  up.  The  image  will 
appear  erect  on  the  drawing  surface  when  the  observer  faces  away 
from  the  light. 


FIG.  215.     EARLY  METHODS  OF  DRAWING  WITH  PROJECTION  APPARATUS. 

In  the  picture  at  the  left  (Fig.  6)  is  shown  a  drawing  tent  or  box  with  a  45° 
mirror  and  vertical  objective  by  which  an  erect  image  is  projected  upon  the 
drawing  table  as  in  figures  88-89.  The  artist  sits  outside,  but  has  his  head 
and  bust  within  and  the  light  is  excluded  by  a  cloth  curtain  over  the  back. 

In  Fig.  5  is  shown  a  drawing  box  composed  of  an  objective  at  the  right  (CD), 
a  45°  mirror  (E  F),  and  a  drawing  surface  (G)  covered  by  a  sloping  roof  of 
opaque  material  to  keep  out  the  light.  With  this  instrument  the  artist  simply 
introduces  the  hand  and  pencil.  The  picture  will  have  the  rights  and  lefts 
reversed  as  the  drawing  is  made  on  the  back  of  the  drawing  paper,  not  on  the 
front  as  with  Fig.  6. 

'Fig.  4  is  to  show  the  course  of  the  rays  from  an  object  (A  B),  and  its  inverted 
image  (G  H):  When  the  mirror  (E  F)  is  introduced  the  image  (I  K)  is  rendered 
horizontal. 


CH.  X]  DRAWING  FOR  PUBLICATION  373 

If  the  observer  faces  the  light  the  object  on  the  stage  will  appear 
like  a  printed  page  upside  down  (fig.  212). 

(B)  For  a  drawing  shelf,  the  mirror  or  prism  being  close  to  the 
ocular  and  the  draughtsman  sitting  with  his  face  toward  the  light 
(fig.  183,  187),  the  object  is  placed  on  the  stage  facing  the  objective 
and  right  edge  up. 

The  image  will  be  erect  on  the  drawing  surface  (fig.  211). 

The  object  on  the  stage  will  also  appear  erect  (fig.  211). 

§  525.  For  a  translucent  screen. — For  this  the  object  is  simply 
turned  around  so  that  it  faces  in  the  opposite  direction  in  each  case 
but  remains  the  same  edge  up. 

§  526.  For  erect  images  on  a  horizontal  drawing  surface  with 
apparatus  like  Edinger's  (fig.  202). — In  this  case  no  mirror  or 
prism  is  necessary.  The  position  of  the  object  on  the  stage  for 
erect  images  is  precisely  the  same  as  for  a  horizontal  microscope 
and  a  vertical  screen  (§  518). 

This  has  the  disadvantage  of  requiring  one  to  turn  the  cover- 
glass  away  from  the  objective,  which  prohibits  the  use  of  high 
powers.  If  the  cover-glass  is  turned  toward  the  objective  the 
drawing  will  be  like  a  mirror  image  (fig.  213). 

DRAWINGS  FOR  PUBLICATION  BY  THE  AID  OF  PROJECTION 
APPARATUS 

§  527.  Projection  apparatus  can  give  much  assistance  in  pro- 
ducing the  outlines  and  main  details  of  drawings  for  publication. 
The  outline  drawings  should  be  made  on  good  drawing  paper  with  a 
medium  lead  pencil.  When  the  ink,  air-brush,  or  crayon  work  is 
added  for  the  finished  drawing,  the  pencil  lines  will  be  covered  or 
they  may  be  erased.  The  finishing  must  be  done  free-hand  and 
constant  reference  made  to  the  actual  specimen,  to  the  image  on 
the  screen,  or  as  looked  at  through  a  microscope.  The  finishing 
cannot  be  done  successfully  with  the  image  of  the  specimen  pro- 
jected on  the  drawing  paper  as  one  cannot  tell  how  the  drawing 
looks  with  the  image  projected  upon  it.  By  means  of  a  suitable 
screen  the  image  may  be  cut  off  of  part  of  the  drawing  surface  while 
doing  the  finishing.  By  removing  the  screen  the  image  can  be 


374  DRAWING  FOR  PUBLICATION  [Cu.  X 

projected  again  upon  the  surface  to  make  sure  that  all  the  details 
have  been  correctly  drawn. 

It  is  always  desirable  that  drawings  accompanying  a  scientific 
article  should  be  at  a  definite  enlargement  or  reduction,  and  that 
the  scale  of  the  drawing  be  definitely  stated  (See  Style  Brief,  of  the 
Wistar  Institute,  pp.  16-17). 

If  the  drawings  have  been  made  without  first  doing  this,  then 
the  magnification  can  be  found  by  arranging  the  apparatus  exactly 
as  when  the  drawings  were  made  and  using  a  micrometer  as  directed 
in  §  508. 

A  plan  frequently  followed  is  to  have  a  few  lines  of  the  microme- 
ter image  drawn  in  one  corner  near  the  picture.  Then  any  one 
can  determine  the  scale  of  magnification  or  reduction  (§510,  5ioa). 

§  528,  Lettering  the  drawings. — After  the  drawings  are  finished 
the  various  parts  can  be  lettered,  or  words  can  be  written  in  where 
needed.  Most  workers,  however,  cannot  letter  neatly  enough  for 
publication.  For  such  it  is  better  to  use  printed  words,  letters  or 
numerals. 

It  is  assumed  here  that  the  drawings  will  be  reproduced  by  some 
photo-engraving  process;  and  for  this  the  letters  or  words  pasted 
on  the  drawing  would  best  be  printed  on  tissue  paper,  (§  528a); 
Gothic  type  is  best.  By  consulting  fig.  216,  one  can  select  the 
proper  size  for  the  reduction  to  be  made  (§  531). 


§  527a.  Tracing  pictures  natural  size  on  drawing  paper. — It  frequently 
happens  in  making  the  drawings  for  a  book  or  for  a  scientific  paper  that  pic- 
tures from  other  books  or  scientific  papers  are  desired.  Of  course,  if  there  are 
to  be  no  modifications,  the  simplest  method  is  to  borrow  an  electrotype  or  to 
have  the  photo-engraver  make  a  new  cut;  but  sometimes  only  an  outline  is 
needed  or  modifications  are  desired. 

If  the  picture  is  to  be  the  same  size  as  the  one  in  the  book  or  periodical  it 
can  be  easily  traced  upon  the  drawing  paper  as  follows :  In  place  of  a  wooden 
shelf  on  the  table  (fig.  183,  §  460)  a  piece  of  thick  glass  is  placed  on  the  brackets 
and  an  incandescent  lamp  of  40  or  60  watts,  surrounded  by  a  lamp  shade  of 
some  kind,  is  turned  so  that  it  shines  directly  upward.  It  is  then  placed  up 
close  to  the  glass  and  the  picture  to  be  traced  is  placed  on  the  glass,  and  over 
it  the  drawing  paper.  The  light  is  so  strong  that  it  traverses  the  picture  and 
the  drawing  paper  and  the  picture  is  clearly  seen  on  the  upper  side  of  the 
drawing  paper.  It  can  be  traced  almost  as  easily  as  if  an  image  were  projected 
upon  the  upper  face.  In  tracing  drawings  for  this  book,  Wattman's  hot  pressed 
paper  and  Reynold's  bristolboard  were  used  in  making  tracings  in  the  way  just 
described.  Even  if  there  is  print  on  the  opposite  side  of  the  page  the  tracing 
of  the  picture  can  be  made  successfully. 


CH.  X]  DRAWING  FOR  PUBLICATION  375 

§  529.  Fastening  the  letters  to  the  drawing. — The  letters, 
numerals,  or  words  are  cut  from  the  printed  sheet,  with  pains  to 
make  straight  edges  and  square  corners.  Then  they  are  turned 
face  downward  and  with  a  camel's  hair  brush  of  small  size  such 
as  is  used  by  artists,  some  freshly  made  starch  paste  is  put  on  the 
back.  As  each  word  or  letter  is  pasted,  one  uses  fine  forceps  to 
pick  it  up  and  place  it  in  the  desired  position,  being  sure  that  the 
letter  or  word  is  arranged  properly.  In  the  beginning  it  is  well  to 
use  a  try-square  or  some  other  instrument  to  make  sure  that  the 
letter  or  word  is  arranged  correctly.  Then  it  is  pressed  down,  using 
some  tissue  paper  over  the  finger  or  some  fine  blotting  paper,  and 
pressing  directly  downward  so  as  not  to  disarrange  the  letter  or 
word  by  a  lateral  thrust. 

§  530.  White  letters  on  a  black  back-ground. — Sometimes  it  is 
necessary  to  use  white  letters  or  numerals  on  a  black  ground  (e.  g., 
see  fig.  211-214).  In  the  largest  printing  houses  one  might  be 
able  to  get  these,  but  they  are  easily  made  as  follows : 

The  desired  letters,  numerals,  abbreviations  or  words  are  printed 
on  the  white  tissue  paper  as  indicated  above.  A  sheet  of  this 
printed  tissue  paper  is  used  as  a  negative  by  putting  a  clean  glass 
in  the  printing  frame,  placing  the  printed  tissue  paper  face  down 
on  the  glass,  and  then  putting  some  Velox,  Cyco,  or  other  photo- 
graphic paper  in  place  and  printing  exactly  as  for  any  negative. 
The  opaque  letters  will  be  in  white,  and  the  practically  transparent 
tissue  paper  between  the  letters  will  give  the  black  back-ground  in 
the  print. 


§  528a.  (i)  The  authors  are  indebted  to  Mr.  George  C.  Stanley,  Ithaca's 
photo-engraver,  for  the  suggestion  to  use  tissue  paper  for  the  printed  letters 
and  words  to  be  pasted  on  drawings  for  photo-engraving.  The  advantage  of 
tissue  paper  is  that  there  is  no  shadow  around  the  edge  of  the  letter  or  word. 
Where  thick,  ordinary  white  paper  is  used  there  is  frequently  left  a  black  line 
due  to  the  shadow,  and  this  line  must  be  cut  out  by  the  engraver  or  it  will  give 
a  black  line  in  the  printed  book. 

(2)     Starch  paste  for  use  in  sticking  on  the  letters  and  words  should  be 
freshly  made.     A  good  paste  is  made  of  dry  laundry  starch  5  grams,  cold  wate  r 
50  cc.     These  are  put  in  a  small  vessel  and  gradually  heated  with  constan  t 
stirring  until  the  paste  is  formed.     Mucilage  and  other  adhesives  make  the 
tissue  paper  yellowish;   and  paste  which  has  been  made  some  time  is  liable  to 
have  fine  lumps  in  it  so  that  the  letters  are  torn  or  distorted  in  pressing  them 
down  on  the  drawing. 


376  DRAWING  FOR  PUBLICATION  [CH.  X 

Paper  and  developer  should  be  of  the  contrast  variety  to  give 
pure  blacks  and  whites. 

These  letters,  etc.,  are  cut  out  and  pasted  on  the  drawing  just  as 
described  above.  The  photographic  paper  being  rather  thick, 
there  will  be  a  white  streak  around  the  letter,  etc.,  where  cut  out. 
This  can  easily  be  blackened  after  being  stuck  in  place  by  the  use 
of  a  pen  or  a  fine  brush,  using  India  ink. 

SIZE  OF  DRAWINGS  AND  THEIR  LETTERING 

§  53 1 .  It  is  wise  to  make  the  drawings  considerably  larger  than 
the  desired  picture.  In  reducing,  the  coarseness  and  some  other 
infelicities  of  the  drawing  become  less  noticeable. 

Of  course  if  the  drawing  is  made  exactly  the  size  of  the  desired 
cut,  then  it  must  look  exactly  as  one  desires  it  in  the  printed  book; 
it  is  not  liable  to  be  improved  by  the  process  of  photo-engraving. 
But  if  the  drawing  is  to  be  reduced,  then  the  lettering,  etc.,  must 
be  coarse  enough  in  the  drawing  to  give  the  proper  appearance  in 
the  finished  cut. 

There  is  some  confusion  in  the  minds  of  the  inexperienced  as  to 
the  appearance  of  a  picture  half  the  size  of  the  original.  To  the 
engraver  half -size  always  means  that  any  given  line  or  part  is  just 
half  the  length  of  the  original.  That  is,  if  any  line  of  the  original 
were  10  centimeters  long,  the  finished  cut  would  show  the  same 
line  5  centimeters  long  if  it  were  reduced  to  half  the  original  size. 
The  appearance  is  well  shown  in  the  accompanying  figure  (fig.  216). 
In  the  upper  half  the  letters  and  numerals  are  of  full  size;  in  the 
middle  they  are  of  half  the  original  size;  and  below  they  are  of 
one-fourth  the  original  size.  This  picture  will  show  one  also  about 
the  size  of  type  to  use  for  the  different  reductions.  The  numerals 
on  the  left  indicate  the  size  of  the  type,  as  24  point,  18  point,  12, 
10,  8,  and  6  point,  respectively. 

The  lettering  of  pictures  in  books  and  periodicals  should  be 
proportioned  to  the  size  of  the  details  of  the  cuts.  It  is  distressing 
to  have  the  letters  and  numerals  on  figures  the  most  prominent 
feature.  On  the  other  hand,  it  is  exasperating  to  have  letters 
so  minute  that  one  needs  a  microscope  to  make  them  out.  As 


24  Point  Type    A  a 
123456789  10 

18  Point  Type  ARS  2  34 

12  Point  Type    ABCabc    1234 
10  Point  Type     ABC     abc     12345 

8  Point  Type    ABCD      abed      12345 
6  Point  Type       ABCDabcd12345       I    II    III    IV 

ABCD  abed         123456789     10  I     II     III     IV     V     VI 


24  Point  Type    A  a 
123456789  10 

18  Point  Type  A  R  S  2  3  4 

12  Point  Type    ABCabc    1234 
10  Point  Type     ABC     abc     12345 

8  Point  Type    ABCO      aOcd      12345 
6  Point  Type      ABCOabed12349       I    II    III    IV 


24  Point  Type    A  a 
123456789  10 

18  Point  Type  A  R  S  2  3  4 


FlG.  216 


378  PHOTOGRAPHIC  ENLARGEMENTS  [Cn.  X 

FIG.  216.     GOTHIC  TYPE  TO  USE  ON  DRAWINGS  AND  THE  APPEARANCE 
WHEN  REDUCED. 

In  the  upper  half  are  shown  letters  and  figures  of  full  size  with  their  designa- 
tions by  the  printer,  i.  e.,  24,  18,  12,  10,  8  and  6  point  type. 

In  the  lower  half  are  shown  the  same  reduced  to  one-half  the  length,  and 
reduced  to  one-fourth  the  length. 

shown  by  the  numerals  and  letters  in  fig.  216,  if  the  drawing  is 
not  to  be  reduced  at  all  one  can  use  6,  8,  or  possibly  10  point  type. 
For  one-half  reduction  (one-half  -off) ,  the  lettering  should  be 
with  10  or  12  point  type.  For  one-fourth  size  ($4  off),  then  the 
lettering  should  be  with  12  or  preferably  with  18  point  type. 

PROJECTION  APPARATUS  FOR  PHOTOGRAPHIC  ENLARGEMENTS 

§  532.  Enlarged  prints  of  negatives. — There  is  great  advantage 
in  making  pictures  of  large  objects  at  a  considerable  distance  so  that 
the  perspective  will  be  correct  and  all  levels  in  focus.  It  is  also 
advantageous  to  make  pictures  of  microscopic  objects  without 
undue  enlargement,  then  there  is  greater  sharpness  of  the  object 
as  a  whole. 

If  now  one  wishes  a  large  picture,  any  good  negative  can  be 
printed  by  the  aid  of  a  photographic  objective  at  almost  any 
desired  enlargement.  This  can  be  done  with  projection  apparatus 
in  a  dark  room  by  the  following  method :  The  management  of  the 
projection  apparatus  is  as  for  drawing.  The  negative  is  placed  in 
some  kind  of  a  holder  and  put  in  the  cone  of  light  from  the  main 
condenser  where  the  part  to  be  enlarged  will  be  fully  illuminated 
(fig.  132,  185).  Care  must  be  taken  to  so  place  the  negative  that 
an  erect  image  will  appear  on  the  printing  paper  (§  512). 

§  533.     Condenser  required  with  negatives  of  different  sizes. — 

Remember  that  the  diameter  of  the  condenser  must  be  somewhat 
greater  than  the  diagonal  of  the  part  of  the  negative  to  be  enlarged 
(§314  and  fig.  114).  For  example,  to  use  the  whole  of  a  lantern- 
slide  negative  (85  x  100  mm.,  3^  x  4  in.)  the  condenser  should 
have  a  diameter  of  14  cm.  (5^2  in.). 

For  a  negative  100  x  125  mm.  (4  x  5  in.),  the  condenser  should 
beiScm.  (7  in.)  in  diameter;  fora  negative  125  x  175  mm.  (5x7^1.), 
the  condenser  should  be  23  cm.  (9  in.)  in  diameter  and  for  a  nega- 
tive 200  x  250  mm.  (8  x  10  in.),  the  condenser  should  be  35  centi- 


CH.  X]  PHOTOGRAPHIC  ENLARGEMENTS  379 

meters  (14  in.)  in  diameter.  Of  course,  if  only  a  part  of  the  nega- 
tive plate  contains  the  picture  to  be  enlarged  then  a  smaller  con- 
denser in  the  given  case  will  answer.  The  above  figures  are  for 
the  diagonal  of  the  respective  sizes.  These  condensers  are  usually 
of  relatively  long  focus,  especially  those  of  the  larger  diameters, 
e.  g.,  the  35  cm.  lens  ordinarily  has  a  focus  of  50  centimeters.  The 
condensers  most  used  for  enlarging  are  usually  of  the  double  form, 
the  convexities  facing  each  other  as  for  the  magic  lantern  condenser 
(%.  185)- 

§  534.  Objectives  to  use  for  printing. — It  is  necessary  to  use 
an  objective  which  has  been  corrected  for  photography.  The 
ordinary  projection  objective  gives  a  good  visual  image,  but  not  a 
good  photographic  image,  hence  it  is  better  to  use  a  photographic 
objective. 

§  535.  In  focusing,  some  white  paper  should  be  put  into  the 
printing  frame  or  pinned  in  place  and  the  image  focused  with  care. 
The  photographic  paper  when  put  in  the  same  place  will  then  give 
a  sharp  picture. 

§  536.  Photographic  paper  for  printing  with  projection  appara- 
tus.— If  one  has  sunlight  or  the  arc  light  the  developing  papers  like 
Velox,  Cyco,  etc.,  are  plenty  rapid  enough.  If  a  weak  light  is  all 
that  is  available,  then  Haloid  or  one  of  the  more  rapid  bromide 
papers  will  be  called  for. 

§  537.  Holding  the  paper  while  printing. — (A)  If  the  pictures 
are  of  microscopic  objects  or  other  pictures  of  relatively  small  size 
(i.  e.,  up  to  30  x  35  cm.;  12  x  14  in.),  a  good  method  is  to  put  a 
clear  glass  in  a  printing  frame  and  press  the  printing  paper  down 
upon  it  just  as  one  does  for  printing  from  a  glass  negative  by  con- 
tact. This  holds  the  paper  perfectly  flat  and  ensures  uniform 
sharpness.  With  the  printing  frame  one  can  lay  it  flat  if  a  mirror 
or  prism  is  used,  or  it  can  stand  on  edge  facing  the  objective  if  no 
mirror  is  used. 

(B)  If  the  printing  paper  is  large  the  usual  method  is  to  have  a 
board  screen  on  a  track.  The  picture  is  then  got  of  the  desired 
size  by  varying  the  distance  between  the  board  and  the  objective, 


380  PHOTOGRAPHIC  ENLARGEMENTS  [Cn.  X 

then  the  image  is  carefully  focused  by  putting  some  white  paper 
on  the  screen  or  by  haying  a  ground-glass  in  the  middle  of  the 
screen.  Then  the  objective  is  covered  with  a  dark  cap  or  with  a 
cap  containing  ruby  glass,  and  the  photographic  paper  is  fastened 
in  place  by  thumb  tacks  or  in  some  other  way,  care  being  taken  to 
stretch  it  smooth. 

§  538.  Exposure. — When  the  paper  is  in  place  the  cap  is 
removed  from  the  objective  and  the  projected  image  will  print  on 
the  paper.  The  time  necessary  depends  upon  the  magnification, 
the  density  of  the  negative,  the  intensity  of  the  light  and  the  sensi- 
tiveness of  the  paper  used .  It  usually  takes  about  one-fourth  the  time 
one  would  print  by  contact  using  a  1 6  candle-power  frosted  incan- 
descent lamp.  A  good  plan  is  to  try  a  small  piece  of  the  paper  and 
determine  the  correct  exposure  before  printing  on  the  large  sheet. 
After  the  exposure  the  objective  is  covered  with  the  cap  and  the 
paper  is  developed  exactly  as  for  contact  printing. 

§  539.  Diaphragm  of  the  objective.— In  printing,  the  diaphragm 
of  the  objective  is  wide  open  if  the  unmodified  cone  of  light  is  used 
for  illumination.  This  has  one  defect  with  the  arc  lamp.  If  there 
are  any  irregularities  in  the  negative,  such  as  minute  scratches,  etc., 
they  would  show  in  the  print,  whereas  if  the  illumination  were  from 
an  extended  instead  of  a  very  small  source  like  the  crater  of  the  arc 
lamp,  the  slight  defects  would  show  very  much  less. 

To  obviate  this  defect  with  the  arc  lamp  one  or  more  plates  of 
ground-glass  or  of  milk  white  glass  are  placed  in  the  path  of  the 
beam  before  the  negative.  It  must  be  put  far  enough  from  the 
negative  so  that  the  grain  of  the  ground-glass  will  not  show. 

With  the  ground-glass  or  the  milky  glass  in  the  beam  the  dia- 
phragm of  the  objective  can  be  closed  as  much  as  desired.  The 
use  of  the  ground-glass  and  the  closure  of  the  diaphragm  will,  of 
course,  necessitate  a  longer  exposure. 

§  540.  Avoidance  of  stray  light. — If  one  is  to  do  considerable 
printing  with  the  projection  apparatus  a  light-tight  lamp-house 
must  be  used  and  light-tight  bellows  between  the  condenser  and 
the  negative  and  objective.  A  special  camera  is  most  satisfactory. 


CH.  X]  PHOTOGRAPHY  AND  PROJECTION  381 

For  the  occasional  use  of  a  laboratory  the  stray  light  can  be  ex- 
cluded by  means  of  asbestos  paper.  Sometimes  the  arc  lamp  is 
put  on  the  outside  of  a  partition,  but  that  necessitates  going  out  of 
the  printing  room  to  adjust  the  lamp.  If  direct  current  is  available 
an  automatic  lamp  is  a  great  convenience. 

PHOTOGRAPHING  WITH  PROJECTION  APPARATUS 

§  541.  Apparatus  which  will  give  good  projection  of  micro- 
scopic specimens  can,  with  slight  modifications  be  used  for  photo- 
micrography. 

There  are  three  possibilities: 

(1)  Printing  the  image  directly  on  a  developing  paper. 

(2)  Exposing  a  dry  plate  directly  to  the  image  as  for  the  paper. 

(3)  Using  a  camera  and  plate  holder. 

§  542.  Printing  the  projected  image  directly  on  a  developing 
paper. — With  the  apparatus  set  up  exactly  as  for  drawing  one  can 
expose  a  sheet  of  developing  paper  to  the  sharply  focused  image  of 
the  specimen  as  described  for  the  enlargement  of  negatives  (§532). 
The  lights  and  shades  will  be  reversed,  but  all  the  outlines  and 
details  will  be  present.  This  is  a  convenient  method  of  getting  an 
enlarged  record  of  the  specimen. 

It  is  also  a  good  method  for  making  pictures  for  models  (§  511) 
especially  when  there  are  many  details.  With  the  cheap  develop- 
ing papers  in  rolls  now  obtainable  the  expense  is  not  greater  than 
for  making  drawings,  and  there  is  liable  to  be  a  gain  in  accuracy. 
The  main  draw-back  is  that  but  a  single  picture  is  made  of  each 
specimen  for  a  single  exposure,  while  in  drawing  it  is  as  easy  to 
make  two  or  three  as  one,  by  using  carbon  paper  (§  511). 

§  543.  Exposing  a  dry  plate  directly  to  the  image. — A  dry  plate 
may  be  exposed  as  just  described  for  the  developing  paper.  The 
object  must  be  so  placed  on  the  stage  of  the  microscope  that  the 
image  on  the  screen  will  be  a  mirror  image  of  the  specimen,  that  is, 
the  rights  and  lefts  will  be  reversed  as  they  should  be  in  a  negative. 
The  image  is  sharply  focused,  and  the  light  cut  off  with  a  deep  red 
glass-  so  that  the  plate  will  not  be  affected. 


382 


PHOTOGRAPHY  AND  PROJECTION 


[CH.  X 


A  Set  screw  holding  the  rod  (S)  in  any  de- 
sired position. 

P  Q  Set  screws  by  which  the  bellows  are 
held  in  place. 

B  Stand  with  tripod  base  in  which  the  sup- 
porting rod  (5)  is  held.  This  rod  is  now  grad- 
uated in  centimeters  and  is  a  ready  means  of  de- 
termining the  length  of  the  camera. 

M    Mirror  of  the  microscope. 

L  The  sleeve  serving  to  make  a  light-tight 
connection  between  the  camera  and  microscope. 

O     Th'e  lower  end  of  the  camera. 

R  The  upper  end  of  the  camera  where  the 
focusing  screen  and  plate  holder  are  situated. 


The  plate  holder  is  then  put  in  po- 
sition, and  the  dark  slide  removed. 
The  red  glass  is  then  removed  for  the 
short  time  necessary  for  the  exposure 
(Vioth  sec.,  more  or  less)  and  then  re- 
placed. The  dark  slide  is  put  back  in 
the  holder.  The  plate  is  developed  and 
printed  as  usual. 

When  working  with  dry  plates  in  this 
way  great  care  is  required  to  avoid  stray 

„  ,,  light.     Stray  light  which  would  not  in- 

FIG.  217.     VERTICAL  PHOTO-      te  J 

MICROGRAPHIC  CAMERA.       jure  printing  papers  will  fog  a  dry  plate. 


holder.  —  When  exact  results  are  required 
or  much  photo-micrography  is  to  be  undertaken,  it  is  better  to  use 
a  camera  in  connection  with  the  projection  apparatus  (fig.  219). 

The  camera  and  projection  apparatus  are  put  on  a  long  labora- 
tory table,  or  the  camera  may  be  put  on  a  second  table  and  adjusted 
to  the  height  of  the  projection  microscope.  The  camera  is  con- 
nected with  the  projection  microscope  by  means  of  a  light-excluding 
sleeve  such  as  that  used  by  Zeiss  with  his  photo-micrographic 
outfit  (fig.  217-218). 

The  camera  serves  to  exclude  all  stray  light  and  to  hold  the 
plate  holder  in  the  correct  position.  The  camera  is  supplied  with  a 
focusing  screen  which  occupies  exactly  the  same  position  as  does 
the  plate  during  exposure. 


CH.  X] 


PHOTOGRAPHY  ANDJPROJECTION 


383 


FIG.  217 A.     VERTICAL  CAMERA  WITH  METAL  SHIELD. 
(From  the  Transactions  of  the  Amer.  Micr.,  Soc.,  Vol.  XXIII,  1901). 

The  camera  is  on  a  low  table  and  a  shield  of  sheet  zinc  or  roofing  tin  is  on  a 
stand  between  the  source  of  light  and  the  camera  to  protect  the  camera  and 
the  eyes  of  the  operator.  Opposite  the  light  source  is  an  opening  with  a'shutter 
The  source  of  light  in  this  case  is  a  kerosene  lamp. 


384  PHOTOGRAPHY  AND  PROJECTION  [Cn.  X 

§  545.  Objectives  to  use. — The  micro-planars  (fig.  123)  or 
other  short  focus  objectives  of  the  photographic  type  are  used 
without  an  ocular.  They  can  be  screwed  into  the  nose-piece  of  the 
microscope  or  the  microscope  can  be  dispensed  with  and  the 
objectives  put  into  the  end  of  the  camera  as  with  photographic 
objectives. 

If  one  wishes  to  use  the  ordinary  microscope  objectives  then  an 
ocular  of  the  projection  type  is  of  great  advantage  although 
Huygenian  oculars  will  give  good  results.  The  apochromatic 
objectives,  and  the  projection  or  compensation  oculars  to  go  with 
them,  give  the  best  results. 

§  546.  Making  the  negative. — The  image  is  first  focused  very 
sharply  on  the  focusing  screen.  For  lights  of  high  intensity  it  will 
be  necessary  to  soften  the  light  in  focusing  so  as  not  to  injure  the 
eyes.  This  can  be  done  by  putting  a  neutral  tint  glass  plate  or 
several  thicknesses  of  ground -glass  or  one  or  more  plates  of  milky 
glass  in  the  path  of  the  beam  before  the  object. 

The  exposure  and  subsequent  development  and  printing  with  the 
negative  are  as  usual. 

§  547.  Photo-micrography  with  a  vertical  camera. — If  a  ver- 
tical microscope  is  to  be  employed  for  photography,  then  it  is  best 
to  use  a  vertical  camera  (fig.  217).  A  parallel  beam  of  light  is 
caused  to  fall  upon  the  plane  surface  of  the  microscope  mirror,  and 
the  mirror  is  turned  to  throw  it  directly  up  through  the  substage 
condenser  upon  the  object.  To  get  the  parallel  or  approximately 
parallel  beam  one  uses  a  condenser  lens  of  very  long  focus  (§  479, 
fig.  154)  or  a  parallelizing  lens  is  used  (fig.  153). 

TROUBLES  MET  IN  CHAPTER  X 

§  548.  The  troubles  liable  to  arise  in  the  work  of  this  chapter 
are  those  common  to  the  preceding  chapters.  Those  discussed  in 
Chapter  I  and  III  are  to  be  especially  reviewed,  as  the  source  of 
light  is  most  likely  to  be  the  electric  arc.  (See  §  i28a  for  the 
blowing  of  fuses) . 

(i)  In  drawing  with  the  microscope  with  the  small  carbon  arc 
lamp  on  the  house  lighting  system,  probably  the  trouble  most 


CH.  X] 


TROUBLES  IX  DRAWING 


385 


FIG.  218.     THE  ZEISS  PHOTO-MICROGRAPHIC  MICROSCOPE. 

(From  Zeiss"  Catalogue). 

This  is  the  parent  form  of  photo-micrographic  stands  with  large  tube  (T), 
handle  in  the  pillar  and  a  special  fine  adjustment  at  the  side  (W).  At  the  top 
is  half  of  the  light -excluding  sleeve. 


386 


TROUBLES  IN  DRAWING 


[On.  X 


likely  to  arise  is  the  lack  of  a  brilliant  picture  on  the  drawing  paper 
owing  to  the  light  in  the  room.  Remember  that  to  get  a  brilliant 
image  the  light  must  come  to  the  eyes  from  the  drawing  surface 
only,  and  the  drawing  surface  must  receive  no  light  except  that 
from  the  specimen.  The  weaker  the  light  and  the  greater  the 
magnification  the  darker  must  the  room  be. 

(2)  In  drawing  from  negatives  or  lantern  slides  remember  that 
it  is  necessary  to  have  a  condenser  somewhat  larger  than  the 
diagonal  of  the  object  to  be  drawn  (§  314,  533). 

(3)  In  drawing  with  the  microscope  where  the  substage  con- 
denser is  used  the  condenser  must  be  in  the  exact  position  to  give 
the  best  results.     If  the  slide  is  thick  the  condenser  is  a  little  higher 


FIG.  219. 


MICRO-PROJECTION  OUTFIT  AND  VERTICAL  CAMERA  ARRANGED 
FOR  PHOTO-MICROGRAPHY. 


(From  The  Microscope}. 

The  apparatus  is  set  up  on  a  long  table  or  on  two  tables  placed  end  to  end. 
The  vertical  camera  (fig.  217)  is  placed  horizontally  and  the  bellows  reversed. 
For  illumination  a  petroleum  lamp  with  large  flat  wick  (38  mm.,  iJ/2  in.) 
answers  well. 

Objects  50  to  60  mm.  in  diameter  may  be  fully  illuminated  with  the  face  of 
the  flame,  the  lamp  being  I  to  2  centimeters  from  the  condenser.  For  powers 
of  100  to  150  diameters  the  flame  is  turned  obliquely  or  edgewise,  and  placed 
5  to  6  centimeters  from  the  condenser.  The  position  shown  in  the  picture 
above  is  for  high  power  work.  No  water-cell  or  specimen  cooler  is  needed. 

A  light-tight  connection  is  made  with  the  large  tube  of  the  microscope  by  a 
double  sleeve  like  that  employed  by  Zeiss  for  the  microscope.  With  low 
magnifications  no  ocular  is  used,  and  the  objective  is  placed  in  the  end  of  the 
camera.  If  one  desires  to  make  pictures  of  a  size  above  the  capacity  of  the 
photo-micrographic  camera  it  is  possible  to  use  an  ordinary  camera,  (fig.  117- 
119),  then  even  quite  large  objects  50  to  60  mm.  long,  can  be  magnified  con- 
siderably. The  petroleum  lamp  has  some  advantages  over  daylight  as  the 
lamp  gives  an  illumination  of  constant  intensity.  It  is  available  during  the 
entire  24  hours  of  the  day,  and  in  all  seasons. 


CH.  X] 


TROUBLES  IN  DRAWING 


387 


than  for  a  slide  which  is  thin.      Attention  to  the  substage  con- 
denser will  make  a  great  difference  with  one's  success. 

(4)  The  right-angled  arc  lamp  should  be   used   in  drawing 
because  if  the  microscope  and  lamp  are  properly  arranged  the  source 
of  light  will  remain  in  the  axis  no  matter  how  long  the  lamp  burns. 
If  an  inclined  carbon  lamp  or  one  with  both  carbons  in  the  vertical 
or  horizontal  position  is  used  the  source  of  light  is  constantly 
getting  out  of  the  axis  from  the  burning  away  of  the  carbons, 
consequently  they  must  be  fed  up  more  frequently  to  keep  the 
source  of  light  in  the  field. 

(5)  The  picture  will  be  distorted  unless  the  axial  ray  strikes  the 
drawing  surface  at  right  angles.     Therefore,  in  using  a  prism  or 
mirror  for  a  horizontal  surface  the  microscope  must  be  horizontal 
and  the  mirror  or  prism  at  45  degrees  to  reflect  the  axial  ray  ver- 
tically downward.     If  the  mirror  or  prism  is  twisted  over  to  one 
side  the  axial  ray  will  not  strike  the  surface  at  right  angles  and  there 
will  be  distortion.     If  one  has  a  micrometer  in  squares  it  is  easy  to 
determine  whether  the  image  is  distorted  or  not. 

(6)  The  image  will  be  erect  only  when  the  object  is  properly 
placed  on  the  stage. 

(7)  If  a  glass  mirror  silvered  on  the  back  is  used,  and  the  object 
is  quite  opaque  the  secondary  image  reflected  from  the  face  is 


FIG.  220.    SLIDE  OF  SERIAL  SECTIONS  WITH  -a,  k-  ON  THE  COVER-GLASS 

TO  ENABLE  ONE  TO  DETERMINE  WHEN  THE  IMAGE  ON  THE  DRAWING 

SURFACE  is  ERECT  (See  fig.  143,  and  §  512,  517). 


388 


DO  AND  DO  NOT  IN  DRAWING 


[CH.  X 


liable  to  cause  confusion.     If  the  mirror  is  silvered  on  the  face  or 
if  a  prism  is  used  there  will  be  no  doubling  of  the  reflected  image. 

(8)  Inverted  images.     One  must  follow  carefully  the  directions 
or  there  is  liable  to  be  an  inversion  of  the  projected  image  (§  512- 
526). 

(9)  In  printing  and  photographing  with  projection  apparatus 
the  difficulties  likely  to  be  met  with  in  photography  are  sure  to  come 
in.     Knowledge  of  the  principles  underlying  photographic  pro- 
cesses will  help  one  to  overcome  the  troubles. 


549.     Summary  of  Chapter  X: 

Do 


i.  Have  a  suitable  room  or  a 
suitable  shield  around  the  draw- 
ing to  keep  out  stray  light 
(§  453-455)- 


Do  NOT 


i.  Do  not  try  to  draw  with 
the  drawing  surface  flooded  with 
stray  light.  Only  the  light 
from  the  specimen  should  reach 
the  drawing  surface. 


2.  Draw  in  the  evening  if  a 
proper  room  is  not  available  in 
the  day  time  (§  453). 


3.  Use  an  arc  lamp  for  light 
if  possible  (§  461-462,  486-487). 

4.  Always  use  a  rheostat  with 
an    arc    lamp,    large    or    small 
(§487,ng.  182,  185). 

5.  One  can  draw  images  pro- 
jected by  all  forms  of  projection 
apparatus  (§452). 


2.  Do  not  forget  that  it  is 
dark  in  all  rooms  in  the  evening 
and,  therefore,  that  is  a  good 
time  to  draw. 

3.  Do  not  use  a  weak  light 
for  drawing  if  you  can  have  an 
arc  light. 

4.  Never  try  to  use  an  arc 
lamp,  large  or  small,  without  a 
rheostat  in  series  with  it. 

5.  Do  not  forget  that  it  is 
possible    to    draw    the    images 
projected  by  any  form  of  appar- 
atus. 


CH.  X] 


DO  AND  DO  NOT  IN  DRAWING 


389 


6.  In  drawing  with  any  form 
of    projection    apparatus    the 
axial  ray  must  strike  the  draw- 
ing surface  at  right  angles  or 
the    image    will    be    distorted 

(§  483). 

7.  Make  sure  that  the  mirror 
or  prism  reflects  the  rays  upon 
the  drawing  surface  so  that  the 
axial  ray  is  at  right  angles  to 
the  surface  (§482-483). 

8.  Use  a  condenser  of  suffi- 
cient diameter  to  fully  light  the 
object  (§  467,  533)- 

9.  Get  the  desired  size  for  the 
drawing  by  making  the  distance 
of  the  drawing  surface  greater 
or  less,  or  by  using  a  different 
optical  system  for  the  projec- 
tion (§465,  507-508). 

10.  Take  great  pains  to  light 
as  brilliantly  as  possible  (§  497, 
and  Chapters  I,  II,  and  IX). 

11.  Take   care   to   have   the 
images  on  the  drawing  surface 
erect  (§512-526). 

12.  In  using  projection   ap- 
paratus   for    photography,    re- 
member the  principles  of  good 
projection,     and    the    require- 
ments for  good  photography. 


6.  Do  not  draw  distorted 
pictures ;  therefore  do  not  have 
the  axial  ray  strike  the  drawing 
surface  obliquely. 


7.  Do  not  forget  to  incline 
the  mirror  used  in  drawing  so 
that  the  axial  ray  will  strike  the 
drawing  surface  at  right  angles. 

8.  Do  not  try  to  project  an 
object  larger  than  the  diameter 
of  the  condenser  lenses  used. 

9.  Do  not  neglect  to  give  the 
scale  at  which  every  drawing  is 
made. 


10.  Do  not  expect  good  light 
unless  the  conditions  for  it  are 
met. 

11.  Do    not    draw    inverted 
images. 

12.  Do  not  expect  projection 
apparatus  to  give  good  photo- 
graphs  unless    sharp,    brilliant 
images      are     projected,     and 
the  photographic  part  is  done 
correctly. 


CHAPTER  XI 
MOVING  PICTURES 

§  550.    Apparatus  and  Material  for  Chapter  XI : 

A  competent  operator  (§  55oa);  Moving  picture  head,  or  mech- 
anism; Rheostat  for  direct  current,  or  rheostat,  inductor  or 
choke-coil,  transformer,  rectifier,  motor-generator  set  for  alter- 
nating current;  Arc  lamp  and  lamp-house ;  Condenser,  assortment 
of  different  sized  plano-convex  lenses  14,  15,  16,  17,  19,  21,  23  cm. 
focus  (5>£,  6,  6>2,  7,  7>£>  8,  9  in.  focus);  meniscus  lens,  23  cm. 
focus  (9  in.  focus) ;  Projection  objective,  equivalent  focus  1 1  to 
15  cm.  (4^2  to  6  in.),  preferably  of  6.3  cm.  (2^  in.)  diameter, 
although  4.5  cm.  (i^  in.)  will  answer;  Moving  picture  films ; 
Tools,  asbestos  gloves,  pliers,  screw  driver,  copper  wire,  pins, 
film  cement;  Supply  of  carbons. 

For  continuous  use  a  special  operating  room  separated  from 
the  auditorium  by  fireproof  walls,  all  openings  into  the  auditorium 
to  have  automatic  shutters  closing  in  case  of  fire,  the  room  to  be 
provided  with  a  large  flue  connecting  to  the  outside  of  the 
building. 

§  550a.  Competent  operator. — As  no  one  can  learn  a  difficult  art  from  book 
directions  alone  without  spending  an  undue  amount  of  time,  we  strongly  advise 
every  one  who  wishes  to  be  a  moving  picture  operator  or  photographer  to  get 
the  help  of  an  expert.  Every  university  and  technical  school  worthy  of  the 
name  now  has  laboratories  in  which  .the  actual  operations  are  learned  by  the 
students  in  repeated  efforts  under  the  direction  of  expert  teachers.  Books  are 
helps,  and  often  give  an  expert  all  that  he  needs  to  enable  him  to  perform 
successfully  some  difficult  or  unfamiliar  operation.  But  the  living  teacher  and 
the  actual  experiment  serve  the  beginner  most  effectively. 

We  strongly  recommend  the  operator  to  possess  the  best  works  on  Moving 
Pictures  and  projection  in  general,  and  to  subscribe  for  one  or  more  periodicals. 
By  studying  these  he  can  keep  himself  informed  of  all  the  advances  in  his  pro- 
fession. In  the  long  run,  the  "man  who  knows"  is  appreciated. 

It  was  inevitable  that  with  the  exceedingly  rapid  development  of  the  moving 
picture  business  many  difficult  operations,  and  the  special  form  of  acting 
requisite  to  the  production  and  exhibition  of  a  photo-play  were  undertaken  by 
persons  without  adequate  training  and  experience.  It  seems  to  the  authors 
that  it  is  highly  creditable  to  human  intelligence  that  the  work  has  been  so  well 
done  and  that  the  improvement  has  been  so  constant  and  rapid.  It  seems  to 
us,  furthermore,  that  an  important  factor  in  the  present  creditable  attainments 
which  have  already  been  reached,  has  been  due  to  the  high  standards  advocated 
by  the  Moving  Picture  World  in  all  branches  of  the  art.  In  particular  the 
authors  wish  to  commend  the  work  of  Mr.  F.  H.  Richardson  in  his  Motion 
Picture  Handbook  and  in  his  weekly  discussions  and  answers  to  questions  in 
the  projection  department  of  the  Moving  Picture  World. 

390 


CH.  XI]  MOVING  PICTURES  391 

§  551.  For  the  historical  development  of  moving  pictures  see 
under  History  in  the  Appendix. 

For  works  on  moving  pictures  see:  Cyclopedia  of  Motion 
Picture  Work,  2  vols.;  Hepworth,  C.  M.,  Animated  Photography; 
Hop  wood,  Living  Pictures ;  Jenkins,  C.  F.,  Handbook  for  Motion 
Pictures  and  Stereopticon  Opera;  Jenkins,  C.  F.,  Picture  Ribbons; 
Richardson,  F.  H.,  Motion  Picture  Handbook,  2d  ed.;  Talbot, 
F.  A.,  Moving  Pictures;  Hints  to  Operators  by  the  Nicholas 
Power  Company;  Periodicals  on  Moving  Pictures,  e.  g.,  the  Mov- 
ing Picture  World  and  catalogues  of  manufacturers  and  dealers 
in  moving  picture  outfits. 


INTRODUCTION 

§  552.  The  steps  that  had  to  be  taken  in  human  experience  and 
knowledge  before  it  was  possible  to  have  moving  pictures  at  all, 
were  many ;  and  the  time  between  some  of  the  steps  was  very  long. 

The  first  step  was  a  knowledge  of  the  physiology  of  vision,  and 
especially  a  knowledge  of  the  persistence  of  visual  impressions. 
Primitive  man  knew  that  a  glowing  torch  would  make  a  circle  of 
fire  if  it  were  whirled  around  rapidly  enough .  He  knew  also  that  he 
could  see  objects  illuminated  by  an  instantaneous  flash  of  lightning. 

From  this  power  of  seeing  by  an  instantaneous  illumination,  and 
the  persistence  of  the  impression  for  a  limited  time  after  the  light 
has  gone,  arise  the  possibility  of  having  moving  pictures.  In  a 
word,  moving  pictures  are  possible  because  we  can  see  instantly, 
but  we  cannot  stop  seeing  instantly. 

To  give  views  rapidly  with  proper  illumination,  involved  the 
discovery  of  means  for  artificial  light  of  great  brilliancy,  and  of  a 
machine  by  which  the  views  could  be  lighted  and  moved  along; 
and  finally  the  long  series  of  discoveries  and  inventions  in  optics 
and  chemistry  before  photography  was  invented  to  make  the  pro- 
duction of  the  views  cheap  and  accurate.  It  was  another  long  step 
taken  by  Newton  when  he  showed  that  white  light  in  nature  is 
composed  of  the  rainbow  colors.  Furthermore,  it  was  shown  by 
him  and  contemporary  and  later  physicists  and  physiologists  that  a 
mixture  of  less  than  the  seven  colors  of  the  rainbow  gave  to  the  eye 
the  appearance  of  white  light.  Even  two  complementary  colors 


392  MOVING  PICTURES  [CH.  XI 

as  red  and  greenish  blue,  yellow  and  indigo  blue,  etc.,  give  the 
appearance  of  white.  With  this  information  it  became  possible 
to  add  to  the  photographic  black  and  white  moving  pictures,  the 
element  of  color.  This  was  accomplished  by  using  isochromatic  or 
panchromatic  film,  and  taking  the  pictures  through  colored  screens, 
the  first  picture  through  a  red,  the  second  through  a  green,  the 
third  through  a  violet  screen  and  this  constantly  repeated  through- 
out the  whole  scene.  In  exhibiting  the  picture  there  is  a  three- 
color  screen  used  so  that  the  picture  exposed  through  the  red  screen 
is  projected  through  a  red  screen,  giving  a  red  image,  and  the  other 
colors  in  like  manner.  If  the  film  is  run  through  the  machine  three 
times  as  fast  as  the  black  and  white  film,  then  the  brain  mixes  the 
colors  of  the  successive  pictures  giving  fairly  true  color  values  and 
black  and  white.  Where  only  two  screens  are  used — red  and  green 
—the  process  is  the  same,  but  the  film  has  to  be  run  through  the 
machine  only  twice  as  fast  as  the  black  and  white  film  as  there  are 
but  two  colors  for  the  brain  to  combine.  Naturally  the  combina- 
tion of  two  colors  gives  a  lower  range  of  possibilities  than  the  mix- 
ture of  three  colors,  but  even  this  is  wonderful,  as  all  will  agree  who 
have  seen  the  colored  moving  pictures  reproducing  the  gorgeous 
scenes  of  nature  or  the  pageants  of  human  splendor  in  all  their 
form  and  movement  and  also  with  a  fair  approximation  to  the  color 
effects. 

So  perfect  have  become  the  materials  and  processes  used  in 
photography,  and  the  accessory  mechanical  appliances,  and  the 
artificial  lights  available,  that  now  the  scientist  can  register 
accurately  the  almost  instantaneous  movements  of  an  insect's  wing, 
the  flight  of  a  cannon  ball,  and  the  numberless  actions  everywhere 
in  nature  which  are  so  rapid  that  the  unaided  eye  cannot  analyze 
them.  On  the  other  hand,  the  movements  in  the  processes  of 
nature  which  are  so  slow  that  one  can  only  see  what  has  been 
accomplished  in  an  hour,  a  day  or  a  year,  can  be  hastened  by  the 
moving  picture  machine  so  that  the  actual  changes  can  be  made  to 
appear  as  if  they  occurred  in  a  brief  time,  and  the  actual  move- 
ments which  were  too  slow  for  the  eye  to  recognize,  are  made  to 
appear  rapidly  enough  for  the  eye  to  follow  them.  In  this  way  the 


CH.  XI]  MOVING  PICTURES  393 

actual  movements  in  a  growing  plant  or  an  opening  flower  are 
revealed  to  the  eye ;  and  the  great  steps  in  the  evolution  of  an  egg 
to  a  complete  animal,  swimming,  walking  or  flying,  stand  out  with 
startling  reality. 

The  last  triumph  is  the  combination  of  the  phonograph  and  the 
moving  picture  machine  so  that  both  the  eye  and  the  ear  are 
appealed  to  as  in  real  life  or  in  the  theater  with  living  actors. 
This  combination  was  suggested  by  Muybridge,  the  first  to  analyze 
and  then  combine  the  movements  of  animals  by  photography  and 
projection.  That  suggestion  was  made  in  1888,  but  it  is  only  now 
after  25  years  that  a  fair  degree  of  success  has  been  obtained.  It 
requires  two  operators,  one  for  the  moving  picture  machine  and 
one  for  the  phonograph.  The  phonograph  is  just  behind  the 
screen,  while  the  moving  picture  apparatus  is  in  the  usual  place  at 
the  back  of  the  theater. 

The  screen  is  sufficiently  transparent  so  that  the  phonograph 
operator  can  see  the  moving  pictures,  and  the  moving  picture 
operator  has  telephonic  connections  with  the  phonograph  so  that 
he  can  hear  accurately  the  sounds.  He  can,  of  course,  see  the 
moving  pictures  on  the  screen.  The  phonograph  is  made  the 
master  machine  and  the  pictures  must  be  made  to  follow  the  sounds. 
This  is  partly  accomplished  by  a  direct  connection  between  the  two 
machines,  and  partly  by  the  intelligent  cooperation  of  the  two 
operators. 

The  first  successful  efforts  in  moving  pictures  were  made  by 
physicists  and  physiologists  who  desired  to  analyze  the  complex 
and  rapid  movements  of  men,  animals,  and  machines.  The  pur- 
pose was  wholly  scientific,  but  it  was  early  seen  that  herein  lay  the 
possibility  of  entertainment  and  general  instruction. 

The  entertainment  or  amusement  feature  is,  perhaps,  now  the 
predominant  one;  but  the  religious,  educational,  economic  and 
scientific  use  of  this  powerful  means  for  portraying  action  has  never 
been  lost  sight  of,  and  to-day  is  more  prominent  than  ever. 

Much  has  been  said  and  written  on  the  moral  or  social  effect  of 
the  moving  picture.  The  writers  and  their  friends  have  visited 
moving  picture  theaters  in  many  cities  and  in  many  lands  to  see 


394  MOVING  PICTURES  [Cn.  XI 

the  kinds  of  scenes  that  were  portrayed,  and  the  kinds  of  people 
who  crowded  the  theaters  to  see  them.  At  the  same  time  they 
have  also  visited  the  regular  theaters  to  see  actual  human  beings  in 
the  plays,  and  the  kind  of  plays  and  the  kind  of  people  who  were 
there  to  see  them. 

To  some  of  us,  at  least,  the  actual  stage  and  the  screen-stage 
seem  equally  real.  The  screen-stage  has  the  advantage  of  a 
boundless,  and  untrammeled  outlook  of  land  and  water,  earth  and 
sky  in  calm  and  sunshine  and  in  the  resistless  action  of  storm  or 
volcanic  eruption. 

In  human  life  it  can  show  actual  scenes,  commonplace  or  heroic; 
scenes  like  a  royal  coronation,  or  the  barbarisms  of  war  and  riot, 
and  on  a  scale  impossible  for  a  regular  theater,  and  at  an  expense 
which  makes  them  available  for  all  mankind  to  see  and  enjoy,  each 
according  to  his  own  knowledge,  experience  and  capabilities. 

That  some  of  the  scenes  in  moving  picture  theaters  are  neither 
inspiring  nor  uplifting,  and  that  the  order  in  which  the  scenes 
appear  is  sometimes  unfortunate,  must  be  admitted.  But  these 
and  all  other  defects  which  have  been  pointed  out  are  not  inherent 
in  the  moving  picture.  They  simply  indicate  human  failings. 
They  can  be  corrected  and  are  being  corrected  all  the  time. 

It  is  perfectly  natural  to  think  of  the  advantages  to  be  gained  by 
impressing  moving  pictures  into  the  service  of  education.  The 
striking  scenes  depicted  by  the  moving  picture  are  well  adapted  for 
arousing  interest  and  giving  the  inspiration  which  lead  to  the  care- 
ful and  painstaking  effort  necessary  for  a  true  education.  For 
example,  in  the  development  of  a  frog  or  a  fish  from  the  egg  the 
moving  picture  shows  the  major  changes  but  not  the  minor  ones 
which  are  the  really  essential  changes.  No  one  would  ever  become 
an  embryologist  by  looking  at  moving  pictures  of  a  developing 
animal  or  plant,  and  so  with  all  the  other  subjects  the  study  of 
which  enters  into  an  education. 

There  are  a  good  many  helps  in  education,  but  there  is  no  way 
to  become  really  educated  in  any  subject  without  the  continuous 
and  concentrated  study  of  details  as  well  as  of  the  subject  as  a 
whole,  any  more  than  a  man  can  become  a  skilled  mechanic  by 


CH.  XI]  MOVING  PICTURES  395 

simply  visiting  the  best  conducted  machine  shop  in  the  world. 
Education  is  personal ;  everything  gained  has  to  be  paid  for  to  the 
last  farthing  in  mental  effort. 

Moving  pictures  are  the  offspring  of  science  through  some  of  the 
finest  minds  that  the  world  has  known.  It  is  simply  for  the  finest 
art,  the  best  science  and  the  highest  aspirations  of  mankind  to  take 
this  powerful  agent — their  offspring — and  put  it  to  the  real  service 
of  humanity.  Let  it  do  what  it  is  so  capable  of  doing  in  the  church, 
in  general  and  technical  schools  of  all  grades ;  in  scientific,  educa- 
tional and  philanthropic  societies;  in  the  theater,  in  the  club,  and 
finally  in  the  home. 

AUDITORIUM,  SCREEN  AND  OPERATING  ROOM 

First,  it  is  necessary  to  consider  the  room  for  projection,  its 
arrangement  for  seats,  lighting  during  and  between  exhibitions,  the 
screen  and  the  position  of  the  machine. 

§  553.  Auditorium  and  screen. — The  auditorium  should  be 
arranged  so  that  everyone  in  the  room  can  get  a  good  view  of  the 
screen,  there  should  be  a  sufficient  number  of  aisles  and  exits  in 
order  that  the  room  can  be  filled  or  emptied  quickly  and  without 
disturbance ;  and  provision  should  be  made  for  giving  a  sufficient 
illumination  during  the  performance  so  that  people  can  find  seats 
or  leave  the  room  without  difficulty. 

The  screen  should  be  dead  white  and  free  from  wrinkles.  If 
simultaneous  sound  effects  are  to  be  produced  it  is  an  advantage  to 
have  the  screen  slightly  translucent  so  that  the  pictures  can  be  seen 
from  behind.  In  a  long  narrow  room  one  of  the  metallic  screens  is 
an  advantage.  These  screens  are  very  poor  for  those  on  the  side 
when  used  in  a  wide  room,  as  the  picture  appears  very  dim  when 
seen  from  the  side.  When  the  hall  is  provided  with  a  stage  it  is  well 
to  hang  the  screen  quite  a  distance  from  the  front  of  the  stage  so 
that  it  will  be  easier  to  avoid  stray  light  and  in  order  that  the  people 
in  the  front  seats  will  not  be  too  close  to  the  picture.  A  dark 
border  or  frame  to  the  screen  is  also  an  advantage.  (For  the  size 
of  screen  and  of  the  screen  images  see  Ch.  XII,  §  633,  638-639). 


396  OPERATING  ROOM  [Cn.  XI 

§  554.  Position  of  the  machine. — The  machine  should  be 
located  so  that  its  optic  axis  is  perpendicular  to  the  screen  or  the 
pictures  will  be  distorted.  If  the  machine  cannot  face  the  screen 
directly  it  is  better  to  have  it  in  the  middle  of  the  room  and  pointing 
upward  or  downward,  or  to  have  it  at  the  same  height  as  the  screen 
and  pointing  slightly  to  one  side.  The  worst  possible  distortion 
occurs  when  the  machine  is  pointed  obliquely  downward  as  it  must 
be  when  placed  in  one  side  of  a  gallery. 

§  555.  Tent  or  booth  for  temporary  operation. — For  a  single 
performance  the  machine  may  be  laid  on  a  table  in  the  middle  of 
the  auditorium  just  as  with  a  magic  lantern  or  it  may  be  enclosed 
with  a  temporary  booth  or  tent  to  enclose  any  stray  light  and  to 
overcome  the  distracting  effect  of  the  machinery. 

§  556.  Permanent  operating  room. — Permanent  installation 
should  include  an  operating  room  large  enough  so  that  the  machine 
or  machines  can  be  operated  without  hinderance  or  loss  of  time 
from  lack  of  sufficient  space.  This  is  very  essential  in  any  place 
where  even  a  short  delay  is  so  disagreeable  to  the  audience.  The 
operating  room  should  be  easy  to  get  to  and  it  should  be  well 
ventilated.  It  should  have  a  large  flue  at  least  50  cm.  (20  in.)  in 
diameter,  connecting  with  the  outside  of  the  building.  All  open- 
ings in  the  operating  room  should  be  provided  with  shutters  which 
will  close  automatically  in  case  of  fire.  The  room  should  be  pro- 
vided with  incandescent  lamps  and  extension  cords  to  use  while 
working  around  the  machine  and  finally  there  should  be  an  electric 
fan  and  a  chair  for  the  operator.  Every  machine  should  be 
accessible  from  all  sides.  Film  boxes  should  be  placed  where  they 
can  be  easily  reached.  Sufficient  tools  for  ordinary  operation,  a 
supply  of  carbons,  pins,  film  cement,  and  extra  condenser  lenses 
should  also  be  handy.  A  shop-room  equipped  for  making  repairs 
to  the  machines  and  for  doing  jobs  of  wiring  should  be  near  the 
operating  room.  It  is  not  advisable  to  try  to  do  such  work  in  the 
operating  room  itself. 

The  operating  room  is  to  be  at  all  times  kept  like  a  battle-ship  in 
time  of  war,  with  the  decks  cleared  for  action,  nothing  there  which 
is  not  actually  required. 


CH.  XI]  OPERATING  ROOM  397 

§  557.     Construction  of  a  modern  operating  room. — For  the 

construction  of  the  operating  room  itself  a  good  description  is  given 
byF.  H.  Richardson  in  the  Moving  Picture  World  of  August  12, 
1911,  p.  372.  See  also  Richardson's  Handbook,  pp.  65-93. 

(1)  "No  operating  room  may  have  less  than  50  square  feet  of 
floor  surface,  or  be  less  than  seven  feet,  in  the  clear,  from  floor  to 
ceiling  at  any  point. 

(2)  All  operating  rooms  shall  have  a  vent  flue  of  not  less  than 
1^2  square  inches  area  to  each  square  foot  of  floor  area,  same  to 
extend  from  the  ceiling,  or  a  point  near  the  ceiling,  to  the  open  air, 
above  the  roof  if  possible ;   provided,  however,  that  no  vent  exceed 
360  square  inches  in  area. 

(3)  All  operating  rooms  shall  be  of  such  fireproof  construction 
as  is  approved  by  the  National  Board  of  Fire  Underwriters  or  the 
City  Fire  Marshal. 

(4)  Every  operating  room  shall  have  a  door,  opening  outward, 
not  less  than  2x6  feet  in  size,  provided  with  an  appropriate  spring 
to  hold  same  shut. 

(5)  Every   opening   from   operating   room   into   auditorium, 
except  door,  shall  be  equipped  with  a  metal  shutter,  sliding  in 
grooves  and  semi-automatic  in  action.     Same  shall  be  so  arranged 
that  all  shutters  are  held  open  by  a  single  cotton  master  cord 
passing  directly  over  front  edge  of  upper  magazine  of  each  machine, 
just  high  enough  to  clear  operator's  head  when  standing.     Shutters 
may  close  by  their  own  weight  or  by  force  of  a  spring.     If  vent 
flue  is  provided  with  damper  it  shall  be  so  weighted  that  it  will 
normally  stand  open  and  shall  only  be  held  shut  by  cord  attached 
by  master  shutter  cord  before  mentioned. 

(6)  Front,  sides,  and  top  of  every  lamp-house  shall  be  tightly 
enclosed,  except  for  vent-holes,  protected  by  wire  gauze  screen,  but 
back  of  lamp-house  may  be  open. 

(7)  All  moving  picture  projection  machines  shall  be  equipped 
with  approved  upper  and  lower  magazines,  doors  of  which  shall  be 
closed  when  machine  is  running. 

(8)  All  rheostats  shall  be  located  outside  the  operating  room, 
but  low  voltage  transformers  (inductors,  economizers,  etc.),  used 
to  control  the  current  may  be  located  inside  the  room. 


398  MOVING  PICTURE  APPARATUS  [Cn.  XI 

(9)  No  wire  of  less  size  than  No.  6  B  &  S  gauge,  shall  be  used  in 
any  projection  arc  circuit. 

(10)  Only  link  fuses,  enclosed  in  suitable  metal  cabinet  with 
spring  door,  shall  be  allowed  in  any  operating  room. 

(1.1)  All  wires,  except  asbestos  covered  from  outlet  to  lamps, 
shall  be  in  conduits. 

(12)  All  switches  shall  be  enclosed  (fig.  278). 

(13)  All   carbon   butts   shall   be   deposited,   immediately  on 
removal  from  lamp,  in  metal  can  containing  water. 

(14)  All  films  shall  be  kept  in  solderless  metal  case  with  ap- 
proved spring-closing  cover,  or  door. 

(15)  Smoking  shall  be  absolutely  prohibited  inside  the  operat- 
ing room. 

(16)  There  shall  be  no  reading  matter  inside  any  operating 
room.     Reading  matter  to  be  construed  to  mean  newspapers, 
novels,  etc.,  but  not  including  catalogues,  or  books  of  instruction,  or 
magazines  helpful  to  the  operator  in  his  work. 

(17)  Not  to  exceed  one  ounce  of  alcohol  or  one  pint  of  lubricat- 
ing oil  shall  be  allowed  in  the  operating  room.      Benzine,  kerosene 
and  like  substances  shall  not  be  kept  in  any  quantity  in  any  theater. 

(18)  Machines  may  be  motor  driven. 

(19)  All  machines  shall  be  firmly  and  effectively  anchored  to 
the  floor." 

§  558.  Source  of  electric  current. — Next  to  be  considered  is  the 
source  of  current  supply.  If  one  is  in  a  place  where  there  is  a  good 
electric  system  in  operation  it  is  usually  much  better  to  buy  the 
power  than  to  try  to  run  a  special  power  plant.  This  is  because  of 
the  greater  certainty  of  the  city  power  and  the  absence  of  responsi- 
bility. It  is,  however,  perfectly  feasible  to  generate  power  with  a 
gasoline,  oil,  alcohol  or  steam  engine.  When  this  is  done  the  power 
is  somewhat  cheaper  but  rather  more  trouble  and  without  careful 
attention  it  is  less  certain  than  a  regular  supply.  Independent 
generation  of  power  in  small  units  makes  possible  the  direct  con- 
nection of  the  arc  to  the  generator  without  the  use  of  a  rheostat  as 
will  be  explained  later.  (Ch.  XIII,  §  680,  see  also  §  562). 


CH.  XI]  MOVING  PICTURE  APPARATUS  399 

§  559.  Wiring. — When  the  supply  is  decided  upon,  the  wiring 
is  next  installed.  This  must  be  heavy  enough  to  carry  the  greatest 
current  which  is  to-be  used  continuously  in  the  lamp.  It  does  not 
need  to  be  designed  for  the  rather  high  current  which  flows  when 
the  carbons  are  brought  into  contact,  as  any  wiring  can  withstand 
a  heavy  overload  for  a  few  seconds  without  injury. 

The  wire  which  enters  the  lamp-house  should  be  flexible  cable, 
asbestos  covered,  and  of  a  carrying  capacity  at  least  double  the 
amount  required  for  use  at  the  arc  (<§  694-695) .  This  is  on  account 
of  the  high  temperature  within  the  lamp-house  and  consequent 
rapid  deterioration  of  a  small  wire. 

§  560.  Fuses. — Fuses  should  be  used  in  every  case  and  not 
circuit  breakers.  This  is  because  a  fuse  will  not  "blow"  instantly 
when  current  is  drawn  greater  than  its  normal  capacity  (as  when 
the  arc  is  started)  but  if  this  overload  is  continued,  it  will  melt  and 
open  the  circuit.  The  circuit  breaker,  on  the  other  hand,  will  open 
the  circuit  instantly  at  the  same  amperage  whether  the  current  is 
momentary  or  long  continued. 

§  561.  Fire  underwriters  and  special  regulations. — The  wiring 
and  installation  must  conform  to  the  fire  underwriters  regulations 
and  any  special  requirements  of  the  city  in  which  the  theater  is 
located.  The  wiring  for  moving  picture  machines  is  neither 
heavier  nor  more  difficult  to  install  than  that  required  for  other 
forms  of  projection,  notably  opaque  projection,  provision  for  25 
amperes  direct  current  or  50  amperes  alternating  current  usually 
being  sufficient  for  small  theaters. 

For  currents  required  in  different  cases;  for  the  size  of  wire 
required  for  these  currents  and  for  fire  underwriters  regulations  see 
Chapter  XIII,  §  691. 

§  562.  Rheostat  or  other  ballast. — As  with  all  forms  of  arc 
lamp,  the  moving  picture  lamp  requires  some  form  of  ballast  or 
regulating  device  to  control  the  current. 

The  simplest  and  cheapest  device  is  of  course  the  resistor  or 
rheostat.  When  the  electric  supply  is  no  volt  direct  current,  a 
rheostat  is  generally  used.  A  rotary  motor-generator  set  or 


400 


MOVING  PICTURE  APPARATUS 


[CH.  XI 


"current  saver"  is  sometimes  used  as  a  ballast  with  direct  current 
and  effects  a  considerable  saving  of  power  especially  when  the 
supply  is  220. or  500  volts.  (See  Ch.  XIII,  §  744).  • 

When  the  supply  is  alternating  current  the  ballast  may  be  in  the 
form  of  a  rheostat  but  reasons  of  economy  exclude  this  form  of 
ballast  when  the  machine  is  used  continuously.  For  continuous 
performance  an  inductor  (choke-coil),  a  special  transformer,  a 
mercury  arc  rectifier  or  a  motor-generator  is  used.  (See  Ch.  XIII, 
§682-683,  723-739)- 


FIG.  221.     EDISON  KINETOSCOPE,  PROVIDED 
WITH  Two- WING  OUTSIDE  SHUTTER. 

(Cut  loaned  by  the  Edison  Manufacturing  Company}. 

When  power  is  independently  generated,  a  special  dynamo  can 
be  connected  directly  to  the  arc  lamp  without  ballast,  the  dynamo 
will  be  its  own  regulating  device.  (See  Ch.  XIII,  §  680). 

Whatever  form  of  ballast  is  used,  the  quality  and  workmanship 
should  be  of  the  best  or  an  endless  amount  of  trouble  may  be 
expected.  The  rheostat  or  other  ballast  must  conform  to  the 
underwriters  regulations  and  must  be  satisfactory  to  the  company 


CH.  XI]  MOVING  PICTURE  APPARATUS  401 

supplying  the  power.  Some  power  companies  object  to  the  use  of 
an  inductor  (choke-coil) .  In  such  cases  a  transformer  can  be  used 
instead. 

§  563.  Stand  or  table. — A  stand  or  table  is  provided  by  the 
makers  of  the  machine.  The  method  used  to  set  up  the  stand  will 
be  fairly  obvious  from  the  illustrations  furnished  by  the  makers  of 
the  particular  machine  used.  Generally  this  stand  is  made  of 
brass  tubes.  One  maker  provides  a  heavy  iron  pillar.  With  this 
make  provision  must  be  made  to  anchor  this  pillar  firmly  to  the 
floor. 

If  the  machine  is  to  be  installed  permanently,  it  is  often  better  to 
use  a  stand  constructed  of  concrete  or  a  very  heavy  wooden  table 
instead  of  the  light  stand  regularly  supplied.  A  very  slight  motion 
of  a  rickety  stand  will  cause  an  enormous  movement  of  the  picture 
on  a  screen  15  to  30  meters  (50  to  100  feet)  away. 

§  564.  Unpacking. — The  moving  picture  machines  coming  from 
the  factory  are  very  carefully  packed.  When  removed  from  the 
box,  it  is  advisable  to  take  careful  notes  of  just  how  the  different 
parts  are  packed  and  to  number  the  wooden  cleats  used  to  hold 
things  in  place,  especially  if  the  machine  will  need  to  be  shipped 
away  again. 

Be  careful  in  unpacking  all  parts,  especially  the  lenses.  Do  not 
throw  away  any  wrapping  material  until  sure  that  no  parts  are 
missing. 

§  565.  The  moving  picture  machine. — When  unpacked  the 
moving  picture  machine  will  be  found  to  consist  of  a  stand  and  base- 
board, arc  lamp,  lamp-house,  condenser,  aperture  plate,  objective, 
shutter,  film  magazines,  and  mechanism  for  moving  the  film. 
There  will  also  be  an  extra  film  reel  and  a  rewinder  (fig.  221-224). 

§  566.  The  arc  lamp. — The  arc  lamp  usually  supplied  with 
moving  picture  outfits  is  of  the  hand-feed  type  with  inclined  car- 
bons. The  handles  for  feeding  the  carbons  and  for  slight  up  and 
down  adjustments  project  backwards  so  they  may  be  manipulated 
without  opening  the  lamp-house.  The  good  makes  of  arc  lamp  are 
adjustable  so  that  the  carbons  can  be  held  in  the  vertical  or  the 


402 


MOVING  PICTURE  APPARATUS 


[CH.  XI 


inclined  position  as  desired  and  each  carbon  holder  can  be  turned  so 
that  the  upper  carbon  is  inclined  and  the  lower  one  is  vertical 
The  right-angle  arc  can  be  used  with  the  moving  picture  outfit  if 
desired,  but  it  should  not  be  used  with  currents  much  above  25 
amperes  Twenty-five  amperes  direct  current  will  be  found 
sufficient  for  all  but  the  largest  rooms. 


FIG  222..     POWER'S  CAMERAGRAPH  No.  6,  SHOWING  THE  LAMP-HOUSE 
IN  POSITION 

(Cut  loaned  by  the  Nicholas  Power  Co.}. 

§  567.  Lamp-house. — The  arc  lamp  is  enclosed  by  a  metal 
house  to  protect  the  operator  from  being  blinded  by  stray  light  and 
to  protect  the  arc  from  air  currents  which  might  blow  it  out  or 
otherwise  interfere  with  its  performance.  The  adjusting  handles 
of  the  lamp  project  so  that  the  lamp  can  be  adjusted  from  time  to 
time  without  opening  the  doors  of  the  lamp-house. 

The  house  should  be  provided  with  doors  to  enable  the  operator 
to  change  the  carbons  and  should  have  a  window  of  dark  glass  so 


CH.  XI]  MOVING  PICTURE  APPARATUS  403 

that  the  arc  can  be  watched.  This  window  should  be  of  fairly 
large  size  and  directly  opposite  the  crater  of  the  arc.  The  glass 
should  be  dark  enough  so  that  the  eyes  will  not  be  tired  by  the  too 
great  brightness  and  yet  light  enough  so  that  the  whole  of  the  hot 
carbon  ends  can  be  seen. 

Another  convenient  way  to  observe  the  arc  is  to  bore  a  fine  hole 
in  the  side  of  the  lamp-house  away  from  the  operator.  This  acts 
like  apinhole  camera  and  an  image  of  the  arc  is  seen  on  the  opposite 
wall.  A  sharper  image  of  the  arc  can  be  formed  by  using  a  long 
focus  lens  over  an  opening  in  the  wall  of  the  lamp-house  to  focus  an 
image  of  the  arc  upon  the  wall.  A  spectacle  lens  of  about  25  cm. 
(10  in.)  focus  (4  diopters)  will  answer.  The  lens  may  be  held  by 
aii\'  convenient  clamp  but  must  be  adjusted  for  distance  to  get  the 
sharpest  image,  otherwise  it  is  no  improvement  over  the  simple 
pinhole. 

The  lamp-house  should  be  well  ventilated  as  from  %  to  2  kilo- 
watts of  power,  .7  to  3  horsepower,  is  converted  into  heat.  While 
the  arc  is  going  there  must  be  some  way  for  this  heat  to  escape, 
otherwise  everything  inside  would  melt.  One  of  the  principal 
causes  of  condenser  breakage  is  poor  ventilation  of  the  lamp-house. 
The  best  ventilation  is  secured  by  having  holes  permitting  air 
circulation  but  no  escape  of  light,  at  the  top  and  near  the  bottom 
of  the  lamp-house.  The  back  of  the  lamp-house  is  sometimes 
removed. 

In  many  places  the  fire  underwriters  or  the  city,  require  that 
these  ventilating  holes  be  covered  with  fine  wire  gauze,  to  prevent 
sparks  flying  out.  This  requirement  was  invented  by  someone 
who  had  the  mistaken  idea  that  an  arc  lamp  was  a  fiery  volcano, 
vomiting  out  sparks  and  lava  in  all  directions  instead  of  a  quiet, 
well  behaved  sort  of  thing.  It  is  true  that  a  minute  spark  some- 
times does  fly  up,  but  it  is  so  light  that  it  cannot  do  any  damage. 
An}'  small  piece  of  the  carbon  tip  which  breaks  off  will  fall  to  the 
bottom  of  the  lamp-house  where  a  suitable  tray  should  be  pro- 
vided to  catch  it.  This  tray  is  also  useful  to  hold  the  short  pieces  of 
hot  carbon  just  taken  out  of  the  lamp  when  new  carbons  are  put  in. 

§  568.  Condenser. — The  condenser  is  usually  in  a  box  which  is 
fastened  to  the  lamp-house  and  moves  with  it.  In  front  of  the  con- 


404 


MOVING  PICTURE  APPARATUS 


[CH.  XI 


denser  is  the  lantern-slide  carrier  for  use  with  the  magic  lantern, 
which  is  usually  found  in  connection  with  moving  pictures. 


FIG.  223. 


NEW  STYLE  CONVERTIBLE  BALOPTICON  WITH  POWER'S 
MOVING  PICTURE  ATTACHMENT. 

(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.}. 


The  condenser  is  usually  provided  with  two  plano-convex  lenses, 
each  of  18  or  19  cm.  focus  (7  to  7>£  in.  focus). 

The  slide-carrier  for  the  magic  lantern  usually  found  connected 
to  the  moving  picture  outfit  is  generally  fastened  to  the  lamp-house 


CH.  XI] 


MOVING  PICTURE  APPARATUS 


405 


directly  in  front  of  the  condenser.  This  is  not  a  good  plan  as  it 
cuts  down  the  light  from  the  condenser,  and  as  the  opening  is  not 
round  but  a  quadrangle  it  often  leads  to  queer  shadows  on  the 
screen.  Some  makers  provide  a  stationary  slide-carrier  opposite 
the  magic  lantern  objective  so  that  the  whole  face  of  the  condenser 
is  free  when  it  is  opposite  the  moving  picture  objective ;  this  is  a 
better  method  than  the  above. 


FIG.  224.     DOUBLE  DISSOLVING  MODEL  C  BALOPTICON  WITH  EDISON 

MOVING  PICTURE  ATTACHMENT. 
(Cut  loaned  by  the  Bausch  &  Lomb  Optical  Co.). 

§  569.  The  moving  picture  head. — This  contains  all  of  the  ele- 
ments of  the  moving  picture  machine  except  the  arrangement  for 
lighting.  The  moving  picture  head  holds  the  objective  and  con- 
tains the  film-moving  mechanism  and  the  aperture  plate. 

§  570.  Aperture  plate. — Considered  optically  the  aperture  plate 
which  serves  as  a  frame  for  the  picture  on  the  film  is  the  most 
important  part  of  the  moving  picture  head. 


406  MOVING  PICTURE  APPARATUS  [Cn.  XI 

The  standard  aperture  plate  has  an  opening  23.08  mm.  wide  x 
17.31  mm.  high  (29/32  in.  x  87/128  in.)  with  rounded  corners. 
When  the  picture  is  in  focus  on  the  screen  the  edges  of  the  aperture 
plate  are  also  in  focus  at  the  same  time  (§  57oa). 

§  571.  The  objective. — The  objective  forms  the  image  of  the 
film  picture  upon  the  screen.  It  is  in  design  exactly  like  an  objec- 
tive for  the  magic  lantern  but  is  of  shorter  focus. 

It  is  better  to  have  the  lenses  of  large  diameter  (see  §  830). 

Moving  picture  objectives  with  lenses  45  mm.  (if£  in.)  and 
65  mm.  (2^2  in.)  in  diameter  are  on  the  market.  The  objectives 
45  mm.  (1^4  in.)  in  diameter  will  answer  but  those  of  65  mm.  (2^ 
in.)  are  to  be  preferred.  The  larger  objectives  will  give  with  less 
trouble  a  screen  image  without  shadows.  (See  §  829,  830). 

One  must  select  an  objective  of  suitable  focal  length  to  give  a 
proper  sized  screen  image  for  the  auditorium  to  be  used.  This  is 
dealt  with  more  fully  in  §  635.  In  most  rooms  a  screen  image  of 
suitable  size  will  be  obtained  with  an  objective  of  between  12.5  to 
13.5  cm.  focus  (5  to  5>^  in.)  when  the  moving  picture  machine  is  at 
the  back  of  the  room. 

§  572.  The  film  mechanism. — This  consists  in  the  proper  gears 
and  sprocket  wheels  for  moving  the  film,  and  for  turning  the  shut- 
ter. The  mechanism  is  complex;  differs  in  different  makes  of 
machines,  and  no  attempt  will  be  made  here  to  describe  it  in  detail. 

§  573.  The  shutter  which  cuts  off  the  light  during  the  time 
when  the  film  is  in  motion  is  located  either  just  beyond  the  aperture 
plate  and  hence  before  the  objective  (fig.  225),  or  just  beyond  the 
objective  (fig.  226,  227).  When  located  between  the  aperture 
plate  and  the  objective,  it  is  called  an  inside  shutter  and  when 
located  beyond  the  objective  it  is  called  an  outside  shutter. 


§  570a.  Standard  aperture. — As  there  was  some  lack  of  uniformity  in  the 
size  of  the  opening  of  the  aperture  plate,  the  Gundlach-Manhattan  "Optical 
Co.  has  selected  a  size  for  a  standard  as  follows:  The  aperture  has  an  opening 
of  23.08  mm.  long  and  17.31  mm.  high  (29/32  x  87/128  inch).  This  standard 
has  been  adopted  by  the  Nicholas  Power  Co.,  the  Edison  Co.,  and  the  Precision 
Machine  Co.  No  doubt  the  other  makers  of  machines  will  adopt  the  standard 
in  due  time.  Moving  Picture  World,  Vol.  20,  April  1 1,  p.  210,  April  25,  p.  512. 


CH.  XI] 


MOVING  PICTURE  APPARATUS 


407 


§  574.  The  film  magazines  are  large  sheet  iron  boxes  which 
hold  the  film  reels.  They  are  big  enough  to  hold  the  standard 
25  cm.  (10  in.)  reel  and  it  is  a  convenience  if  they  are  large  enough 
to  hold  the  larger  reels  of  30  cm.  (12  in.)  diameter.  The  film 
magazines  are  fitted  with  fire  traps  to  prevent  any  fire  getting  into 
the  magazine  if  the  film  should  start  to  burn. 


FIG.  225. 


MOVING  PICTURE  MECHANISM  WITH  INSIDE  SHUTTER,  I  S. 
For  full  explanation  see  Fig.  231. 


INSTALLATION  OF  A  MOVING  PICTURE  OUTFIT 

§  575.  After  the  wiring  to  the  operating  room  has  been  installed 
in  accordance  with  the  fire  underwriters  regulations  and  any  special 
regulations  of  the  city  in  which  the  work  is  done,  all  is  ready  to 
connect  in  the  rheostat,  transformer,  or  other  regulating  device 
(§  728,  736)  and  to  attach  the  wires  to  the  arc  lamp. 

These  connections  are  exactly  like  those  for  the  magic  lantern 
(fig.  3)  when  a  rheostat  or  inductor  (choke-coil)  is  used.  When  a 
transformer  or  mercury  arc  rectifier  is  used,  the  primary  side  is 


408 


MOVING  PICTURE  APPARATUS 


[CH.  XI 


connected  to  the  line,  and  the  secondary  side  is  connected  to  the 
arc  lamp.  (See  Ch.  XIII,  §  683,  739). 

The  switches  should  be  in  a  convenient  location,  so  that  the 
current  can  be  turned  on  or  off  without  moving  from  the  operating 
position. 

As  soon  as  the  connections  are  made  it  is  well  to  use  an  ammeter 
and  to  find  what  current  the  arc  will  draw  with  the  different  set- 
tings of  the  controlling  lever  of  the  rheostat  or  transformer.  It  is 


FIG.  226. 


MOVING  PICTURE  MECHANISM  WITH  OUTSIDE  SHUTTER,  O  S. 
For  full  explanation  see  Fig.  231. 


a  good  thing  also  to  use  a  voltmeter  to  determine  the  line  voltage 
on  open  circuit,  also  the  voltage  across  the  line,  between  the  arc 
terminals,  across  the  rheostat  or  choke-coil,  or  if  a  transformer  is 
used,  the  voltage  given  by  the  secondary  both  on  open  circuit  and 
when  the  arc  is  running.  The  voltmeter  or  ammeter  must  be 
designed  for  the  kind  and  amount  of  current  for  which  it  is  to  be 
used,  that  is,  alternating  or  direct  current.  When  a  rectifier  or  a 
motor-generator  is  used  it  will  be  necessary  to  have  both  direct 


CH.  XI]  OPTICS  OF  MOVING  PICTURES  409 

current  and  alternating  current  instruments.     (See  Chap.  XIII 
for  using  these  instruments  §  662-674). 


OPTICS  OF  MOVING  PICTURE  PROJECTION 

§  576.  For  purposes  of  description  the  projection  of  the 
individual  pictures  of  a  film  can  be  considered  apart  from  the 
mechanism  which  moves  the  film. 

The  projection  of  the  film  picture  has  much  in  common  with  that 
of  the  ordinary  lantern  slide  but  it  is  somewhat  more  difficult. 

A  theoretical  treatment  of  the  proper  method  of  lighting  the  film 
is  found  in  §  825.  Briefly  stated  it  is  this:  Light  from  the  arc  is 
collected  by  the  condenser  so  as  to  illuminate  the  film.  This 
illumination  must  be  very  intense  and  at  the  same  time  must  be 
evenly  distributed  over  the  entire  area  of  the  film.  To  secure  this 
result  with  the  ordinary  large  condensers  (4^2  in.  in  diameter) 
requires  the  condenser  to  be  quite  a  distance  away  from  the  film, 
the  crater  of  the  arc  to  be  of  considerable  size,  and  the  projection 
objective  to  be  of  fairly  large  diameter. 

Fig.  228  shows  the  optical  arrangement  most  commonly  used. 
Light  from  the  arc  is  collected  by  the  condenser  upon  the 
film  at  s,  passing  through  the  transparent  parts  of  the  film,  it  is 
bent  by  the  objective  in  such  a  way  as  to  form  a  sharp  image  of 
the  film  s,  upon  the  screen. 

Only  one  picture  of  the  film  is  seen  at  a  time,  the  rest  being 
carried  in  the  magazines  or  covered  with  shields.  The  picture 
to  be  shown  is  just  in  front  of  the  opening  of  the  aperture  plate. 
Optically  we  are  concerned  only  with  the  aperture  plate  and  the 
short  section  of  film  behind  it.  It  is  this  short  section  of  film  which 
"must  be  evenly  illuminated  and  projected  upon  the  screen. 

Beyond  the  film  is  the  objective  (fig.  229).  The  objective  should 
be  of  good  quality  as  it  is  the  objective  which  determines  the 
sharpness  of  the  screen  picture.  Moreover,  the  objective  must 
not  be  of  too  small  diameter,  for  if  it  is  too  small  there  is  danger 
that  the  screen  image  will  not  be  evenly  lighted  although  the 
illumination  of  the  film  may  be  perfectly  even.  The  focal  length 


4io 


OPTICS  OF  MOVING  PICTURES 


[Cn.  XI 


FIG.  227.     MECHANISM  OF  POWER'S  No.  6  CAMERAGRAPH,  SHOWING  THE 
THREE-WING,  OUTSIDE  SHUTTER. 

(Cut  loaned  by  the  Nicholas  Power  Company). 

of  the  objective  determines  the  size  of  the  screen  picture  for  a  given 
screen  distance. 

§  577.  Lining  up  the  moving  picture  machine ;  Adjustment  of 
the  light. — The  machine  being  assembled  on  the  board,  the  parts 
lined  up  mechanically  as  well  as  possible  (§  51+),  the  final  steps  to 


CH.  XI] 


OPTICS  OF  MOVING  PICTURES  411 

get  a  good  light  on  the 
screen  must  now  be  taken. 
The  moving  picture  head 
as  it  comes  from  the  fac- 
tory    should    have    the 
aperture  plate   and   the 
center  of  the   objective 
mount     at     the     same 
height.     If  they  are  not, 
the  aperture  plate  must 
be  moved  up    or   down 
until  its  center  is  on  the 
same  axial  line  as  the  ob- 
jective.   The  adjustment  can  prob- 
ably be  done  with  sufficient  accuracy 
with  the  eye,  when  looking  through 
the  lens  opening,  the  lens  being  in 
place.     This  is  a  matter  which  al- 
ways should  be  looked  after  by  the 
manufacturer. 

Another  method  would  be  to  re- 
move the  condensers  and  adjust  the 
arc  lamp  to  exactly  the  same  height 
as  the  aperture  plate.  A  piece  of 
paper  put  in  place  of  the  lens 
should,  when  the  arc  is  lighted, 
show  the  shadow  of  the  center  of 
the  aperture  plate  in  the  exact  cen- 
ter of  the  circular  piece  of  paper. 

FIG.  228.  OPTICAL  SYSTEM  AND  ILLUMIN- 
ATION OF  MOVING  PICTURES. 

Lamp. 

Condenser. 

S     Lantern-slide  holder. 

Fire  Shutter  This  is  open  only  when 
the  machine  is  running. 

5     Aperture  plate. 

Objective. 

See  fig.  231  for  full  explanation  of  the 
mechanism. 


412  OPTICS  OF  MOVING  PICTURES  [Cn.  XI 

The  center  of  the  condenser  and  the  center  of  the  aperture  plate 
are  adjusted  to  the  same  height  above  the  baseboard.  This  is 
attended  to  by  the  manufacturer,  but  if  a  head  of  one  machine  is 
used  with  the  arc  and  condenser  of  another  make,  adjustment 
might  have  to  be  made.  Make  a  spot  with  a  pen  or  a  wax  pencil 
exactly  in  the  center  of  the  front  lens  of  the  condenser,  measure  the 

height  of  this  above  the  baseboard. 
Make  a  similar  mark  on  the  aperture 
plate  at  the  height  of  the  middle  of 
the  opening  and  measure  its  distance 
above  the  baseboard.  If  the  aperture 
plate  is  too  low  the  head  should  not 

FIG.  229.    LARGE  DIAMETER      be  screwed  directly  to  the  baseboard 
PROJECTION  OBJECTIVE  FOR        ,     ,    ,       1  j  i_    IT,    j  re  •     ^.1        -,i 

MOVING  PICTURES  ^u^  should  be  lilted  up  sufficiently  with 

(Cut  loaned  by  the  Gundlach-  a  thin  piece  of  board.  If  the  aperture 
'  Manhattan  Optical  Co.}.  plate  is  too  high>  the  front  of  the  base. 

board  can  be  cut  down  or  the  lamp-house  and  condenser  can  be 
raised  by  using  a  piece  of  wood  or  asbestos  board  between  the  base- 
board and  the  lamp-house  fastenings. 

After  getting  the  objective,  aperture  plate,  and  condenser  at  the 
correct  height,  it  only  remains  to  get  the  arc  at  the  right  height. 
This  is  done  from  time  to  time  by  raising  or  lowering  the  arc  lamp 
until  the  light  spot  falls  exactly  over  the  aperture  plate. 

The  sidewise  adjustment  of  the  lamp-house  is  now  made  in  the 
same  way  by  measuring  the  distance  from  the  edge  of  the  base- 
board to  the  center  of  the  condenser  and  then  to  the  center  of  the 
aperture  plate.  This  measurement  can  be  made  by  using  a  vertical 
board  (§  52). 

When  the  same  arc  and  condenser  are  used  for  both  moving  pic- 
tures and  lantern  slides,  the  lamp-house  should  be  in  the  correct 
position  when  pulled  on  its  lateral  rods  as  near  as  possible  to  the 
operator.  If  it  is  not,  stops  can  be  fastened  on  the  side  rods  to  hold 
the  lamp-house  in  the  correct  position. 

§  578.  Back  and  forth  adjustment  of  the  arc  lamp  and  con- 
denser.— This  is  one  of  the  most  important  and  troublesome  adjust- 
ments to  make.  There  would  be  but  little  difficulty  in  getting  an 


CH.  XI]  OPTICS  OF  MOVING  PICTURES  413 

even  illumination  of  the  film  picture  and  the  screen  image  if  con- 
densers were  obtainable  entirely  free  from  spherical  aberration, 
but  this  is  not  practical.  No  rule  can  be  given  as  to  the  best 
position  of  the  arc  and  condenser  but  the  best  position  must  be 
determined  for  each  particular  case.  Some  general  hints  can, 
however,  be  given. 

First — The  objective  should  be  of  large  diameter.  This  will 
allow  of  a  greater  range  of  adjustment  through  which  good  illumin- 
ation can  be  obtained  (§  829-830). 

The  lenses  of  the  double-lens  condenser  (fig.  i)  should  be  as  near 
together  as  possible  without  actually  touching.  The  convex  sides 
of  the  condenser  lenses  should  face  each  other,  the  plane  sides 
should  face  the  lamp  and  the  objective. 

The  condenser  lenses  should  be  of  fairly  long  focus,  18  to  19 
cm.  (7  to  7^  in.). 

If  the  condenser  is  as  far  away  from  the  aperture  plate  as 
possible  the  illumination  is  usually  more  even,  though  less  intense 
than  when  the  condenser  is  close  to  the  aperture  plate. 

When  first  setting  up  the  machine,  it  is  a  great  help  to  have  a 
series  of  condenser  lenses  to  try,  say  such  a  series  as  two  lenses  of 
14  cm.  focus,  one  each  of  15,  16,  17  cm.,  two  of  19  cm.  focus,  (two 
of  5>^  in.,  one  each  of  6,  6>£,  7  in.,  two  of  7^  m-)-  Tne  two  con' 
denser  lenses  should  be  of  the  same  focus,  then  only  one  kind  of 
condenser  lens  will  need  to  be  kept  in  stock  to  supply  breakage. 

When  the  adjustment  for  distance  is  to  be  made,  move  the  lamp- 
house  with  its  condenser  close  to  the  aperture  plate,  fasten  it 
in  position,  move  the  arc  in  the  lamp-house  nearer  to  and  farther 
from  the  condenser  until  the  best  light  is  obtained  on  the  screen. 
Note  how  this  light  appears  and  whether  there  are  any  ghosts  or 
shadows.  Then  fasten  the  lamp-house  and  condenser  slightly 
farther  from  the  aperture  plate  and  move  the  arc  until  the  best 
light  is  again  obtained.  After  repeating  this,  for  every  position 
of  the  condenser,  the  condenser  is  set  at  the  distance  which  was 
found  to  be  best.  It  may  be  necessary  to  try  a  different  set 
of  condenser  lenses  before  the  best  possible  result  is  obtained. 
This  is  a  rather  tedious  process  but  is  well  worth  while  doing. 


414  MAGIC  LANTERN  AND  MOVING  PICTURES         [Cn.  XI 

ADJUSTMENT  OF  THE  MAGIC  LANTERN  ATTACHMENT  FOR  USE  IN 
CONNECTION  WITH  THE  MOVING  PICTURE  MACHINE 

§  579.  The  adjustment  of  the  arc  lamp  and  condenser  for  the 
moving  picture  part  is  of  much  greater  importance  and  is  more 
difficult  than  that  for  the  magic  lantern  attachment,  hence,  no 
attention  should  be  paid  to  the  projection  of  lantern  slides  until  the 
projection  of  moving  pictures  is  perfect. 

In  most  outfits  the  lamp-house  moves  sidewise  on  some  lateral 
rods.  When  pulled  towards  the  operator  the  lamp  is  in  line  with 
the  moving  picture  objective,  and  when  pushed  away  from  the 
operator  until  it  hits  a  stop,  it  is  in  line  with  the  lantern  objective. 

Push  the  lamp-house  on  these  lateral  rods  until  it  is  held  by  the 
stops.  A  lantern  slide  is  put  in  the  holder  and  the  lantern  objec- 
tive support  is  loosened  and  the  lantern  objective  moved  sidewise 
until  it  is  over  the  spot  of  light  from  the  arc  and  moved  back  and 
forth  until  the  image  of  the  slide  is  in  focus  on  the  screen.  If  there 
are  shadows  on  the  screen  not  due  to  malposition  of  the  carbons, 
use  an  objective  with  larger  lenses. 

If  the  lantern  picture  does  not  occupy  the  same  place  on  the 
screen  as  the  moving  picture  it  may  be  the  fault  of  the  side  adjust- 
ment of  the  slide-holder,  or  it  may  be  due  to  faulty  alignment  of  the 
arc  lamp  and  moving  picture  head.  If  this  should  be  the  case 
move  the  lamp-house  sidewise  until  the  lantern-slide  picture 
occupies  the  proper  position  on  the  screen.  Then  move  the  arc 
sidewise  until  the  screen  is  well  lighted  and  clamp  it  in  position. 
When,  now,  the  lamp-house  is  pulled  into  position  in  front  of  the 
moving  picture  objective  the  spot  of  light  may  not  fall  upon  the 
aperture  plate  but  to  one  side.  If  it  is  not  in  the  right  position  do 
not  alter  the  adjustment  of  the  arc  lamp  but  move  the  lamp-house 
as  a  whole  to  one  side  until  the  spot  exactly  covers  the  aperture 
plate.  Then  fasten  the  stop,  so  that  the  lamp-house  will  always 
occupy  the  same  position  when  pulled  toward  the  operator. 

§  580.  Management  of  the  arc  lamp. — During  an  exhibition  it 
is  necessary  to  watch  the  arc  lamp  to  see  that  it  is  burning  prop- 
erly. There  are  several  ways  of  burning  the  arc  which  will  give  a 
good  light : 


CH.  XI]          MAGIC  LANTERN  AND  MOVING  PICTURES 


415 


The  carbons  may  be  at  right  angles  (fig.  23  C). 

The  carbons  may  be  inclined  backwards  about  30°  (fig.  230  a). 

The  upper  carbon  may  be  inclined  backward  45°,  the  lower 
carbon  being  vertical  (fig.  230  c). 

The  carbons  may  come  together  in  the  form  of  a  horizontal  V 
with  the  point  towards  the  condenser  (fig.  23  D). 

Both  carbons  may  be  vertical  (fig.  230  b). 

Whatever  carbon  setting  is  used,  the  arc  must  be  held,  so  that  the 
crater  or  craters  face  the  condenser. 

The  form  of  the  arc  can  be  watched  by  observing  it  through  the 
smoky  glass  window  or  by  the  pinhole  or  lens  image  on  the  wall 
(§  567).  When  using  alternating  current  the  sound  will  give  an 
indication  as  to  whether  the  arc  is  of  the  right  length. 

Constant  vigilance  in  watching  the  arc  is  one  of  the  requirements 
for  success  is  showing  moving  pictures.  During  an  exhibition, 
never  let  the  arc  go  out. 

§  581.  Supply  of  carbons  for  the  arc  lamp. — A  good  supply  of 
carbons  should  be  provided  and  placed  where  they  rray  easily  be 
reached.  The  carbons  are  soft-cored  and  their  size  should  be 
suited  to  the  current  used  (see  §  753a).  Generally  16  mm.  carbons 
(?/8  in-)  are  used,  both  being  of  the  same  size. 

§  582.  Position  of  the  film  in  the  machine. — When  a  film  is 
passing  through  the  machine  the  rule  for  its  position  is  the  same 
as  with  the  lantern  slides,  that  is,  the  picture  should  appear  correct 
when  one  looks  through  it  toward  the 
screen  but  it  must  be  upside  down. 
To  accomplish  this  one  should  bear  in 
mind  that  as  the  films  are  printed  they 
will  appear  correct  when  one  looks  at 
the  emulsion  side  just  as  with  a  lan- 
tern slide  or  an  ordinary  paper  print. 
Therefore,  the  light  is  made  to  strike 
the  emulsion  side  of  the  film. 


\ 


\ 


\ 


§  583.     Mechanism. — Without    go- 
ing into  the    details    of    the   special 


FIG.  230.     POSITION  OF  CAR- 
BONS FOR  MOVING  PIC- 
TURE PROJECTION. 
a     Inclined. 
b     Vertical. 
c     Upper  carbon  inclined, 

lower  carbon  vertical. 


416 


MOVING  PICTURE  FILM  AND  MECHANISM         [Cn.  XI 


arrangements  employed  in  the  different  makes  of  machine,  the 
principle  is  simple,  although  the  mechanical  problems  in  work- 
ing out  these  principles  require  much  care. 


FIG.   231.     FIGURE   TO   REPRESENT   THE   PRINCIPLE   OF  THE   MOVING 
PICTURE  MACHINE  MECHANISM. 

a  b     Sprocket  wheels  moving  with  uniform  velocity. 

c     Intermittent  sprocket  wheel  which  jerks  down  the  short  section~of  film 
between  L  and  M. 

i     Idlers  to  hold  the  film  on  the  sprocket  wheels. 

D     Gate  which  holds  the  film  in  place  in  front  of  the  aperture  plate. 

F    Upper  film  reel,  unwinding. 

G     Lower  film  reel,  winding  up. 

5    Aperture  plate. 

Objective. 


CH.  XI] 


MOVING  PICTURE  FILM  AND  MECHANISM 


417 


The  essential  part  of  the  mechanism  consists  in  three  sprocket 
wheels,  a,  b,  and  c,  (fig.  231),  the  two  wheels  a  and  b  move  con- 
tinuously at  the  average  rate  at  which  the  film  is  passing  (30  cm., 
i  foot,  per  second),  and  serve  to  unwind  the  film  from  the  upper 
reel  F  and  feed  the  film  to  the  take-up  reel  G  at  a  uniform  rate. 
The  sprocket  wheel  c,  located  between  the  other  two,  is  inter- 
mittent in  its  movements,  being  stationary  for  about  %  of  the  time 
and  being  in  rapid  motion  for  about  Yt>  of  the  time.  The  effect 
is,  that  after  the  film  has  been  in  position  for  exposure  on  the  screen 
this  sprocket  wheel  jerks  the  small  section  of  film  between  L  and  M 
forward  to  the  next  picture.  In  fig.  232  is  shown  one  form  of 
mechanism  for  causing  the  intermittent  movement  of  the  sprocket 
wheel. 

When  the  film  is  stationary  it  is  projected  on  the  screen  by  the 
objective,  but  during  the  short  time  when  the  film  is  in  motion  a 
shutter  either  before  or  behind  the  objective  cuts  off  the  light  and 
prevents  any  blurring  due  to  the  movement  of  the  picture. 

The  films  are  made  in  such  a  way  that  if  the  pictures  are  right 
side  up,  the  later  picture  will  be  below  the  earlier  ones,  but  as  in 
passing  through  the  machine  the  pictures  are  upside  down,  the 
later  pictures  are  above  and  it  is  necessary  to  move  the  film  down- 
ward to  bring  the  pictures  on 
the  screen  in  due  order. 

§  584.  Threading  the  film  in 
the  machine. — The  film  as 
wound  on  the  reel  usually  is 
wound  in  the  correct  direction, 
so  that  the  first  pictures  are  on 
the  outside.  If  this  is  not  the 
case,  the  film  must  be  rewound 
on  another  reel  to  reverse  its 
direction.  If  the  direction  is 
correct  the  pictures  will  be  up- 
side down  when  the  film  is  in 
FIG.  232.  INTERMITTENT  MOVEMENT  the  machine,  that  is,  when  the 

7c  ^WER^;6  vT,RAG£APH'       film  is  passing  downward  from 

(Cut  loaned  by  the  Nicholas  Power 


Company). 


F  (fig.  231). 


418  MOVING  PICTURE  FILM  AND  MECHANISM        [Cn.  XI 

Next,  it  is  necessary  to  get  the  film  right  side  out,  otherwise, 
everything  will  be  reversed  and  appear  as  if  seen  in  a  mirror,  an 
especially  troublesome  state  of  affairs  when  titles  or  letters  are 
shown.  The  side  of  the  film  which  has  the  emulsion  appears 
rough,  the  other  side  is  smooth  and  shiny.  The  film  often  has  a 
tendency  to  curl,  the  emulsion  being  on  the  concave  side.  The 
film  is  turned  so  that  the  rough,  emulsion  side  bearing  the  picture 
is  toward  the  light.  When  it  is  wound  correctly  on  the  reel,  and 
the  emulsion  side  is  turned  so  it  will  face  the  light  as  the  film 
unwinds,  the  reel  of  film  is  put  in  the  upper  magazine.  The  end 
of  the  film  is  pushed  through  the  opening  in  the  magazine  between 
the  rollers  of  the  fire-trap.  This  can  best  be  done  by  using  the 
index  and  middle  fingers  to  hold  the  film. 


FIG.  233.     EDISON  KINETOSCOPE  MECHANISM. 
(Cut  loaned  by  the  Edison   Manufacturing  Company) 

The  magazine  doors  are  open  showing  the  film  reels. 
The  film  is  in  place  ready  to  project. 


CH.  XI]         MOVING  PICTURE  FILM  AND  MECHANISM  419 

The  gate  D  is  then  opened  and  the  idlers,  iii  are  pushed  away 
from  the  sprocket  wheels  a,  b  and  c.  A  sufficient  length  of  film 
is  unrolled  from  F  to  reach  to  the  take-up  reel  G  and  the  film 
is  put  under  the  sprocket  wheel  a,  so  that  the  teeth  fit  into  the  holes 
at  the  edges  of  the  film.  Care  must  be  taken  that  the  film  goes 
over  or  under  the  sprocket  wheels  in  such  a  way  that  as  the  crank 
is  turned  forward  all  of  the  sprocket  wheels  tend  to  move  the  film 
in  the  same  direction,  otherwise  they  will  tear  it  apart.  The 
arrangement  may  differ  in  different  machines. 

After  putting  the  film  on  the  sprocket  wheel  a,  so  that  the  teeth 
pass  through  the  holes  of  the  film,  the  idler  i,  is  pushed  over  to  hold 
the  film  in  place.  This  can  be  done  with  one  of  the  fingers  while 
holding  the  film  in  place  with  the  thumb  and  forefinger.  The  film 
is  then  engaged  with  the  lower  sprocket  wheel  b,  leaving  an  extra 
length  of  film  to  form  the  two  loops  L  and  M.  This  can  best  be 
determined  by  experience,  it  must  be  enough  so  that  the  inter- 
mittent sprocket  will  not  jerk  the  film  in  two  and  not  long  enough 
so  that  the  loops  will  strike  any  shields  there  may  be  to  cover  them. 

The  film  is  held  against  the  intermittent  sprocket  c,  so  the  loops 
L  and  M,  are  about  equal  in  size  and  held  straight  on  the  tracks  of 
the  aperture  plate  when  the  gate  D,  is  closed. 

The  end  of  the  film  is  now  pushed  through  the  fire-trap  opening 
in  the  lower  magazine  and  fastened  to  the  take-up  reel  G.  This  is 
accomplished  by  slipping  the  end  under  the  spring  on  the  spindle 
of  the  reel,  in  such  a  direction  that  the  film  will  not  be  folded  as  the 
reel  is  turned.  The  reel  is  turned  to  insure  the  end  of  the  film 
being  well  fastened.  Fig.  233  shows  a  mechanism  with  the  film  in 
position  and  ready  to  operate  as  soon  as  the  magazine  doors  are 
closed. 

If  the  picture  is  not  directly  in  front  of  the  aperture  plate  but  is 
above  or  below  (misframed) ,  it  can  be  put  in  its  proper  position  by 
a  lever  which  lowers  the  mechanism  and  film  without  disturbing 
the  position  of  the  aperture  plate  and  objective. 

§  585.  Direction  of  motion. — The  normal  direction  of  motion 
to  secure  the  proper  sequence  of  events  in  the  order  in  which  they 
occurred  is  secured  by  moving  the  film  downward,  and  results 


420  MOVING  PICTURE  FILM  AND  MECHANISM       [Cn.  XI 

from  a  right-hand  rotation  of  the  crank.  If  the  crank  is  turned  to 
the  left  the  film  will  be  pushed  upward  by  the  intermittent  sprocket 
instead  of  being  pulled  downwards  as  it  should  be.  This  would 
most  likely  result  in  crumpling  and  breaking  the  film. 

§  586.  Operation  and  speed. — After  the  machine  is  threaded 
the  lamp  is  pulled  toward  the  operator  so  that  the  light  shines  upon 
the  aperture  plate. 

In  starting  the  machine  do  not  start  with  a  jerk  but  start  grad- 
ually (1-2  seconds),  otherwise  an  unnecessary  strain  is  put  upon  the 
gears.  The  crank  is  turned  in  a  right-hand  (clockwise)  direction 
at  such  a  speed  that  the  film  passes  at  the  rate  of  16  pictures  per 
second.  If  the  gearing  is  arranged  so  that  the  intermittent 
sprocket  would  move  16  times  for  each  revolution  of  the  crank,  this 
would  require  i  revolution  per  second  or  10  revolutions  of  the 
crank  every  ten  seconds.  One  should  practice  the  speed  for  a 
while  with  no  film  in  the  machine,  looking  at  the  second  hand  of  a 
watch  and  turning  with  a  uniform  speed  until  there  are  just  10 
revolutions  every  time  the  second  hand  passes  a  ten  second  division. 
This  should  be  practiced  for  some  time  until  the  proper  speed  can 
be  maintained  with  certainty.  After  the  film  is  in,  the  action  in 
the  scene  will  serve  as  a  guide  for  the  proper  speed,  as  some  films 
are  improved  by  being  shown  at  a  slower  or  faster  rate  than  they 
were  taken,  i.  e.,  the  standard  given  above. 

See  Richardson's  Handbook,  p.  219. 

§  587.  Automatic  fire  shutter. — As  the  machine  starts,  the 
automatic  fire  shutter  (fig.  228)  opens  and  allows  the  light  to  fall 
upon  the  film.  If  the  picture  is  not  at  the  right  height  on  the 
screen  it  can  be  "framed  up"  by  moving  a  lever  which  raises  or 
lowers  the  mechanism  and  film. 

If  an  old  machine  is  used  that  does  not  have  an  automatic  fire 
shutter,  one  must  be  extremely  careful  never  to  allow  the  light  to 
fall  upon  the  film  except  when  it  is  in  motion,  otherwise  one  or  two 
seconds  will  suffice  either  to  ruin  the  film  if  non-inflammable  film 
is  used  or  to  start  a  conflagration  if  celluloid  film  is  used.  The 
danger  from  this  source  is  so  great  that  we  strongly  recommend 


CH.  XI]  MOVING  PICTURE  SHUTTER  421 

that  a  water-cell  be  used  (§  848)  in  cases  where  an  automatic  fire 
shutter  is  not  provided;  where  a  motor  is  used  to  drive  the 
machine ;  for  all  experimental  work  and  for  every  person  running 
a  moving  picture  machine  who  has  not  had  abundant  experience  in 
operating.  It  is  so  easy  to  let  the  film  stop  for  a  second,  or  to  have 
the  film  break  leaving  a  tag  end  of  film  in  the  aperture  plate,  and 
wonder  afterward  what  started  the  fire. 

§  588.  Setting  or  "timing"  the  shutter. — The  shutter  should 
be  mounted  on  the  spindle  used  to  turn  it  in  such  a  way  that  it  will 
cut  off  the  light  from  the  screen  during  the  time  when  the  film  is  in 
motion.  If  the  shutter  is  not  set  exactly  right  in  the  beginning  it 
is  often  a  rather  tedious  job  to  correct  its  position,  but  by  going  at 
the  matter  systematically  the  difficulty  is  greatly  lessened. 

Shutters  of  the  one-wing  type  can,  of  course,  be  set  in  only  one 
way  but  shutters  of  the  two-  or  three-wing  types  may  have  wings  of 
different  widths.  In  this  case  the  widest  wing  is  the  one  which 
should  intercept  the  light  while  the  film  moves. 

The  easiest  way  to  set  the  shutter  would,  of  course,  be  to  run 
the  machine  very  slowly  and  watch  the  picture  on  the  screen.  If 
no  shutter  were  used  the  picture  would  seem  to  jump  up,  and  be 
replaced  by  a  picture  which  comes  up  from  below.  When  the 
shutter  is  in  place,  if  the  picture  seems  to  jump  up  just  before  the 
light  is  out,  the  shutter  is  said  to  be  too  "late"  and  it  must  be 
loosened  on  its  shaft  and  turned  slightly  forwards,  that  is,  in  the 
direction  in  which  it  is  turning.  The  shutter  is  then  fastened 
securely  in  position.  If  the  picture  jumps  into  place  from  below 
just  after  the  light  comes  on,  the  shutter  is  said  to  be  too  "early" 
and  it  must  be  turned  slightly  backwards.  That  the  shutter  may 
be  correctly  set  when  it  is  turning  rapidly  as  well  as  when  it  is 
moving  slowly,  it  is  well  to  hold  the  outside  of  the  shutter  or  the 
shaft  on  which  it  turns  with  the  finger  so  as  to  take  up  lost  motion. 
When  in  rapid  rotation  all  the  lost  motion  is  taken  up  on  account 
of  air  friction. 


§  587a.     With  a  two-lens  condenser  the  water-cell  can  be  put  between  the 

condenser  and  the  aperture  plate  (fig.  206). 


422  MOVING  PICTURE  SHUTTER  [Cn.  XI 

Running  the  machine  slowly  with  a  film  in  the  machine  is  entirely 
practical  provided  the  arc  current  is  not  extremely  heavy,  and 
provided  a  water-cell  is  used  (See  §  596,  779-782). 

When  no  water-cell  is  at  hand  the  machine  must  be  run  rapidly. 
In  this  case  the  rule  for  changing  the  position  of  the  shutter  is 
exactly  the  same  but  the  motion  of  each  individual  picture  cannot 
be  seen.  If  one  has  a  film  which  is  nearly  opaque,  but  has  a  few 
spots  in  it,  as  a  period  on  a  title  for  example,  there  is  an  effect 
known  as  "travel  ghost"  which  is  seen  if  no  shutter  is  used  or  if 
the  shutter  is  incorrectly  timed.  This  is  caused  by  the  persistence 
of  vision.  As  the  white  spot  moves  upward,  it  appears  to  be  a 
streak  instead  of  a  spot.  If,  now,  the  shutter  is  too  late,  the  light 
is  not  cut  off  until  the  spot  starts  to  move  upwards  and  a  streak  is 
seen  above  the  spot.  If  the  shutter  is  too  early,  the  light  is  turned 
on  while  the  spot  is  still  moving  upward  and  before  it  comes  to 
rest;  the  streak  is  then  seen  below  the  spot. 

If  the  shutter  is  too  narrow  the  motion  of  the  spot,  both  before 
and  after  the  light  is  cut  off  and  the  streak  will  be  seen  both  above 
and  below  the  spot  of  light. 

§  589.  Rule  for  setting  or  timing  the  shutter. — If  the  streak  or 
travel  ghost  appears  above  the  letters  of  the  title,  the  shutter  is  too 
late,  turn  it  slightly  forward  on  the  shaft. 

If  the  streak  or  travel  ghost  appears  below  the  letters  of  the  title 
the  shutter  is  too  early,  turn  it  slightly  backwards  on  the  shaft. 

If  the  streak  or  travel  ghost  appears  both  above  and  below  the 
letters  of  the  title,  the  shutter  blade  is  too  narrow..  Use  a  shutter 
with  a  wider  blade. 

§  590.  The  best  position  of  the  shutter  and  the  speed  to  prevent 
flicker. — The  shutter  may  be  placed  in  either  of  two  positions ;  it 
may  be  just  beyond  the  film  and  between  it  and  the  objective 
(inside  shutter)  or  it  may  be  placed  beyond  the  objective  (outside 
shutter).  There  is  a  difference  in  the  effect  produced  depending 
on  which  of  these  positions  is  chosen  (fig.  225-226). 

With  the  inside  shutter,  when  the  machine  is  turned  slowly  the 
image  of  the  shutter  can  be  seen  somewhat  out  of  focus  traveling 
from  one  side  of  the  picture  to  the  other. 


CH.  XI)  FLICKER  WITH  MOVING  PICTURES  423 

With  the  outside  shutter,  beyond  the  objective,  the  wing  of  the 
shutter  as  it  advances  removes  light  from  the  whole  of  the  picture,  a 
phenomenon  which  tends  to  reduce  flicker. 

The  diameter  of  the  inside  shutter  is  limited  by  the  size  of  the 
mechanism,  while  the  outside  may  be  made  as  large  as  is  desired. 

As  will  be  seen  below,  the  diameter  of  the  shutter  has  an  effect 
on  the  light. 

The  picture  should  be  entirely  covered  by  the  shutter  before 
it  commences  to  move,  and  it  should  not  be  uncovered  until  it  has 
ceased  to  move.  This  requires  that  the  wings  of  the  shutter  need 
to  be  about  3  cm.  (ij<  in.)  wider  than  the  theoretical  >6th  of  the 
circumference  of  the  circle. 

The  larger  the  circle  the  nearer  to  Yt>  of  the  circle  is  the  width 
of  the  shutter  wing. 

With  a  shutter  of  large  diameter,  the  actual  velocity  is  greater 
and  the  interruption  of  the  light  is  more  sudden,  therefore  a 
shutter  of  large  diameter  is  to  be  preferred. 

§  591.  Flicker. — The  standard  speed  of  the  film  is  given  as 
1 8  meters  (60  ft.)  per  minute,  30  cm.  (i  ft.)  per  second.  There 
being  16  films  per  30  cm.  (foot),  this  gives  16  pictures  per  second. 

It  is  the  general  intention  to  run  films  at  this  speed  though  they 
are  often  run  either  faster  or  slower  to  get  the  best  effects.  The 
time  during  which  one  picture  is  shown  (Vie  second)  can  be  divided 
into  6  equal  periods,  during  five  of  these  periods  the  picture  is 
stationary  and  during  the  6th  the  film  is  moved  and  the  next 
picture  substituted. 

One  complete  change  will  be  called  a  cycle. 

The  films  could  be  run  through  the  machine  with  no  shutter  at 
all,  the  film  being  in  place  an  instant  and  then  moved  and  the  next 
picture  substituted  by  a  quick  movement.  This  will  cause  a 
spreading  out  of  white  patches  into  a  vertical  streak  called  travel 
ghost,  and  will  also  give  a  general  gray  appearance  and  lack  of 
contrast  to  the  screen  image. 

To  avoid  this  appearance  some  kind  of  a  shutter  is  used  to 
obliterate  the  pictures  while  the  film  is  in  motion.  The  shutter 
can  be  either  translucent  or  opaque. 


424 

80 


FLICKER  WITH  MOVING  PICTURES  [Cn.  XI 


00          I  Z          3          4 

Logarithm  of  Illumination 

FIG.  234.     THE  RELATION  BETWEEN  THE  ILLUMINATION  OF  THE  SCREEN 

AND  THE  NUMBER  OF  FLASHES  PER  SECOND  AT  WHICH  FLICKER  JUST 

DISAPPEARS. 

If  the  flashes  are  more  frequent  than  indicated  by  the  curve  for  a  given 
illumination  there  will  be  no.  nicker,  but  if  less  frequent,  flicker  will  be  seen. 

The  solid  line  represents  the  observation  of  T.  C.  Porter  and  the  dotted  line 
represents  some  rough  observations  made  by  the  authors. 


CH.  XI]  FLICKER  WITH  MOVING  PICTURES  425 

If  the  shutter  is  translucent  the  appearance  during  the  change 
of  pictures  is  that  of  a  screen  lighted  to  a  uniform  gray.  This  kind 
of  shutter  is  not  much  used  in  practice  as  it  has  the  disadvantage 
of  slightly  illuminating  the  parts  of  the  screen  which  should  be 
absolutely  black. 

The  opaque  shutters  were  originally  made  to  cover  the  picture 
during  the  time  the  picture  was  in  motion,  i.  e.,  from  >£  to  >6  of  the 
cycle,  the  rest  of  the  cycle  the  screen  was  lighted,  but  this  was 
found  to  give  a  very  bad  flicker. 

Recently  to  get  rid  of  the  flicker  the  shutters  have  been  made 
with  2  or  3  opaque  wings. 

With  the  one-wing  shutter  a  cycle  is  made  up  with 

1 .  Picture  on  the  screen — screen  light — %  to  %  cycle. 

2.  Picture  changed — screen  dark — >^  to  Yt>  cycle. 

There  are  16  cycles  per  second.  The  average  transmission  is 
}4  to  %  of  the  incident  light. 

It  has  been  found  that  with  a  one-wing  shutter  the  flicker  is 
nearly  as  troublesome  when  the  opaque  part  is  >£  as  when  it  is 
^/2  of  the  shutter.  To  avoid  this,  extra  dark  wings  are  added  to 
the  shutter,  the  form  with  3  wings  being  the  best  With  a  three- 
wing  shutter  a  cycle  is  made  up  of : 

1 .  Picture  on  the  screen — screen  light — %  cycle. 

2.  Same  picture  on  the  screen  but — screen  dark — 3^  cycle. 

3 .  Same  picture  on  the  screen  but — screen  light — Yt>  cycle. 

4.  Same  picture  on  the  screen  but — screen  dark — Yt>  cycle. 

5.  Same  picture  on  the  screen  but — screen  light — >6  cycle. 

6.  Picture  changed — screen  dark — Y(>  cycle. 

The  screen  is  dark  }£  and  light  >2  of  the  time :  Transmission  of 
incident  light,  50%. 

Each  picture  is  thrown  on  the  screen  three  times  before  it  is 
changed  for  the  next.  Thus,  while  there  are  16  cycles  per  second; 
there  will  be  48  flashes  per  second. 

At  this  speed,  48  flashes,  flicker  will  altogether  disappear  (See 
§  592). 


426  FLICKER  WITH  MOVING  PICTURES  [Cn.  XI 

THEORY  AND  EXPERIMENTS  ON  FLICKER 

§  592.  Experiments  have  been  made  to  determine  the  speed  at 
which  flicker  disappears,  that  is,  the  speed  at  which  the  eye  is  un- 
able to  distinguish  between  a  continuous  and  an  intermittent 
light. 

These  experiments  show  that  at  a  certain  speed  the  appearance 
of  flicker  disappears.  This  speed  is  practically  the  same  for 
different  people. 

As  the  speed  is  increased  the  flicker  disappears  for  the  center  of 
the  field  of  vision  before  it  does  for  the  edge.  Thus,  the  light  on  a 
screen  may  not  appear  to  flicker  when  looked  at  directly  but  it  may 
appear  to  flicker  when  looked  at  out  of  the  "tail  of  the  eye." 

As  the  brightness  of  illumination  is  increased  the  appearance  of 
flicker  is  increased  and  a  higher  speed  is  required  for  flicker  to 
disappear.  Thus,  when  showing  a  very  dark  film,  the  light  may 
not  appear  to  flicker  at  all,  while  with  a  very  transparent  film  or 
no  film  at  all  the  light  may  appear  to  flicker  violently  although  the 
speed  is  the  same. 

If,  instead  of  having  the  dark  period  and  the  light  period  equal, 
either  the  dark  period  or  the  light  period  is  made  less  in  proportion 
the  flicker  appears  less  violent,  and  it  disappears  entirely  at  a  lower 
speed.  This  effect  is,  however,  not  very  great. 

Thus,  the  flicker  with  a  shutter  in  which  >6  is  light  and  %. 
is  dark,  is  the  same  as  one  in  which  %  is  light  and  /^  is  dark 
(§ 


§  592a.  A  formula  to  express  these  factors  numerically  was  worked  out  by 
T.  C.  Porter  of  Eton  College  and  published  in  the  Proceedings  of  the  Royal 
Society,,  Vol.  63,  p.  347;  Vol.  70,  p.  313-329  (1902). 

The  constants  have  been  recalculated. 

Let  f  =  number  of  light  flashes  per  second  at  which  flicker  disappears  when 
light  and  dark  flashes  are  equal. 

Let  n  =  number  of  flashes  per  second;  light  and  dark  flashes  are  unequal. 

w  =  angle  of  white  space  in  disc. 

(360°  —  w)  =  angle  of  dark  space  in  disc. 

I       =  intensity  of  illumination  in  meter  candles. 

b  =  constant  depending  on  illumination. 

From  experimental  data  the  formula  comes  out 

f  =  26  -|-  12.2  log  I 

b  =  12.04  +  2.378  log  I 

n  =  f  +  b  [logw  —  log  (360°  —  w)  —  4.5106]. 


CH.  XI]  PRECAUTIONS  FOR  MOVING  PICTURES  427 

§  592b.     Table  showing  Speed  at  which  flicker  just  disappears. — 
FROM  T.  C.  PORTER 

Flashes  per  second 

Illumination  Logarithm  of  at  which  flicker 

meter  candles  illumination  just  disappears 

.0625  8.796-10  1775 

.in  9.046-10  18.08 

.25  9-398-IO  18.50 

i. oo  o.ooo  25.08 

4.00  0.602  33.50 

1.56  0.193  28.00 

2.70  0.431  32.00 

6.30  0.799  35-50 

25.00  1.398  42.66 

100.00  2.000  50.16 

100.00  2.000  50.83 

178.00  2.250  55.08 

400.00  2.602  56.42 

1600.00  3-204  65.00 

6400.00  3.806  71.00 

RESULTS  FOUND  BY  THE  AUTHORS  WITH  A  MOVING"  PICTURE  OUTFIT 

32.00  1.500  36 

100.00  2.000  41 

1000.00  3.000  50 

3200.00  3.500  54 

The  curves  (fig.  234)  are  drawn  to  show  the  speed  at  which 
flicker  disappears  for  equal  light  and  dark  flashes.  There  is  not  a 
great  advantage  as  far  as  the  speed  at  which  flicker  disappears  in 
having  the  duration  of  the  dark  flash  very  short.  The  actual 
appearance  of  flicker  is  much  less  violent,  however,  when  the  dark 
section  is  narrow. 

GENERAL  PRECAUTIONS 

§  593.  Inspection  of  films. — Before  attempting  to  show  films 
to  an  audience,  it  is  well  to  inspect  them  carefully  to  see  that  they 
are  in  good  condition  and  wound  on  the  reel  correctly. 

Use  the  rewinder  to  roll  the  film  from  the  new  reel  upon  an  empty 
reel.  Turn  the  handle  slowly  with  one  hand  while  holding  the 
edge  of  the  film  between  the  fingers  of  the  other  hand ;  do  not  touch 
the  face  of  the  film.  When  a  patch  is  met  in  the  film  inspect  it 
carefully  to  see  that :  (i)  The  same  side  of  the  film  is  on  top.  (2) 
The  patch  is  made  at  the  right  place  so  there  will  not  be  a  misframe, 


428  PRECAUTIONS  FOR  MOVING  PICTURES  [Cu.  XI 

i.  e.,  see  that  the  pictures  are  evenly  spaced.  (3)  The  sprocket 
holes  match  evenly.  (4)  That  the  patch  is  strong  and  no  loose 
corners  stick  up. 

If  the  patch  is  not  good  in  all  these  particulars,  it  must  be 
remedied. 

There  should  be  no  torn  sprocket  holes  or  torn  places  in  the  film 
or  bad  scratches  in  the  emulsion.  If  any  such  defects  are  found, 
they  should  be  cut  out  and  the  film  patched  together  again.  Places 
may  be  found  where  the  film  broke  and  was  pinned  together. 
Remove  the  pin  and  cement  the  film. 

When  the  whole  film  has  been  inspected  in  this  way,  rewind  it, 
so  that  it  will  go  through  the  machine  correctly. 

See  that  there  is  a  '  'leader"  or  strip  of  blank  film  i  to  2  meters 
(4  to  5  ft.)  long  to  thread  through  the  machine,  so  the  entire  title 
of  the  film  may  be  shown.  The  part  of  the  film  used  to  thread  the 
machine  often  becomes  broken  and  a  good  "leader"  saves  the  film 
itself  from  damage. 

If  there  is  time,  it  is  well  to  run  the  film  through  the  machine  and 
watch  the  screen  picture  before  showing  it  to  an  audience. 

§  594.  Splicing  the  film. — When  moving  pictures  are  to  be 
shown  the  operator  will  need  to  patch  the  film  occasionally.  Often 
a  film  breaks  or  an  old  splice  comes  in  two.  A  splice  is  made 
by  qementing  the  two  ends  of  the  film  with  "Film  Cement." 

Cut  one  end  of  the  film,  b,  (fig.  235),  exactly  on  the  line  between 
two  pictures  and  scrape  the  back  (shiny  side)  of  the  film  with  a 
sharp  knife.  There  may  be  oil  on  the  film.  It  must  be  removed; 
cement  will  not  hold  otherwise.  Cut  the  other  end  of  the  film  a, 
about  4  mm.  (J^j  in.)  longer  than  a  dividing  line  between  two  pic- 
tures. Then  scrape  off  the  emulsion  between  the  picture  division 
and  the  ends  of  the  film.  This  emulsion  can  be  scraped  off 
accurately  to  the  line  by  holding  a  straight  edge  over  the  picture 
on  a,  and  letting  the  end  of  the  film  project.  Scrape  the  emulsion 
off  and  right  down  into  the  film  stock.  Scrape  the  corners  as  well 
as  the  middle,  as  the  corners  usually  are  the  first  to  work  loose. 
Film  cement  is  then  spread  on  the  back  of  b,  and  the  front  of  a, 
with  a  brush  or  stick,  never  use  the  fingers.  Be  sure  to  get  plenty 


CH.  XI] 


PRECAUTIONS  FOR  MOVING  PICTURES 


429 


of  cement  on  the  corners  of  the 
film.  Then  immediately  press 
the  two  ends  of  the  film  together 
firmly  for  a  few  seconds,  being 
careful  not  to  push  the  ends  of 
the  film  sidewise  in  doing  so. 

Several  points  must  be  care- 
fully observed  in  order  to  get  a 
splice  which  is  satisfactory  and 
durable. 

i .  Cut  the  film  so  that  the  di- 
viding lines  between  two  pictures 
come  exactly  together  or  there 
will  be  a  "misframe"  when  the 
film  is  running  through  the  ma- 
chine. 


FIG.  235A.     EDISON  FILM  MENDER. 

(Cut  loaned  by  the  Edison  Manu- 
facturing Company"). 

It  has  three  gates  or  hinges — those 
on  the  sides  clamp  down  and  hold 
the  film  while  the  ends  are  cut  and 
prepared  and  the  cement  is  applied. 
The  narrow  middle  clamp  is  then 
closed  holding  the  ends  of  the  film 
firmly  in  contact  while  the  cement 
dries.  The  gauge  shown  at  the  left 
enables  the  operator  to  cut  true  edges 
on  the  film  and  scrape  the  proper 
width  for  the  cementing. 


FIG.  235.    METHOD  OF  PATCHING  A 
MOVING  PICTURE  FILM. 

One  end  of  the  film  B,  b  is  cut  on 
the  line  between  two  pictures  and 
the  other  end  A ,  a,  is  cut  a  short  dis- 
tance beyond  the  line  between  two 
pictures.  The  film  side  of  one  and 
the  shiny  side  of  the  other  are 
scraped,  cement  is  applied  and  the 
two  ends  are  placed  together  so 
that  the  sprocket  holes  will  match. 


2.  Scrape    the    film   well, 
both  the  back  side  of  b,  and 
the  emulsion  side  of  a. 

3.  Apply  the  cement   and 
work  rapidly. 

4.  Be    sure    to    hold   the 
emulsion    side-  of  both   films 
either  up  or  down. 


430  PRECAUTIONS  FOR  MOVING  PICTURES  [Cn.  XI 

5.  Get  the  film  together  so  that  the  two  parts  of   the  film  are 
in  a  straight  line  and  not  at  an  angle. 

6.  Get  the  sprocket  holes  together,  so  that  they  will  match 
accurately. 

7.  Press  the  film  firmly  together  without  any  sidewise  motion. 
It  is  well  to  practice  on  short  pieces  of  scrap  film  until  strong 

splices  fitting  together  accurately  can  be  made  quickly. 

There  are  two  kinds  of  film  cement,  one  which  is  good  for  cellu- 
loid films  only,  the  other  (NI  cement)  will  work  equally  well  on 
non-inflammable  film  and  celluloid  film. 

For  making  permanent  patches  in  a  routine  way  there  is  a  film 
mender  (fig.  23 5 A), consisting  of  a  guide  and  a  pressure  clamp,  so 
that  the  film  maybe  accurately  held  while  being  cemented  together. 

All  splices  should  be  as  far  as  possible  made  before  beginning  a 
performance.  Any  old  splices  which  appear  weak  and  likely  to  pull 
apart  should  be  pulled  apart  and  cemented  together  again. 

With  the  greatest  precaution  a  film  will  sometimes  come  apart 
during  an  exhibition.  When  this  occurs  the  film  is  pinned  together 
to  be  spliced  permanently  later.  Be  sure  to  remove  pins  and  make 
permanent  splices  before  attempting  to  run  the  film  through  the 
machine  again. 

WINDING  AND  REWINDING 

§  595.  A  device  to  wind  the  film  from  one  reel  to  another  is  a 
part  of  any  moving  picture  outfit. 

While  passing  through  the  machine  the  film  is  always  wound  on 
the  lower  reel  in  the  wrong  direction  for  use,  and  it  is  necessary  to 
rewind  it,  so  that  it  will  be  right  side  out  again. 

While  rewinding  is  the  time  to  remove  pins  and  splice  per- 
manently any  breaks  in  the  film  which  occurred  during  an  exhibi- 
tion. 

In  most  moving  picture  theaters  one  film  is  rewound  while  the 
next  film  is  being  shown,  the  operator  turning  the  moving  picture 
crank  with  one  hand  and  the  rewinder  with  the  other  hand.  When 
the  rewinding  is  done  this  way  very  rapidly  and  the  rewinder  is 
fastened  to  the  walls  of  a  sheet  iron  booth  a  decidedly  terrifying 
sound  may  be  produced. 


CH.  XU  DANGER  OF  FIRE  431 

DANGER  OF  FIRE 

§  596.  Before  the  introduction  of  non-inflammable  films,  all 
films  were  made  by  coating  the  emulsion  upon  celluloid.  This  is  a 
nitrate  (the  trinitrate)  of  cellulose  to  which  is  added  a  certain 
amount  of  camphor.  A  more  highly  nitrated  cellulose  is  called  gun 
cotton. 

There  is  sufficient  oxygen  in  the  nitrated  cellulose  to  partially 
support  combustion  and  it  is  the  cause  of  the  highly  inflammable 
nature  of  celluloid.  This  was  strikingly  shown  in  some  experi- 
ments made  to  ascertain  the  possible  danger  from  an  ignited  film. 
A  small  reel  of  film  was  lighted  and  put  under  a  tin  box  so  that  no 
air  could  get  at  it.  A  fire  in  ordinary  combustibles,  such  as  paper 
or  wood,  would  soon  be  smothered,  but  the  roll  of  film  continued 
to  decompose  in  the  closed  box.  This  shows  that  if  a  roll  of  film, 
even  in  a  closed  fire  proof  magazine,  once  catches  fire  it  will  con- 
tinue to  burn  as  long  as  there  is  anything  left  of  it. 

The  gases  given  off  from  the  film  decomposing  in  a  closed  box  are 
very  disagreeable  and  will  burn  in  contact  with  air  if  they  are  once 
lighted.  If  celluloid  will  burn  so  vigorously  in  a  closed  box,  what 
would  be  the  effect  of  a  large  reel  of  film  lying  uncoiled  in  a  waste 
basket  or  on  the  floor  should  it  once  catch  fire  ?  This  was  the  prac- 
tise in  the  early  days  of  the  art  of  projecting  moving  pictures. 
Seven  to  ten  meters  (twenty  or  thirty  feet)  of  film  piled  loosely,  will 
be  completely  consumed  in  a  few  seconds,  burning  with  a  fierce 
flair,  e  while  it  lasts. 

In  view  of  this  very  evident  danger,  modern  apparatus  is 
designed  to  make  it  as  safe  as  possible.  To  the  good  design  of  the 
machine  must  be  added  the  cooperation  of  the  operator  to  prevent 
a  fire. 

The  fire  shutter  (fig.  228),  automatically  closes  whenever  the 
machine  is  not  running.  This  shutter  is  placed  in  front  of  the  film 
and  prevents  the  light  of  the  arc  from  striking  it  except  when  it 
is  in  motion.  If  the  film  should  break,  however,  a  tag  end  might 
remain  in  the  aperture  plate  and  be  ignited,  the  fire  shutter  remain- 
ing open  while  the  crank  was  being  turned.  To  prevent  this 
trouble  the  light  should  be  instantly  shut  off  whenever  a  film 
breaks. 


432  DANGER  OF  FIRE  [Cn.  XI 

The  time  required  for  igniting  a  film  was  examined.  It  was 
found  that  an  ordinary  film,  partly  black  and  partly  transparent 
when  held  in  the  condenser  focus  would  first  curl  and  later  burst 
into  flame.  The  time  required  for  each  was  noted,  first  with,  then 
without  a  water-cell. 

No  water-cell  With  water-cell 

Image  of  arc  Curl  Burn  Curl  Burn 

20  Ampere  D.  C.  Arc 

Concentrated  spot  1.3  sec.     2.6  sec.     5  sec.        10  sec. 

Small  spot 2     sec.     3.5  sec.     7  sec.        12  sec. 

24  Ampere  A.  C. 

Concentrated  spot 6     sec.     zosec.      over  30  sec. 

35  Ampere  A.  C. 

Spot  large  enough  to  project 

picture,  film  dead  black  ...3     sec.     12  sec.      over  60  sec. 

With  35  amperes  alternating  current  and  the  crater  image  large 
enough  to  project  the  full  size  of  picture,  the  film  curled  in  3  seconds 
and  burst  into  flame  in  12  seconds.  When  a  water-cell  was  used 
the  film  was  merely  slightly  warped  and  not  in  the  least  injured 
after  an  indefinite  exposure.  With  larger  installations  the  water- 
cell  could  not  be  relied  on  to  protect  the  film  indefinitely,  though  it 
would  much  reduce  the  risk. 

The  data  given  in  §  848  (fig.  342),  shows  the  effects  of  the 
water-cell  in  reducing  the  radiant  energy. 

Examination  was  made  of  the  probable  security  afforded  by  the 
fire-trap  of  a  fire-proof  film  magazine.  A  short  piece  of  film  was  put 
through  the  fire-trap  of  a  film  magazine.  This  fire-trap  consists  in 
a  flat  tube,  the  lower  end  of  which  is  nearly  closed  by  a  pair  of 
metal  rollers.  The  flame  would  not  follow  the  film  through  the 
metal  tube.  When,  however,  the  film  was  pulled  rapidly  through 
the  fire-trap  it  might  or  might  not  be  extinguished  by  the  rollers. 

With  the  upper  magazine,  where  the  film  hangs  down,  the  rising 
flames  heated  the  film  to  such  an  extent  that  when  pulled  upward 
through  the  fire-trap  it  continued  to  burn  on  the  other  side.  When 
the  film  projecting  from  the  lower  magazine  was  ignited  and  pulled 
down  through  the  fire-trap,  it  was  extinguished  just  as  a  strip  of 


CH.  XI]  MOVING  PICTURE  EXHIBITION  433 

paper  would  be.  The  end  of  the  film  did  not  get  as  hot  as  that 
projecting  from  the  upper  magazine  because  the  rising  flames  did 
not  tend  to  play  around  the  unburned  part.  It  would  seem,  there- 
fore, that  the  fire  would  probably  not  be  carried  into  the  lower 
magazine  along  with  the  film.  Of  course,  with  the  upper  magazine 
the  film  is  going  out  of  the  opening  in  normal  operation.  What 
would  be  the  effect  of  the  sharp  blaze  from  a  meter  or  more  (three 
feet)  of  loose  film  which  would  quickly  unwind  if  the  film  broke  can 
only  be  conjectured.  It  would  be  likely  to  get  the  magazine  red 
hot  and  set  the  film  inside  on  fire. 

With  these  possibilities  of  risk  in  mind,  one  will  naturally  be  very 
careful  in  handling  the  apparatus,  so  that  nothing  shall  start  to 
burn  and  to  follow  the  precautions  of  keeping  all  of  the  films  not 
in  use  inside  of  fire-proof  boxes.  The  two  films  in  use  are :  the  film 
in  the  machine,  and  the  film  which  has  just  been  run  through  and  is 
being  rewound. 

When  non-inflammable  film  is  used  the  above  precautions  are  not 
necessary  from  the  standpoint  of  fire  risk,  but  the  films  might  be 
spoiled.  It  is,  however,  a  good  plan  to  be  careful  even  if  non- 
inflammable  films  are  used,  so  that  habits  of  carelessness  will  not 
lead  to  accident  should  one  of  the  celluloid  films  be  included  with- 
out the  knowledge  of  its  nature. 

THE  CONDUCT  OF  AN  EXHIBITION 

§  597.  Inspection  of  the  plant. — Is  the  exhibition  going  to  go 
smoothly,  without  hitches,  or  will  the  light  be  poor  and  go  out,  the 
film  be  out  of  focus,  and  break  and  everything  go  wrong?  This 
depends  largely  upon  the  operator  and  a  very  careful  inspection  of 
all  the  apparatus  before  the  exhibition  begins. 

The  principle  things  to  look  out  for  are: 

(1)  See  that  all  wiring  is  in  good  shape,  no  binding  posts  loose, 
no  wires  almost  burned  out  in  the  lamp. 

(2)  See  that  the  carbons  in  the  lamp  are  long  enough,  that  extra 
carbons  are  ready,  that  tools  to  change  carbons  are  handy. 

(3)  Burn  the  arc  a  little  while  till  the  carbon  ends  are  properly 
shaped. 


434  MOVING  PICTURE  EXHIBITION  [Cn.  XI 

(4)  See  that  the  optical  parts  are  clean,  and  free  from  dust. 
See  that  everything  is  in  line  and  the  light  is  even  on  the  screen. 

(5)  See  that  the  objective  is  in  focus. 

(6)  See  that  the  mechanism  is  oiled  and  in  good  order,  no  screws 
loose. 

(7)  See  that  the  films  are  properly  mended  and  that  there  are 
no  misframes. 

(8)  See  that  the  rolls  of  film  are  in  the  proper  order. 

The  first  reel  of  film  is  put  in  the  magazine  and  the  machine  is 
threaded. 

The  arc  is  either  pushed  away  from  the  operator  so  it  will  not 
shine  on  the  moving  picture  head  or  else  the  dowser  in  front  of  the 
condenser  is  let  down. 

The  arc  lamp  is  lighted.  When  all  is  ready  the  crank  of  the 
machine  is  started,  the  arc  lamp  pulled  toward  the  operator  into 
position,  the  dowser  is  raised,  and  the  house  lights  turned  off 
all  at  the  same  time. 

During  the  exhibition  there  should  be  but  two  things  to  watch. 

1.  The  adjustment  of  the  carbons.     The  carbons  need  occa- 
sional attention  to  keep  a  good  light. 

2 .  The  action  on  the  screen.     The  action  on  the  screen  should 
be  very  carefully  followed.     It  will  serve  as  a  guide  to  the  proper 
speed  to  turn  the  crank  of  the  machine.    The  lighting  of  the  picture 
and  the  focus  of  the  objective  may  need  attention  occasionally  as 
can  be  seen  by  watching  the  screen. 

If  the  machine  or  the  film  is  poor  various  mishaps  may  occur  and 
require  a  short  stop. 

The  most  frequent  is  a  misframe.  This  occurs  when  a  patch 
has  not  been  properly  made  and  the  pictures  not  properly  matched. 
The  difficulty  is  remedied  by  raising  or  lowering  the  framing  lever. 
Note  the  place  where  the  misframe  occurs  and  remove  it  before  the 
film  is  shown  again. 

The  film  may  break.  Turn  off  the  light  instantly,  or  push  the 
lamp  over  to  the  lantern-slide  side  or  lower  the  dowser.  If  a  tag 
end  of  film  is  left  in  the  aperture  plate,  it  may  catch  fire  if  the  light 
is  not  turned  off.  The  film  is  now  threaded  through  the  machine 


CH.  XI]  MOVING  PICTURE  EXHIBITION  435 

again  and  the  ends  pinned  together  in  the  lower  film  magazine. 
Splice  permanently  later. 

When  the  end  of  the  film  is  reached,  turn  up  the  house  lights  and 
put  out  the  arc  light,  or  push  the  lamp  over  to  the  lantern-slide  side 
as  the  case  may  require.  Turn  the  crank  a  few  times  until  the  film 
has  all  rolled  into  the  lower  film  magazine. 

The  lower  reel  is  taken  out  and  put  on  the  re  winder,  the  empty 
reel  from  the  upper  magazine  put  in  its  place  and  a  new  roll  of  film 
is  put  in  the  upper  magazine. 


FIG.  236.  THE  EDISON  HOME  KINETOSCOPE. 
(Cut  loaned  by  Thomas  A.  Edison,  Inc.). 

At  the  end  of  the  exhibition  all  of  the  films  are  rewound  and  put 
in  the  box  to  be  kept  until  the  next  day  or  to  be  sent  away. 

§  598.    Home  projectors  and  advertising  magic  lanterns. — In 

addition  to  the  regular  moving  picture  machines  there  have  been 
two  side-line  developments.  One  of  these  is  a  relatively  cheap 
moving  picture  machine  with  a  small  arc  lamp  for  the  house  light- 
ing system  (§  127)  or  some  other  form  of  radiant  (Ch.  IV,  V). 
Some  of  these  small  instruments  like  the  "Phantoscope"  of  Jenkins, 
take  the  standard  size  of  motion  picture  film.  Edison  has  put  out 
another  form,  the  "Home  Kinetoscope,"  (fig.  236).  This  does  not 
project  the  ordinary  size  of  moving  picture,  but  very  small  pic- 
tures. Instead  of  one  row  of  pictures  on  the  film  there  are  three 
rows.  With  the  small  pictures  in  three  rows,  a  film  80  feet  (24.38 
meters)  long  contains  as  many  pictures  as  1000  feet  (304.8  meters) 
of  the  ordinary  moving  picture  film,  and  the  mechanism  is  so 
arranged  that  the  three  rows  are  shown  without  a  break. 


436  TROUBLES  WITH  MOVING  PICTURES  [Cn.  XI 

The  automatic  magic  lanterns  are  devised  to  show  automatically 
a  series  of  ordinary  lantern  slides.  One  of  these  instruments  is 
called  the  "Advertigraph"  by  Williams,  Brown  &  Earle  and  has  a 
capacity  of  24  lantern  slides.  Another  form,  designated  a 
"Stereomotorgraph"  by  the  Charles  Besler  Co.,  has  a  capacity 
of  52  lantern  slides.  These  instruments  are  very  effective  for 
advertising  and  for  exhibitions  in  museums. 

TROUBLES 

§  599.  There  are  two  main  troubles  confronting  the  moving 
picture  operator:  A  poor  screen  image,  and  fire  in  the  operating 
room. 

A  poor  screen  image.  This  may  be  due  to  any  one  or  a  combina- 
tion of  the  following: 

(1)  An  operator  with  insufficient  knowledge  and  experience. 
This  is  probably  the  most  common  cause. 

( 2 )  A  poor  pro j  ection  apparatus . 

(3 )  A  bad  light  due  to  insufficient  current  or  to  a  wrong  relative 
position  of  the  carbons. 

(4)  The  parts  of  the  projection  apparatus  not  on  one  axis. 

(5)  The  film  may  be  poor;  too  dark  or  not  sharp,  or  worn  out, 
or  badly  perforated,  or  scratched,  giving  rainstorm  appearances. 

(6)  The  film  may  be  wrong  side  up  or  wrong  side  out  in  the 
machine. 

(7)  There  may  be  a  "misframe"  (§  584,  597). 

(8)  The  apparatus  or  the  floor  may  vibrate,  giving  a  jerky 
appearance  on  the  screen. 

(9)  The  shutter  may  not  be  in  the  right  position  or  of  the  right 
design,  hence  flicker,  travel  ghost,  etc. 

(10)  The  general  light  in  the  room  may  be  too  great,  hence,  a 
gray  picture  without  sufficient  contrast.     The  same  effect  is  pro- 
duced by  a  single  room  light  or  the  light  from  a  door  or  window 
shining  directly  on  the  screen. 

Fire  in  the  operating  room.  This  seems  inexcusable,  but  may 
occur.  To  avoid  loss  of  life  and  of  property  the  operating  room 
must  be  (i)  truly  fire-proof;  (2)  it  must  have  a  large  flue  leading 


CH.  XI]  TROUBLES  WITH  MOVING  PICTURES  437 

to  the  open  air  outside  the  building;  (3)  all  the  openings  in  the 
operating  room  must  be  closed  by  fire-proof  shutters  the  instant  a 
fire  starts.  In  this  way  the  smoke  and  gases  will  escape  through 
the  flue,  and  no  one  in  the  audience  will  know  that  anything  is 
wrong. 

From  the  standpoint  of  the  operator,  if  a  fire  should  start  he 
should  turn  off  the  arc  light  and  turn  on  the  room  lights  as  soon  as 
possible.  If  there  is  a  pail  of  water  or  a  small  fire  extinguisher 
of  the  wet  form  in  the  room  the  water  or  the  fire  extinguisher  can 
be  used  to  good  advantage  to  prevent  the  fire  from  spreading. 
The  cooling  effect  will  sometimes  put  out  the  film,  although,  as 
stated  above  exclusion  of  oxygen  does  no  good  for  the  celluloid 
contains  enough  oxygen  to  support  combustion.  The  real  way 
after  all  is  to  be  so  careful  that  a  fire  never  starts.  (See  Richard- 
son's Handbook,  26.  edition,  pp.  65-93). 


438 


DO  AND  DO  NOT  WITH  MOVING  PICTURES        [Cn.  XI 


§  5991.     Summary  of  Chapter  XI: 


Do 

1.  Learn  the  principles,  and 
perfect  yourself  in  the  practice 
under  expert  guidance,   before 
you  assume  the  responsibility  of 
an  independent  operator. 

2 .  Keep  your  operating  room 
in  perfect  order. 

3.  Light  the  theater  so  that 
the  lights  cannot  shine  directly 
in  the  eyes  of  the  spectators  or 
upon  the  screen. 

4.  Have  a  perfect  screen.     If 
it  is  a  painted  screen,  add  a  fresh 
coat  occasionally. 

5.  Use  direct  current  for  the 
arc  lamp  if  possible  (Ch.  XIII). 

6.  Inspect  wiring  and  appara- 
tus daily. 

7 .  Keep  the  lenses  of  the  con- 
denser   and    of    the    objective 
clean,  and  in  the  right  relative 
position. 

8.  Keep  in  mind  the  precau- 
tions (§  593-594)- 

9.  Learn  to  conduct  the  ex- 
hibition  in    the    best    possible 
manner. 

10.  Remember  that  it  is  far 
easier  to  avoid  a  fire  than  to  put 
it  out. 


Do  NOT 

1.  Do  not  pretend  to  be  a 
competent   operator  until   you 
have   the   requisite   knowledge 
and  experience,  and  then  never 
stop  learning. 

2.  Do  not  have  your  operat- 
ing room  in  disorder. 

3.  Do  not  install  room  lights 
so  that  they  can  glare  in  the 
eyes  of  the  spectators  or  shine 
on  the  screen. 

4.  Do  not  project  on  a  dirty 
screen. 

5.  Do    not    use    alternating 
current   for   projection   if   you 
can  use  direct. 

6.  Do  not  neglect  a  careful 
daily  inspection  of  wiring  and 
apparatus. 

7.  Do  not  use  dirty  lenses  or 
objectives. 


8.  Do  not  fail  to  study  care- 
fully the  precautions  (§  593). 

9.  Do  not  neglect  the  direc- 
tions   for    the    conduct    of   an 
exhibition. 

10.  Never  forget  the  danger 
from  fire. 


CHAPTER  XII 
PROJECTION   ROOMS  AND  SCREENS 

§  600.     Apparatus  and  Materials  for  Chapter  XII: 

1.  Room  which  can  be  made  entirely  dark,  or  which  can  be 
partly  lighted,   depending  on  the  kind  of    projection  and  the 
radiant. 

2.  If  for  exhibitions,  the  room  should  have  plenty  of  aisles  and 
exits,  and  there  should  always  be  lights  (red  lights)  near  the  exits, 
and  these  lights  should  be  independent  of  the  projection  circuit. 
The  room  should  be  well  ventilated,  and  of  a  form  found  suitable 
for  audiences,  e.  g.,  like  a  church,  theater  or  university  lecture 
room.     The  room  should  be  tinted  and  decorated  with  light-absorb- 
ing colors  (§  604). 

3.  The  lantern  or  other  projection  apparatus  should  be  so 
placed  that  it  does  not  interfere  with  the  audience  (§  612—620). 

4.  Special  room  for  the  projection  apparatus.     If  in  a  moving 
picture  theater,  there  should  be  a  fire-proof  room  for  the  apparatus. 
This  should  have  a  large  ventilator  extending  through  the  roof  or 
side  of  the  building  (§  556-557). 

5.  Screen  upon  which  the  images  are  projected.     This  should 
receive  the  image  at  right  angles  to  avoid  distortion  (fig.  241),  and 
be  of  sufficient  size  for  the  room  (§633). 

§  601.  For  the  historical  consideration  of  rooms  and  screens 
see  under  history  in  the  Appendix.  See  also  the  works  referred  to 
in  Chapter  I,  §  2,  and  the  catalogues  of  manufacturers  of  projection 
apparatus  and  materials.  Periodicals  on  moving  pictures  like  the 
Moving  Picture  World;  F.  H.  Richardson's  Motion  Picture  Hand- 
book; and  F.  A.  Talbot's  Moving  Pictures. 

§  602.  Suitable  room  for  projection. — Any  room  which  can  be 
darkened  may  be  used  for  projection,  but  to  be  satisfactory  it 
should  have  the  qualities  of  a  good  auditorium. 

(1)  There  should  be  plenty  of  aisles  and  passages,  so  that  the 
auditors  can  easily  reach  their  seats. 

(2)  There  should  be  plenty  of  exits,  so  that  the  room  can  be 
quickly  and  safety  emptied. 

(3)  There  should  be  plenty  of  fresh  air. 

439 


440  PROJECTION  ROOM  [Cn.  XII 

(4)  Each  seat  should  have  a  good  view  of  the  stage  and  the 
screen. 

(5)  There  should  be  enough  diffuse  light  in  the  room  so  that 
people  can  find  their  way  around  easily  and  after  gaining  twilight 
vision,  be  able  to  take  notes. 

§  603.  Form  of  the  room. — In  general  that  shape  of  room  which 
has  been  found  most  satisfactory  for  churches  and  theaters  and  for 
science  lecture  rooms  in  colleges  and  universities  is  well  adapted  for 
projection.  As,  however,  the  entire  attention  must  be  given  to 
the  images  on  the  screen  in  the  middle  of  the  stage  there  is  a  ten- 
dency to  make  the  rooms  used  especially  for  projection  longer  than 
they  are  wide.  In  a  room  which  is  approximately  square,  the 
spectators  who  sit  at  the  sides  of  the  room  near  the  front  do  not 
have  so  good  a  view  of  the  screen  as  those  in  the  middle  of  the  room 
and  farther  back. 

With  a  long  narrow  room  either  the  picture  must  be  magnified 
excessively  to  enable  those  on  the  back  seats  to  see  the  details, 
while  for  those  on  the  front  seats  the  pictures  seem  very  coarse,  or 
there  must  be  a  compromise  so  that  only  for  those  in  the  middle  of 
the  hall  are  the  screen  pictures  of  the  most  favorable  size. 

We  strongly  advise  any  person  having  the  responsibility  of 
planning  a  lecture  hall  for  educational  purposes  or  for  exhibitions, 
to  take  advantage  of  human  experience  and  see  a  considerable 
number  of  halls  in  various  places,  and  get  hints  of  what  not  to  do 
as  well  as  of  what  to  do  from  those  who  have  had  experience. 
Then  he  can  combine  excellencies  and  avoid  mistakes  in  planning 
his  own  building  or  room. 

§  604.  Tint  and  decoration  of  the  room. — In  order  to  get  the 
best  possible  results  in  projection,  no  light  whatsoever  should  reach 
the  eyes  of  the  spectators  except  that  reflected  from  the  screen. 
With  the  moderate  light  available  for  the  earliest  users  of  the  magic 
lantern  it  was  advised  that  the  walls  and  ceiling  be  made  black  so 
that,  as  they  put  it,  "the  room  would  be  as  sombre  as  possible." 
For  some  experiments  in  projection  with  polarized  light,  the 
spectroscope,  and  the  highest  power  micro-projection  such  a  room 


CH   XII]  LIGHTING  THE  PROJECTION  ROOM  441 

would  still  be  an  advantage;  but  for  ordinary  magic  lantern  and 
moving  picture  exhibitions  total  darkening  of  the  room  is  unneces- 
sary and  undesirable.  But  for  all  projection  it  is  a  great  advan- 
tage to  prevent  any  light  from  falling  upon  the  screen  except  that 
from  the  projection  apparatus.  The  room  should  therefore  be 
tinted  with  some  light-absorbing  color.  Nothing  is  better  than  the 
brownish  color  of  natural  wood,  such  as  oak  or  pine.  If  natural 
wood  is  not  used,  the  walls  and  ceilings  can  be  tinted  brownish 
or  olive.  For  decorations,  rich,  dark  red,  orange,  green,  and  blue 
may  be  used.  Light  orange,  green,  and  blue  reflect  too  much  light 
but  the  dark,  rich  colors  give  the  pleasing  effect  without  making 
the  room  too  light. 

For  mixing  these  tints,  if  oil  colors  are  used,  much  turpentine 
should  be  employed  to  give  a  flat  or  dull  finish,  not  a  shiny  or  glossy 
one.  If  the  finish  is  shiny  it  will  act  like  a  mirror  and  give  an 
undesirable  glare,  and  shine  in  the  face  of  some  of  the  auditors. 

§  605.  Light  in  the  exhibition  room. — For  magic  lantern  and 
moving  picture  exhibitions,  the  room  should  be  light  enough  so 
that  the  spectators  can  easily  find  their  way  about ;  and  after  the 
twilight  vision  is  established,  the  spectators  should  be  able  to  take 
notes  easily. 

If  the  room  is  finished  and  decorated  with  light-absorbing  colors 
and  tints  as  indicated  above,  there  is  no  danger  of  making  the 
screen  images  gray  and  dull  from  reflections  from  the  walls  and 
ceiling.  One  has  simply  to  guard  against  direct  light  shining  on  the 
screen  from  a  window  or  from  a  lamp.  (For  lighting  a  black-board 
in  a  lecture  room  see  fig.  240). 

§  606.  Lamps  for  general  lighting. — The  lamps  to  give  the 
needed  light  should  be  so  arranged,  and  with  such  shades  that: 

(1)  They  cannot  shine  directly  in  the  eyes  of  the  spectators;   and 

(2)  That  they  cannot  send  any  of  their  rays  directly  upon  the 
screen.     This  is  best  accomplished  by  placing  the  lights  along  the 
sides  of  the  room  or  on  the  ceiling  or  both,  and  shading  them  so  that 
none  of  their  light  can  extend  directly  to  the  screen. 

The  arrangement  sometimes  used  of  a  row  of  lights  around  the 
screen  is  bad;  for,  while  no  light  can  reach  the  screen  from  them, 


442 


LIGHTING  THE  PROJECTION  ROOM 


[CH.  XII 


the  glare  in  the  eyes  of  the  spectators  will  detract  from  the  effect. 

If  ceiling  lights  are  used  they  should  be  placed  close  to  the  ceiling 
and  on  the  side  of  the  construction  work  (stringers,  etc.)  away  from 
the  screen.  Then  the  light  will  extend  obliquely  downward  and 
backward,  but  none  of  it  will  fall  directly  upon  the  screen. 

Lights  along  the  sides  of  the  room  can  be  placed  behind  the 
projecting  construction  work,  or  shaded  so  that  the  light  cannot 
extend  toward  the  screen. 


FIG.  237.     METHODS  OF  INDIRECT  LIGHTING. 
(Cuts  loaned  by  the  National  X-Ray  Reflector  Co.). 

A  Shows  an  opaque  bowl  containing  the  electric  light.  The  light  is 
reflected  upward  and  is  diffused  throughout  the  room. 

B  and  C  Illustrate  the  indirect  lighting  where  the  bowl  containing  the 
electric  light  allows  a  certain  amount  of  the  light  to  extend  downward.  The 
light  is  also  reflected  upward  as  in  A. 

In  C  the  bowl  is  cut  away  to  show  the  electric  bulb,  the  reflector  for  throwing 
the  light  upward,  and  the  opal  glass  diffuser  below  to  give  the  soft  luminous 
effect  of  a  very  large  source  in  the  "luminous  bowl." 


CH.  XII]  DARKENING  THE  PROJECTION  ROOM  443 

The  indirect  or  concealed  light  sources  which  have  been  recently 
developed  answer  all  the  requirements  for  suitably  lighting  a  mov- 
ing picture  theater  or,  indeed,  any  other  place  where  a  soft  light  is 
required  and  the  light  should  not  shine  directly  in  the  eyes  of  the 
spectators  (fig.  237  A,  B,  C). 

It  is  also  an  advantage  to  have  the  screen  in  a  kind  of  alcove  i  to 
2  meters  (3-6  ft.)  deep  and  the  walls  on  the  sides,  the  floor  and  the 
ceiling  dark  brown  or  dark  red  or  olive  to  absorb  any  light  reflected 
upon  them  (6o6a). 

For  exhibitions,  it  also  adds  brilliancy  to  the  picture  to  have  a 
black  border  around  the  screen.  It  gives  also  the  effect  of  a  framed 
picture. 

With  such  an  arrangement  of  the  lights  in  a  suitably  tinted  room, 
no  light  will  reach  the  screen  directly  to  destroy  the  contrast  and 
render  the  image  vague.  There  can  be  sufficient  diffused  light  in 
the  room  to  enable  one  on  entering  to  see  the  aisles  and  seats,  and 
go  about  without  stumbling.  In  a  short  time  twilight  vision  will 
be  established  and  it  will  then  be  possible  to  read  or  to  take  notes. 

§  607.  Red  lights  near  all  exits.  Fire  escapes. — In  public 
halls,  and  especially  in  moving  picture  theaters,  it  is  an  advantage, 
and  often  a  requirement  in  city  regulations,  to  have  red  lights  near 
every  exit  so  that  the  audience  can  see  exactly  where  it  is  possible 
to  get  out  of  the  hall. 

The  manager  of  every  public  hall  should  look  to  it  every  day  that 
the  fire  escapes  are  in  working  order  and  before  every  exhibition 
that  the  doors  or  gates  to  the  fire  escapes  are  unlocked  and  easily 
opened. 

§  608.  Relative  darkness  of  the  room  for  different  kinds  of 
projection. — The  amount  of  diffuse  light  permissible  in  the  pro- 

§  606a.  While  it  is  a  great  help  to  have  a  screen  in  a  dark  alcove,  still  the 
general  light  of  the  room,  although  none  extends  directly  upon  the  screen, 
tends,  if  too  great,  to  make  the  image  less  brilliant  and  definite.  Every  one 
who  has  studied  astronomy  at  all  with  a  telescope  knows  full  well  how  the 
definiteness  of  the  image  of  a  nebula  or  dim  star  cluster  diminishes  when  the 
moon  rises  and  floods  the  heavens  with  its  diffuse  light.  One  can  also  see  the 
effect  of  too  much  diffused  light  by  observing  a  lighted  clock  face  on  a  dark 
night,  and  the  same  face  with  the  same  light  shining  from  it  on  a  moonlight 
night  or  early  in  the  evening  twilight  before  complete  darkness. 


444  DARKENING  THE  PROJECTION  ROOM  [Cn.  XII 

jection  room  depends  entirely  upon  the  brilliance  of  the  screen 
image.  In  order  to  see  the  screen  image  clearly  there  must  be 
strong  contrast  between  it  and  surrounding  objects.  With  trans- 
parent lantern  slides  and  sunlight  or  the  electric  light  to  illuminate 
them  one  can  see  the  screen  images  well  in  a  room  so  light  that 
everything  in  the  room  is  visible  provided  no  direct  light  reaches 
the  screen  except  that  from  the  projection  apparatus.  If  the 
lantern  slides  are  less  transparent  or  the  light  used  for  projection 
less  brilliant,  then  the  room  must  be  relatively  darkened  to  give 
the  needed  contrast.  Keeping  the  principle  of  contrast  in  mind, 
one  readily  understands  that  for  some  of  the  experiments  in  physics 
where  the  light  on  the  screen  is  very  dim,  with  kinemacolor  moving 
pictures  and  with  Lumiere  colored  lantern  slides,  and  with  high 
power  micro-projection,  the  room  must  be  very  dark  in  order  to  get 
the  screen  image  clearly  visible.  In  like  manner  if  the  source  of 
light  for  projection  is  relatively  weak,  like  the  acetylene  flame  or 
some  other  less  brilliant  light  than  the  electric  arc,  the  room  must 
be  darker  than  with  a  more  brilliant  radiant. 

§  609.  Daylight  and  twilight  vision. — It  has  been  known  for 
time  out  of  mind  that  with  most  people  the  eyes  can  adapt  them- 
selves to  a  dim  light  or  to  a  bright  light.  If  one  goes  into  a  dimly 
lighted  room  from  full  daylight  the  room  will  at  first  appear  per- 
fectly black,  but  in  a  few  minutes  objects  can  be  seen  fairly  well, 
and  within  half  an  hour  the  room  will  appear  comparatively  light. 
On  the  other  hand,  in  passing  from  a  comparatively  dark  room  to 
full  sunlight  the  eyes  are  so  dazzled  at  first  that  hardly  anything 
can  be  seen,  but  soon  the  eyes  become  adapted  to  the  bright  light. 
It  has  been  found  by  careful  experiments  on  large  numbers  of 
people  that  the  main  adaptation  of  the  eyes  for  bright  light  after 
being  in  a  dark  room  requires  only  about  6  minutes,  while  the 
adaptation  for  a  dim  light  after  being  in  full  daylight  requires 
about  30  minutes,  although  after  10  minutes  the  eye  is  about  100 
times  as  sensitive  in  a  dark  room  as  it  is  in  full  daylight.  While 
the  pupil  expends  normally  in  dim  light,  thus  increasing  the  aper- 
ture of  the  eye,  this  is  not  the  fundamental  thing  in  adaptation,  but 
there  is  some  change  in  the  retina  which  gives  it  greater  sensitive- 
ness. 


CH.  XII]  DARKENING  THE  PROJECTION  ROOM 


445 


Sh 


f 


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U          A 

W 

y- 

1              SI 

i 

\ 
;.Sh 

^ 

^ 

CrG 

V 

i 

t 

P 

CJ  I*-1 


FIG.  238.     FACE  AND  SECTIONAL  VIEW  OF  WINDOW  SHADES  PLANNED  FOR 
IN  THE  CONSTRUCTION  OF  THE  BUILDING. 

A  Cross  section  showing  the  window  shade  (Sh)  in  the  grooves  (W  W)  at 
the  sides. 

B     Face  view  of  the  window  with  the  shade  (Sh)  shown  by  dotted  lines. 

C  Sectional  view  of  the  window  showing  the  window  "sashes  (W  S),  the 
ordinary  window  curtains  (Cr)  close  to  the  sash,  and  the  window  shade  (Sh) 
considerably  in  front  of  the  curtain,  i.  e.,  near  the  front  of  the  window  frame. 

The  coping  over  the  window  is  shown  by  dotted  lines  as  turned  down. 
This  exposes  the  shade  roller  so  that  it  can  be  adjusted  if  it  gets  out  of  order. 

The  window  shade  is  shown  as  drawn  down,  and  the  shade  string  goes  over  a 
pulley  (P)  and  is  caught  in  a  fork-like  holder  (C)  in  front  of  the  window  frame. 


446  DARKENING  THE  PROJECTION  ROOM  [Cn.  XII 

Much  of  the  projection  at  the  present  time  requires  daylight 
rather  than  twilight  vision  from  the  brilliancy  of  the  screen  images, 
but  one  should  keep  in  mind  that  good  screen  images  may  be 
obtained  by  two  methods  (i)  brilliant  illumination  and  daylight 
vision;  or  (2)  moderate  illumination  and  twilight  vision. 

§  610.  Method  of  darkening  a  room. — As  many  rooms  used  for 
projection  are  well  supplied  with  windows  there  must  be  some 
method  of  excluding  daylight  or  other  outside  light.  The  two 
means  usually  employed  are  wood  or  metal  shutters  and  opaque 
cloth  curtains. 

Shutters  may  be  on  hinges  and  swing  sidewise,  or  they  may  be 
hung,  and  by  means  of  pulleys  raised  and  lowered.  In  many 
laboratories  where  the  shutters  are  opened  and  closed  several  times 
during  a  lecture,  there  is  a  water  or  electric  motor  to  move  the 
shutters. 

If  curtains  are  used  they  should  be  of  dark  colored  opaque  cloth 
on  a  spring  roller,  so  that  they  can  be  opened  or  closed  as  much  or  as 
little  as  desired.  These  are  usually  opened  and  closed  by  hand 
(fig.  238). 

§  611.  Excluding  light  at  the  window  margins. — As  curtains 
are  usually  hung,  there  is  a  space  admitting  light  at  the  top,  bot- 
tom, and  sides  of  the  window.  This  can  be  avoided  by  having  the 
edges  of  the  curtain  in  a  groove  at  the  sides  and  bottom  of  the 
window  frame,  and  having  the  curtain  roller  above  the  opening  of 
the  window  frame  (fig.  238).  If  one  has  the  designing  of  the 
building,  proper  grooves  can  be  planned  for  and  put  in  when  the 
window  frames  are  made.  If  this  has  not  been  planned  for  in 
designing  the  building,  then  the  light-excluding  devices  can  be 
added  afterwards.  That  is,  a  light-excluding  shield  can  be  put  all 
around  the  window  frame  (fig.  239).  This  will,  of  course,  cut  down 
somewhat  the  opening  of  the  window  frame. 

POSITION  OF  THE  PROJECTION  APPARATUS  IN  THE  ROOM 
§  612.     The  best  position  for  the  projection  apparatus  in  a  lec- 
ture room  or  exhibition  room  is  at  the  back  of  the  room,  where  it  is 
entirely  free  from  the  audience.     This  also  gives  the  operator 
greater  freedom  (fig.  240). 


CH.  XII] 


DARKENING  THE  PROJECTION  ROOM 


447 


Sh 

B 


"sir 


FIG.  239.     FACE  AND  SECTIONAL  VIEW  OF  A  WINDOW  SHOWING  HOW  THE 

LIGHT-EXCLUDING  SHADE  CAN  BE  INSTALLED  AFTER  THE  BUILDING  is 

CONSTRUCTED. 


448 


POSITION  OF  PROJECTION  APPARATUS  [Cn.  XII 


A  Cross  section  showing  the  window  shade  (Sh)  behind  the  thin  boards 
(WW]  which  serve  to  exclude  the  light  at  the  top,  sides  and  bottom  of  the 
shade. 

B  Face  view  of  the  window  with  the  light-excluding  shade  (Sh)  shown  in 
dotted  lines,  (L)  indicates  the  size  of  the  window  frame.  The  sash  cuts  this 
down  somewhat  and  the  thin  board  frame  to  cut  out  the  light  around  the  edge 
of  the  curtain  cuts  it  down  considerably  more. 

C  Lateral  view  of  the  window  with  the  shade  in  dotted  lines.  The  light- 
excluding  frame  around  the  edge  is  in  full  lines  in  B  and  C. 

§  613.    Position  of  the  projection  apparatus  with  a  level  room. — 

In  a  level  room,  the  projection  apparatus  at  the  back  of  the  room 
must  be  at  such  a  level  that  the  projection  beam  goes  over  the 
heads  of  the  spectators.  This  can  be  accomplished  by  building  a 
platform,  or  by  using  a  high  table.  In  case  the  image  is  still  not 
high  enough  on  the  screen,  the  lantern  can  be  tilted  slightly  upward 
by  putting  a  wedge  under  the  end  of  the  baseboard  supporting  it 
(fig.  240). 


FIG.  240.     SECTIONAL  VIEW  OF  A  LECTURE  ROOM  HAVING  A  GALLERY. 

B  Black-board.  This  is  lighted  by  incandescent  lamps  behind  a  curved, 
metal  shield  (H  L).  This  gives  plenty  of  light  for  the  black-board  without  in 
any  way  injuring  the  brilliancy  of  the  screen  image. 

L  T    Lecturer's  table  on  the  platform  (P). 

Ml     The  magic  lantern  in  the  gallery  on  its  table  and  special  support  (T). 

Sc     Screen  forjjthe  image  above  the  black-board. 


CH.  XII] 


POSITION  OF  PROJECTION  APPARATUS 


449 


§  614.    Level  room  with  the  apparatus  near  the  screen. — It  is 

sometimes  desirable  to  put  the  apparatus  near  the  screen.  Then 
provision  must  be  made  by  removing  some  of  the  seats  if  the  center 
aisle  is  not  wide  enough. 

The  apparatus  must  usually  be  raised  somewhat  also,  and  some- 
times the  objective  inclined  more  or  less  upward.  In  case  it  is 
desired  to  have  the  apparatus  very  near  the  screen  it  must  be 
pointed  upward  considerably  and  then  the  screen  should  be  hinged 
at  the  bottom  so  that  it  can  be  inclined  toward  the  lantern  till  it  is 
perpendicular  to  the  optic  axis.  The  simplest  way  to  fix  the 
screen  in  any  position,  and  to  change  the  position  is  by  means  of 
ropes  and  pulleys  at  the  top. 


FIG.  241.     LECTURE  ROOM  WITH  RISING  SEATS,  AND  THE  LANTERN  IN  THE 
MIDDLE  OF  THE  ROOM,  NOT  AT  THE  BACK. 

B  Black-board  lighted  by  the  hidden  lights  (H  L)  behind  a  curved  metal 
shield. 

L  T    Lecturer's  table  in  front  of  the  audience. 

Ml  E  The  magic  lantern  (Ml) ;  its  rays  shown  in  full  lines,  and  the  episcope 
or  opaque  lantern  (£)  with  its  rays  shown  extending  from  the  mirror  (M)  in 
dotted  lines. 

Sc  The  screen  for  receiving  the  image.  As  the  magic  lantern  must  be 
elevated  the  screen  is  tipped  toward  it  to  meet  the  axial  ray  at  right  angles. 

For  such  a  position  of  the  magic  lantern  the  projection  objective  must  be  of 
shorter  focus  to  give  the  desired  size  of  image  than  when  the  lantern  is  at  the 
back  of  the  room  (§  636). 


450 


POSITION  OF  PROJECTION  APPARATUS  [Cn.  XII 


The  lantern  should  be  fastened  to  a  hinged  board  when  it  is 
elevated  considerably  (fig.  118,  242). 

§  615.  Magic  lantern  on  the  lecture  table. — Occasionally  it  is 
an  advantage  to  have  the  magic  lantern  on  the  lecture  table ;  then 
the  lecturer  can  manipulate  it  himself. 

There  are  three  arrangements  possible:  (i)  The  lantern  is 
pointed  toward  a  screen  at  the  side  of  the  room  (fig.  243).  (2)  It 
is  pointed  obliquely  upward  toward  the  screen  in  front  of  the 
audience.  In  this  case  the  screen  must  be  inclined  toward  the 
lantern  as  indicated  above  (§614).  (3)  Occasionally,  for  ease  of 
manipulation,  the  lantern  is  pointed  obliquely  upward  toward  the 
audience  and  a  plane  mirror  reflects  the  image-forming  rays  back- 
ward .to  the  screen  (fig.  244).  If  a  mirror  is  used,  the  lantern  slides 
must  be  inserted  with  their  faces  toward  the  objective. 


FIG.  242.     MAGIC  LANTERN  TABLE  WITH  HINGED  BASEBOARD. 
Hinges  connect  the  baseboard  to  the  table  at  the  left.     By  putting  a  block 
under  the  board  at  the  right,  it  can  be  elevated  to  bring  the  screen  picture 
higher  up  (fig.  118). 


CH.  XII]          POSITION  OF  PROJECTION  APPARATUS 


§  616.  Projection  with  inclined  seats  or  gallery. — If  the  seats 
in  the  auditorium  are  raised  after  the  manner  of  an  amphitheater  or 
if  a  gallery  is  present,  in  many  cases  the  apparatus  can  go  to  the 
back  of  the  room  or  in  the  gallery.  This  may  make  it  necessary  to 
point  the  projection  apparatus  somewhat  downward  towards  the 


FIG.  243.     GROUND  PLAN  OF  A  LECTURE  ROOM  WITH  THE  MAGIC  LANTERN 

ON  THE  LECTURER'S  TABLE  AND  THE  SCREEN  AT  THE  SIDE  OF  THE  ROOM. 

B     Black-board. 

Ml  Magic  lantern  on  the  lecture  table  (L  T)  and  pointing  up  to  the  screen 
(Sc)  on  the  side  of  the  room. 

Sc    The  screen  is  shown  tipped  forward  to  avoid  distortion. 

Such  a  position  of  the  lantern  enables  the  lecturer  to  perform  experiments  or 
show  lantern  slides  conveniently. 


452 


POSITION  OF  PROJECTION  APPARATUS  [Cn.  XII 


screen,  but  as  the  distance  is  usually  considerable,  the  screen  image 
will  be  good  on  a  vertical  screen.  The  position  of  the  lantern 
should  never  be  so  high  that  the  screen  image  will  be  distorted. 

§  617.  Apparatus  in  the  middle  of  the  auditorium  with  raised 
seats. — If  the  apparatus  cannot  be  at  the  back  of  the  room  in  an 
amphitheater  then  a  space  or  alcove  must  be  made  somewhere  in 
the  middle  by  omitting  a  certain  number  of  seats.  The  machine 
is  liable  to  be  more  or  less  distracting  if  in  the  middle  of  the  room, 
but  sometimes  this  cannot  be  avoided  on  account  of  distance  or  the 
form  of  the  amphitheater  (fig.  241). 


FIG.  244.  PART  OF  A  LECTURE  ROOM  WITH  THE  MAGIC  LANTERN  ON  THE 

LECTURE  TABLE  DIRECTED  TOWARD  THE  AUDIENCE  AND  A  MIRROR  TO 

REFLECT  THE  IMAGE  ON  THE  SCREEN  IN  FRONT  OF  THE  AUDIENCE. 

B     Black-board  with  hidden  light  behind  the  curved  metal  shield  (H  L). 

Ml  M  Magic  lantern  pointing  toward  the  audience.  The  mirror  reflects 
the  image  back  to  the  screen  in  front  of  the  audience.  The  mirror  also  serves 
as  a  shield. 

Sc  The  image  screen.  By  means  of  the  pulley  and  cord  it  is  inclined  on  its 
hinges  at  the  lower  edge  toward  the  mirror  of  the  magic  lantern.  In  this  case 
it  is  not  inclined  sufficiently  to  meet  the  axial  ray  at  right  angles,  hence  there 
will  be  some  distortion  of  the  image  and  the  upper  edge  will  not  be  in  sharp 
focus  when  the  lower  edge  is. 

T  Lecturer's  table.  With  such  an  arrangement  the  lecturer  can  demon- 
strate with  the  lantern  conveniently,  and  still  have  the  screen  in  front  of  the 
audience.  If  he  uses  lantern  slides  they  must  be  put  in  the  holder  facing  the 
objective,  not  the  light  or  there  would  be  a  mirror  image  on  the  screen  (fig.  213). 


CH.  XII] 


POSITION  OF  PROJECTION  APPARATUS 


453 


Occasionally  when  the  seats  are  on  a  steep  incline  there  is  left  a 
space  through  which  the  projection  objective  can  send  its  beam  to 
the  curtain,  the  apparatus  and  operator  being  under  the  seats  of 
the  amphitheater. 

§  618.  Apparatus  on  one  side  of  the  room. — Occasionally  the 
apparatus  is  put  on  one  side  of  the  room  and  instead  of  projecting 
directly  in  front  of  the  audience  the  projection  is  on  one  side  of  the 
room.  The  auditors  simply  turn  in  their  seats  to  face  the  screen. 


L  T 


FIG.  245.  SECTIONAL  VIEW  OF  A  LECTURE  ROOM  SHOWING  THE  POSITION  OF 
THE  PROTECTION  APPARATUS  WITH  A  TRANSLUCENT  SCREEN. 

Ml  Magic  lantern  or  other  projection  apparatus  on  its  table  (T)  and 
raised  platform  (PI)  in  a  room  outside  the  lecture  room. 

L  T    Lecturer's  table. 

Tr  Sc  Translucent  screen.  The  audience  does  not  see  the  apparatus; 
only  the  screen  image  is  visible. 

Lantern  slides  must  be  inserted  in  the  holder  facing  the  objective,  not  the 
light,  or  the  image  will  have  the  rights  and  lefts  changed  like  fig.  213. 

This  is  not  so  satisfactory  as  when  the  screen  is  directly  in  front 
(fig.  243). 

§  619.  Apparatus  wholly  without  the  room. — Regardless  of  the 
form  of  the  room,  the  apparatus  may  be  placed  in  a  room  just  back 
of  the  lecture  table  in  front  of  the  audience  and  a  translucent  screen 
employed.  This  arrangement  has  decided  advantages,  but  a 
translucent  screen  is  not  so  satisfactory  as  a  white  opaque  screen 
(see  fig.  245). 

§  620.  Special  operating  room. — With  the  ordinary  magic 
lantern  and  projection  microscope  the  apparatus  and  operator  are 


454  WHITE  IMAGE  SCREENS  [Cn.  XII 

usually  in  the  general  exhibition  room,  and  there  is  no  .special 
boxing  or  enclosure  of  the  apparatus.  But  in  moving  picture 
theaters,  where  there  is  some  danger  from  the  inflammability  of  the 
picture  films,  both  the  fire  underwriters  and  the  municipal  regula- 
tions, usually  require  some  form  of  fire-proof  operating  room. 

IMAGE  SCREEN 

§  621.  Next  in  importance  to  a  suitable  room  for  exhibitions 
with  projection  apparatus  is  a  good  screen  upon  which  to  project 
the  image. 

No  one  has  ever  more  briefly  and  clearly  stated  the  qualities  of  a 
good  image  screen  than  Goring  &  Prit chard:  "It  should  reflect 
the \greatest  possible  quantity  of  light  and  absorb  the  least," 
"Every  care  should  be  taken  to  render  the  surface  as  smooth,  white 
and  opaque  as  it  can  be  made"  .  .  .  "inasmuch  as  the  bril- 
liancy and  perfectness  of  the  picture  will  greatly  depend  on  the 
whiteness,  and  the  sharpness  of  its  outline  upon  the  smoothness  of 
the  screen."  The  screen  should  be  dull  white,  never  shiny. 

§  622.  Screens  of  plaster  paris  upon  the  wall. — A  screen  ful- 
filling all  the  requirements  just  given  is  a  wall  coated  with  a  smooth 
finish  of  pure,  fine  plaster  of  Paris. 

§  623.  Painted  wall  screen. — -While  a  plaster  of  Paris  wall 
screen  is  perhaps  the  best,  a  smoothly  plastered  wall,  if  properly 
painted,  gives  almost  as  good  results  and  is  much  cheaper.  The 
wall,  as  stated,  should  be  finished  as  smoothly  as  possible  by  the 
plasterers,  then  it  is  coated  with  pure  linseed  oil  if  porous,  or  with 
a  mixture  of  equal  parts  of  linseed  oil  and  turpentine  if  the  wall  is 
hard  and  non-porous.  When  this  is  dry,  the  wall  is  painted  with 
either  white  lead  ground  in  oil  and  thinned  with  turpentine,  or 
with  "sanitary  paint"  thinned  with  turpentine.  The  sanitary 
paint  has  the  advantage  that  it  does  not  turn  yellow  with  age,  and  - 
that  it  is  more  easily  cleaned  with  soap  and  water. 

When  the  paint  is  properly  thinned  it  should  be  strained  through 
one  or  two  layers  of  gauze  (cheese  cloth)  to  get  out  any  lumps  or 
coarse  particles.  • 


CH.  XII]  WHITE  IMAGE  SCREENS  455 

In  spreading  the  paint  on  the  wall  one  should  use  a  soft  brush 
and  apply  only  the  tip  of  the  brush.  This  will  give  a  smooth  finish 
and  if  one  uses  plenty  of  paint  there  will  be  no  joints,  but  the 
whole  will  appear  like  one  uniform  coat.  Practical  painters  call 
this  "flowing  on  the  paint." 

After  one  coat  is  well  dried  another  can  be  put  on  until  the  wall  is 
perfectly  white.  If  plenty  of  turpentine  is  used  the  surface  will  be 
dull.  It  should  not  be  glossy  or  shiny. 

Whenever  the  surface  becomes  dirty  it  can  be  washed  off  with 
soap  and  water.  If  it  is  not  up  to  standard  whiteness  after  the 
washing  and  drying,  put  on  another  coat  of  the  paint. 

Sometimes  hot  glue,  15%  to  20%  in  water,  is  used  for  sizing  the 
wall.  This  answers  well  if  the  wall  is  perfectly  dry  and  not  subject 
to  moisture.  In  general  it  is  safer  to  use  the  linseed  oil  sizing. 

In  our  experiments  several  white  paints  were  used,  but  the  pure 
white  lead  (sometimes  called  "flake  white")  and  the  non-lead  con- 
taining paint  called  "sanitary  paint"  were  found  most  satisfactory. 
The  latter  has  the  advantage  over  white  lead  that  it  does  not  yellow 
with  age,  and  gives  a  very  opaque  and  white  surface  which  stands 
washing  with  soap  and  water  very  well. 

§  624.  Whitewashed  wall  screens. — A  smoothly  plastered  wall 
that  has  been  carefully  whitewashed  with  milk  of  lime  gives  a  good, 
dull  white  surface  for  a  projection  screen.  It  rubs  off  rather  easily 
and  cannot  be  cleaned.  Of  course  a  fresh  coat  of  whitewash  will 
renew  the  screen.  It  is  cheap  as  well  as  good.  One  should  take 
pains  to  strain  the  whitewash,  and  to  apply  it  smoothly  so  that  a 
uniform  surface  will  be  produced. 

We  did  not  find  a  kalsomined  wall  satisfactory  for  projection. 
It  is,  or  soon  becomes,  too  yellow. 

§  625.     Painted  cloth  screens. — A  good  screen  can  be  made  by 

stretching  some  smoothly  woven,  strong  cotton  cloth  (strong 
muslin)  upon  a  frame  and  painting  it  as  for  the  wall  (§623).  The 
frame  must  be  strong  and  the  cloth  stretched  tight  so  that  there 
will  be  no  wrinkles,  and  it  must  not  rest  against  anything.  . 

One  could  paint  directly  on  the  cloth,  but  it  is  more  satisfactory 
to  size  the  cloth  in  some  way  first.  One  of  the  best  methods  is 


456  WHITE  IMAGE  SCREENS  [Cn.  XII 

to  use  white  linseed  oil,  raw  or  boiled.  The  oil  is  put  on  with  a 
soft  brush  like  paint.  It  is  well  to  make  all  the  brush  strokes  in 
one  direction,  so  that  the  lint  or  nap  on  the  surface  of  the  cloth  will 
be  smoothed  down  in  one  direction.  After  the  linseed  oil  is  dry  the 
cloth  is  painted,  preferably  with  sanitary  paint  and  turpentine, 
although  white  lead  thinned  with  turpentine  answers  well.  One 
coat  should  be  allowed  to  dry  before  adding  another.  It  takes 
from  one  to  two  days  for  each  coat  to  dry.  The  screen  will  be 
white  and  opaque  with  three  to  five  coats.  Care  should  be  taken 
to  strain  the  paint  as  for  the  walls  (§623),  then  there  will  be  no 
rough  spots  (§  625a,  625b). 

If  the  curtain  gets  grimy  it  can  be  wiped  off  with  soap  and 
water,  and  if  necessary  after  it  is  dry,  a  fresh  coat  of  the  paint  can 
be  put  on. 

§  626.  Roller  screens. — Cloth  screens  which  have  been  painted 
as  just  described  make  excellent  roller  curtains,  for  the  sizing  and 

§  625a.  Amounts  of  sizing  oil  and  paint  for  a  cloth  screen. — For  oil-sizing 
and  painting  a  muslin  screen  the  following  times  for  drying  in  the  summer,  and 
the  following  amounts  of  oil  and  paint  were  used  to  make  a  perfect  screen. 
For  sizing,  white  raw  linseed  oil  was  used,  and  only  one  coat  was  applied. 

For  this  it  required  220  cubic  centimeters  of  the  linseed  oil  per  square  meter 
of  cloth,  or  about  one- tenth  of  this  amount  per  square  foot. 

For  painting,  a  preparation  of  sanitary  paint  known  as  "Artists'  Scenic 
White,"  ready  for  use  on  screens  was  used,  two  coats  were  applied.  It 
required  no  cc.  of  the  paint  for  each  square  meter  of  surface. 

It  required  about  36  hours  for  the  raw  oil  sizing  to  dry;  24  hours  was 
sufficient  time  for  a  coat  of  the  white  paint  to  dry.  The  finished  screen  was 
flexible  and  easily  rolled. 

For  a  screen  3  meters  or  10  feet  square  it  would  require  for  sizing  and  paint- 
ing about  two  quarts  of  linseed  oil  and  about  the  same  amount  of  the  "Artists' 
Scenic  White"  or  any  other  white  paint  for  two  coats  of  the  paint. 

§  625b.  The  cloth  may  be  sized  by  the  use  of  white  shellac.  This  is  thinned 
about  half  with  denatured  alcohol  and  painted  on  the  surface  just  as  described 
for  the  oil  size.  It  gives  a  good  surface  to  paint  on,  but  does  not  leave  the 
curtain  so  flexible. 

A  hot  15%  to  20%  solution  of  white  glue  in  water  may  also  be  used  as 
described  for  the  oil  or  shellac  size.  This  has  the  advantage  of  pasting  down 
the  nap  of  the  cloth  and  of  giving  a  very  good  surface  to  paint  on.  It  has  the 
disadvantage  of  expanding  and  contracting  greatly  with  different  conditions  of 
moisture.  If  the  glue  size  is  used  the  curtain  should  have  at  least  one  coat  of 
paint  on  the  back,  so  that  the  glue  size  cannot  be  so  easily  affected  by  moisture. 

§  625c.  The  authors  wish  to  express  their  appreciation  for  information  on 
paints  and  the  painting  of  wall  and  cloth  screens  for  projection,  to  Mr.  A.  E. 
Nash,  Superintendent  of  the  Cornell  University  paint  shop. 


CH.  XII]  WHITE  IMAGE  SCREENS  457 

painting  leave  the  cloth  flexible,  and  without  liability  of  cracking 
and  peeling.  They  are  mounted  on  heavy  spring  rollers  as 
ordinary  window  curtains  are  so  commonly  mounted,  and  can  be 
rolled  up  when  not  in  use. 

§  627.    White  cloth  screens  without  paint  or  other  facing.— 

White  cloth  such  as  a  bed  sheet  has  always  been  and  still  is  used. 
The  cloth  should  be  as  white  as  possible,  and  of  good  thickness. 
It  is  also  advantageous  to  have  the  screen  of  one  piece  without 
seams.  Bed  sheets  may  be  obtained  in  large  dry  goods  houses 
about  3  meters  square  (10  ft.  sq.)  without  seams.  These  make 
very  good  curtains  when  the  folds  are  ironed  out,  and  the  sheet 
stretched  to  hold  it  flat.  It  is  not  easy  to  stretch  a  sheet  so  evenly 
that  there  will  be  no  folds  or  wrinkles.  Fortunately,  a  slight 
unevenness  is  not  noticeable  in  the  screen  image.  A  screen  which 
appears  quite  uneven  to  the  naked  eye  in  daylight  may  give  very 
good  screen  images  and  appear  perfectly  smooth,  when  giving  an 
exhibition. 

Cloth  screens  have  the  disadvantage  that  they  are  not  suffi- 
ciently opaque.  If  one  goes  behind  the  screen  the  image  is  almost 
as  well  seen  as  in  looking  at  the  face  of  the  screen.  This  means 
that  almost  as  much  light  traverses  the  screen  as  is  reflected  from 
the  face.  Naturally,  it  takes  much  more  light  for  a  brilliant  screen 
image  than  with  an  opaque  screen  (§  632). 

For  some  purposes  it  is  advantageous  to  be  able  to  see  the  image 
on  the  back,  then  assistants  behind  the  screen  can  make  the 
appropriate  noises  to  make  the  scene  seem  more  real.  For  exam- 
ple, in  a  moving  picture  scene,  sounds  can  be  made  to  imitate 
the  breaking  of  the  waves  on  the  shore,  the  clatter  of  horses  hoofs 
on  a  pavement,  etc.,  etc.  Unless  the  assistant  could  see  the 
image  it  would  not  be  possible  to  suit  the  sound  so  accurately  to 
the  scene. 

Sometimes  so  large  a  screen  is  needed  that  strips  of  white  cloth 
are  sewed  together.  If  this  must  be  done  the  seams  should  be  very 
smooth.  On  such  screens  the  seams  show  like  lighter  streaks  on 
the  image,  as  more  light  is  reflected  from  the  double  thickness  of 


458  WHITE  IMAGE  SCREENS  [Cn.  XII 

cloth.     Behind  the  screen  the  seams  show  as  black  or  dark  streaks, 
as  less  light  traverses  the  screen  along  the  seam  (§  62ya). 

§  628  Paper  screens. — The  suitability  of  white  paper  screens 
has  been  recognized  for  a  long  time.  One  of  the  best  possible 
screens  is  a  large  sheet  of  white  cardboard.  As  shown  by  photo- 
metric measurements,  the  reflections  from  a  white  cardboard  are 
almost  as  great  as  from  the  standard  surface  of  oxide  of  mag- 
nesium (§  632).  The  white  cardboard  is  especially  suitable  for 
the  images  of  the  high  power  projection  microscope,  and  if  it  could 
be  had  in  sufficiently  large  sheets  it  would  make  an  almost  perfect 
screen  for  large  rooms.  (In  large  paper  stores  one  can  get  sheets 
71x112  cm.  (28x44  in-)-  The  paper  used  for  drawings  by 
architects  and  engineers  and  69  x  102  cm.  (27  x  40  in.)  in  size  is 
also  excellent  for  screen  purposes.  It  is  not  so  easy  to  get  a  smooth 
surface  as  with  the  cardboard). 

Finally,  cloth  is  sometimes  faced  with  paper  to  give  a  more 
opaque  and  perfect  screen. 

SCREENS  WITH  METALLIC  SURFACES 

§  629  Dull  white  surfaces  reflect  almost  equally  throughout 
the  whole  hemisphere  (fig.  248)  and  therefore  the  image  appears 
almost  equally  brilliant  in  any  position.  Those  near  the  axis  of 
the  projection  apparatus  in  the  middle  of  the  room  do  not  see  the 


§  627a.  Screens  for  traveling  exhibitions. — When  exhibitions  must  be 
given  in  school-houses  and  in  halls  where  there  is  no  lantern  and  no  screen,  the 
exhibitor  must  supply  both.  In  traveling  it  is  inconvenient  to  carry  a  roller 
screen,  and  usually  the  screen  is  folded  so  that  it  can  be  packed  in  a  small 
space.  Thisi  of  course,  makes  creases  in  the  screen,  and  besides  there  is  noth- 
ing to  support  it  so  that  it  will  hang  smooth  and  even. 

For  a  traveling  screen  a  heavy,  seamless  bed  sheet  is  excellent.  Bed  sheets 
in  one  piece  as  large  as  needed  are  to  be  had.  To  hang  these  sheets  there 
should  be  a  strong  cord  along  the  upper  edge  either  in  a  hem  or  in  curtain  rings. 
From  the  corners  of  the  sheet  should  be  strong  cords  by  which  the  sheet  can  be 
stretched  out  smooth  and  held  in  position  by  passing  the  cords  through  screw 
eyes  or  attaching  them  to  other  fixed  supports. 

It  is  well  also  to  have  rings  along  all  the  edges  to  attach  strings  to,  to  pull 
the  edges  taut,  and  to  support  the  curtain  at  the  upper  edge. 

For  temporary  use,  a  sheet  may  be  stretched  and  held  in  position  by  tying 
strings  to  the  corners  and  by  fastening  the  strings  along  the  edges  by  safety 
pins. 


CH.  XII] 


SCREENS  WITH  METALLIC  FACING 


459 


image  much  more  brilliantly  illuminated  than  those  at  the  side. 
Sir  David  Brewster  in  1832  advocated  and  used  the  bright  metallic 
surface  on  the  back  of  looking  glasses,  which  at  that  time  was 
composed  of  mercury  and  tin.  Later,  surfaces  covered  with 
silver-leaf,  silver  particles  or  particles  of  aluminum  have  been  tried. 
Last  of  all,  plate  glass  has  been  ground  on  one  side,  and  the  smooth 
side  silvered.  The  ground  surface  of  the  glass  is  turned  toward  the 
projection  apparatus  and  facing  the  spectators  who  get  the  image 
reflected  from  the  mat  surface  of  the  glass  and  transmitted  from 
the  mirror  through  the  mat  (§  62ga). 


FIG.  246.     DISTRIBUTION  OF  LIGHT 
REFLECTED  FROM  A  WHITE 

SCREEN. 

It  is  approximately  uniform  through- 
out the  entire  hemisphere. 


FIG.  247.     DISTRIBUTION  OF  LIGHT 
FROM  A  SEMI-DIFFUSELY  RE- 
FLECTING SCREEN. 

The  closeness  of  the  arrows  indicates 

the  apparent  brightness  as  seen 

from  different  directions. 


§  630.  Suitability  of  metallic  screens. — Metallic  screens  are 
not  suitable  for  micro-projection,  or,  indeed,  for  any  projection  if 
fine  details  are  to  be  studied  close  to  the  screen,  but  details  which 
can  be  seen  at  a  distance  of  2  to  3  meters  are  very  well  brought 
out  on  the  mirror  screen,  and  other  metallic  screens.  In  com- 
paring a  mirror  screen,  an  aluminum  bronze  screen  and  one  of 
plaster  of  Paris  or  cardboard  if  the  image  was  observed  within 
the  narrow  angle  of  15  degrees  to  the  right  or  left  of  the  axis,  30 
in  all,  the  mirror  screen  was  brightest,  the  aluminum  next,  and 
finally  the  plaster  of  Paris  or  cardboard,  the  screens  being  in  the 
field  at  the  same  time  so  that  the  comparison  was  under  identical 

§  62  9a.  The  authors  wish  to  acknowledge  their  indebtedness  to  The  Motion 
Picture  Screen  Company  of  Shelbyville,  Indiana,  U.  S.  A.,  for  their  courtesy  in 
sending  a  sample  of  their  "Mirror  Screen"  for  experiment;  to  the  Bausch  & 
Lomb  Optical  Company  for  the  loan  of  the  two  metallic  screens  of  Zeiss;  to 
the  J.  H.  Gentner  Company  of  Newburgh,  N.  Y.,  for  samples  of  Mirroroide; 
and  to  other  screen  manufacturers  for  courteous  answers  to  inquiries. 


460 


BRIGHTNESS  OF  SCREENS 


[On.  XII 


CH.  XII]  TRANSLUCENT  SCREENS  461 

On  the  curved  surface  of  the  diagram  are  given  the  degrees  of  inclination  of 
the  light.  On  the  diameter,  and  on  the  radius  at  right  angles  to  the  diameter 
are  given  the  percentage  of  apparent  brightness.  Magnesium  oxide  is  taken 
as  the  standard  and  called  100%. 

The  data  shown  on  the  diagram  are  given  in  figures  in  the  table,  §  632. 

Curve  i.  Screen  coated  with  magnesium  oxide.  It  is  to  be  noted  that  it  is 
only  in  the  central  region  that  the  full  100%  of  reflection  occurs. 

Points  2222     Plaster  of  Paris  screen. 

Curve  3     Cardboard  screen. 

Points  4  4     Screen  painted  with  white  lead. 

Points  5  5     Screen  painted  with  Artists'  Scenic  White. 

Points  6  6     Screen  painted  with  zinc  white. 

Curve  7     Cardboard  screen  painted  with  aluminum. 

Curve  8     Zeiss  metallic  screen. 

For  9  see  the  table,  §  632. 

For  the  Mirror  Screen,  see  the  table,  §  632. 

Points  10  iv  10  Reflection  and  transmission  of  a  white  muslin  screen. 
Note  its  uniformity  (§  632). 

Points  ii  ii  ii  Reflection  and  transmission  of  white  gauze  (Griswoldville 
gauze,  No.  10).  With  this  screen  more  light  is  transmitted  than  reflected. 

Point  12     Transmission  of  ground-glass. 

Point  13     Reflection  of  bristolboard. 

conditions.     At  an  angle  of  30  degrees  and  upward  the  metallic 
screens  appeared  almost  black,  and  the  white  screens  pure  white. 

§  631.  Translucent  screens. — For  the  old  phantasmagoria  and 
for  many  appearances  given  by  shadow  pictures  it  is  necessary  to 
have  a  translucent  screen  like  ground-glass  or  translucent  cloth 
or  paper.  The  paper  or  cloth  is  rendered  as  translucent  as  desired 
by  the  use  of  water,  water  and  glycerine,  or  oil.  Tracing  cloth 
makes  good  translucent  screens  of  moderate  size. 

With  a  translucent  screen  the  apparatus  is  entirely  out  of  sight 
behind  the  screen  and  only  the  picture  shining  through  the  screen 
is  seen  by  the  audience.  This  is  not  so  good  and  effective  a  method 
of  showing  projection  images  as  the  opaque  white  screen  or  the 
metallic  screen,  for  much  more  light  is  lost  (fig.  248).  It  is  still 
used  in  some  institutions,  as  it  entirely  eliminates  the  projection 
apparatus  and  the  operator  from  the  auditorium  (§  631  a). 

The  ground-glass  screen  is  excellent,  but  this,  like  a  metallic 
screen  restricts  the  brilliant  image  to  a  rather  narrow  angle  (see 
§  630,  63  2  and  fig.  250) .  The  ground  surface  should  be  fine  or  there 
is  given  the  appearance  of  looking  toward  a  bright  light  in  a  snow 
storm,  this  is  especially  marked  if  one  is  near  the  ground-glass  and 
looking  nearly  along  the  axis. 


462  REFLECTION  OF    DIFFERENT  SCREENS  [Cn.  XII 

The  cloth  screens  were  not  so  satisfactory  as  the  ground-glass 
because  the  crossing  threads  make  a  kind  of  grating  and  one  sees 
diffraction  images ;  and  if  one  is  in  direct  line  with  the  arc  lamp 
the  cloth  acts  almost  as  if  it  were  transparent.  The  translucent 
mercerized  paper  used  in  making  tracings  is  practically  as  good  as 
ground-glass,  but  it  is  difficult  to  hold  it  smooth  and  even.  The 
tracing  cloth  used  by  architects  and  engineers  is  good  for  a  trans- 
lucent screen. 

There  is  a  practical  difficulty  with  all  translucent  screens.  On 
account  of  the  poor  reflection,  the  operator  cannot  tell  with  the 
same  certainty  when  the  image  is  in  focus  as  with  a  white,  opaque 
screen. 

§  632.  Table  of  the  reflection  of  different  screens  compared 
with  magnesium  oxide. 

No.  At  15°  At  45°          At  60° 

1  Magnesium  Oxide 100  83 

2  Plaster  of  Paris .95.6         88.7          78 

3  Cardboard 84.5  67 

4  White  Lead 88.5  79.4 

5  Century  Company *s  White 89.4  81 

6  Zinc  Paint 84.4  76.5 

7  Aluminum  Paint  on  Card 210  18 

8  Zeiss  Metallic  Screen,  smooth  ....  136  14 

9  Mirror  Screen 200 

10     White  Muslin,  Reflection 73.4  69.1          66.9 

Transmission 39  30 

,11     Gauze,  Reflection    33  27 

Transmission 35 

12  Ground-Glass,  Transmission 300  14.2 

13  Bristolboard,  Reflection    91.5 


§  631a.  For  example,  in  the  anatomical  institute  at  Munich.  Here  all  the 
projection,  whether  with  the  magic  lantern,  the  projection  microscope  or  the 
opaque  lantern,  is  upon  a  translucent  screen;  also  in  some  of  the  lecture  rooms 
in  Holland. 


CH.  XII] 


BRIGHTNESS  OF  SCREENS 


463 


?  o 


FIG.  249.     DISTRIBUTION  OF  APPARENT  BRIGHTNESS  WITH  DIFFERENT 
SCREENS    WHEN   VIEWED   FROM    DIFFERENT    DIRECTIONS. 

On  the  sides  and  curved  surface  are  given  the  degrees  of  inclination  of  the 
reflected  light. 

The  numbers  along  the  central  radius  indicate  the  relative  brightness  of  each 
screen,  magnesium  oxide  being  used  as  the  standard  and  called  100%. 

A  Magnesium  oxide  screen.  It  gives  the  standard  brightness  of  1 00%  and 
reflects  nearly  equally  throughout  the  entire  180  degrees. 

B     White  cardboard. 

C  Screen  with  aluminum  bronze  facing.  This  gives  3.2  times  the  bright- 
ness of  magnesium  oxide  in  the  center,  but  it  falls  off  rapidly  at  the  sides. 

E  Mirror  screen.  This  gives  7.1  times  the  brightness  of  magnesium  oxide 
in  the  center. 

It  is  to  be  noted  in  general  that  the  mirror  screens,  (C.  E.}  give  great  inten- 
sity when  seen  near  the  center,  and  that  this  intense  light  is  restricted  to  an 
angle  of  about  25  degrees.  Farther  to  the  side  the  light  falls  off  rapidly, 
being  in  marked  contrast  with  the  white  screens  (A  B}. 


464  SIZE  OF  SCREENS  CH.  XII] 

SIZE  OF  SCREENS  AND  SCREEN  IMAGES 

§  633.  The  size  of  screen  images  which  will  give  the  best 
results  in  a  given  case  can  only  be  determined  by  trial.  The  size 
should  be  great  enough  so  that  the  people  sitting  on  the  back  seats 
can  see  all  the  details  to  be  shown  and  still  not  so  large  that  those 
sitting  near  the  front  will  be  repelled  by  the  coarseness  of  the  image. 

As  a  result  of  experiments  to  determine  the  best  size  of  screen 
picture  for  the  average  seat  in  a  room  the  following  general  rules 
have  been  worked  out  so : — 

§  634.  Size  of  the  screen  for  lantern  slides. — The  screen  image 
must  be  large  enough  so  that  details  are  visible  to  the  most  distant 
spectator.  For  example,  in  teaching  work  and  in  demonstrations 
at  scientific  meetings,  etc.,  lantern  slides  often  contain  tables  of 
figures  and  printed  sentences.  Naturally,  the  farthest  sitter 
should  be  able  to  see  the  figures  and  to  read  the  words  easily. 

This  could  not  be  done  by  those  on  the  back  seats  if  the  letters 
were  much  smaller  than  six  point.  Of  course,  if  the  letters  on  the 
slide  are  as  large  as  eight  or  ten  point  type  (fig.  216),  they  can  be 
read  at  a  glance. 

In  long,  narrow  rooms  the  magnification  necessary  to  enable  the 
people  on  the  back  seats  to  see  the  details  well  will  make  every- 
thing gigantic  for  those  .sitting  near  the  screen. 

For  a  well  ai ranged  auditorium,  if  the  letters  and  numerals  on  the  slide  are  of  the  size  of 
6  point  type,  such  as  shown  in  this  sentence,  and  the  screen  image  is  from  one-fourth  to  one- 
fifth  as  wide  as  the  distance  from  the  farthest  seat  in  the  room  to  vthe  screen,  all  in  the 
audience  should  be  able  to  read  the  print  on  the  lantern  slide  with  ease. 

§  635  Projection  objectives  necessary  to  give  the  proper 
screen  image  with  the  magic  lantern. — If  the  lantern  can  be  at  the 
extreme  rear  of  the  room,  and  the  image  of  the  slide  is  to  be  one- 
fourth  or  one-fifth  as  wide  as  the  room  is  long,  as  stated  above 
(§  634),  a  projection  objective  of  30  cm.  (12  in.)  focus  will  give  the 
desired  screen  image  for  a  properly  made  lantern  slide,  no  matter 
what  the  size  of  the  room.  This  is  because  the  30  cm.  objective 
gives  an  image  on  the  screen,  regardless  of  its  distance,  which  will 
appear  to  the  observer  standing  by  the  lantern,  like  the  same  lan- 
tern slide  held  30  cm.  (12  in.)  in  front  of  the  observer's  eyes.  If 
the  lantern  slide  is  well  made  and  properly  proportioned  all  the 


CH.  XII] 


BRIGHTNESS  OF  SCREENS 


465 


details  should  be  plainly  visible  when  the  slide  is  30  cm.  in  front  of 
the  eyes,  and  therefore  are  plainly  visible  in  the  screen  image  as 
far  back  as  the  lantern. 


FIG.  250.     DISTRIBUTION  OF  BRIGHTNESS  OF  TRANSLUCENT  AND  REFLECT- 
ING SCREENS  WHEN  SEEN  FROM  DIFFERENT  DIRECTIONS 

Reflected  Light  from  Magnesium  Oxide  and  Mirror  Screen. 
Light  Transmitted  through  Ground  Glass. 

Note  that  the  mirror  screen  when  seen  perpendicularly  reflects  7  times 
as  much  light  as  does  magnesium  oxide,  and  ground-glass  19  times  as  much. 

But  this  great  brightness  of  the  mirror  screen  and  the  ground-glass  is 
limited  to  a  very  narrow  angle,  while  the  white  magnesium  oxide  reflects 
nearly  equally  throughout  the  entire  hemisphere. 

Brightness  of  MgO  is  taken  as  unity  and  the  figures  on  the  radius  indicate 
the  number  of  times  brighter  the  screen  appears  than  this. 


466 


SIZE  OF  SCREENS 


[On.  XII 


If  the  letters  and  numerals  and  other  details  on  the  slide  are  too 
small  to  be  seen  by  the  normal  eye  when  held  30  cm.  (12  in.)  away, 
then  they  will  not  show  clearly  in  the  screen  image  with  this  objec- 
tive at  the  back  of  the  room,  although  they  may  be  plainly  visible 
to  those  near  the  screen. 

As  the  lantern  is  frequently  not  quite  at  the  extreme  back  of  the 
room,  an  objective  of  25  cm.  (10  in.)  focus  is  more  commonly  used 
than  the  one  of  30  cm.  (12  in.).  It  makes  the  image  somewhat 
larger,  and  for  many  people  is  more  satisfactory. 

§  636.  Objective  to  use  when  the  lantern  is  not  at  the  back  of 
the  room. — Regardless  of  the  position  of  the  lantern  a  screen  image 
must  be  large  enough  for  all  in  the  room  to  see  the  details  as  stated 
above  (§  633-634). 


FIG.  251. 


SIZES  OF  SCREENS  NECESSARY  FOR  DIFFERENT 
SCREEN  DISTANCES. 


This  shows  that  the  same  object  and  objective  will  give  a  screen  image  of  a 
size  directly  proportional  to  the  screen  distance. 


CH.  XII]  SIZE  OF  SCREENS  467 

If  the  lantern  cannot  be  at  the  back  of  the  room,  but  must  be 
closer  to  the  screen,  then  the  projection  objective  must  be  of  shorter 
focus  than  25  to  30  cm.  (10-12  in.). 

To  determine  the  proper  objective  to  use  to  give  the  desired  size 
of  image  in  any  case  one  must  proceed  as  follows : 

(1)  The  size  of  screen  image  is  decided  on  by  remembering  that 
it  should  be  between  one-fourth  and  one-fifth  the  distance  to  the 
farthest  seat  in  the  room. 

(2)  The   distance  from  the  screen  to  the  lantern  must  be 
measured. 

(3)  Following  the  simple  optical  law  founded  on  the  geometry 
of  similar  triangles  that:     "The  size  of  object  and  image  vary 
directly  as  their  distance  from  the  center  of  the  objective,"  one 
can  by  simple  proportion  get  the  focus  which  the  objective  should 
have  for  a  given  screen  image. 

§  637.  Examples. — For  example,  suppose  the  distance  from  the 
screen  to  the  farthest  seat  is  20  meters  (66  ft.),  the  width  of  the 
screen  should  be  not  less  than  one-fourth  this  distance,  i.  e.,  five 
meters  (16.5  ft.). 

Now  suppose  that  instead  of  the  lantern  being  20  meters  from 
the  screen  it  is  only  n  meters  (36  ft.)  from  it,  what  should  be  the 
focus  of  the  projection  objective  to  give  a  screen  image  5  meters 
(16.5  ft.)  wide? 

The  formula  best  adapted  for  this  calculation  is: 

JL     A 
o        i 

where  f  is  the  distance  of  the  object  from  the  center  of  the  objective 
(focus  of  the  objective). 

0  is  the  size  of  the  object. 

d    is  the  distance  from  the  objective  to  the  screen. 

1  is  the  size  of  the  screen  image. 

It  is  assumed  in  all  the  calculations  for  the  magic  lantern  that 
the  width  of  the  lantern-slide  opening  or  picture  is  7.5  cm.  or  3 
inches. 


468  SIZE  OF  SCREENS  [Cn.  XII 

In  the  above  example 
f     is  unknown. 

0  the  size  of  the  object  is  7.5  cm.  3  in. 
d     the  distance  of  the  screen  image  is     n  meters         36ft. 

1  the  size  of  the  screen  image  is  5  meters          16.5  ft. 
Substituting  the  values  in  the  formula  we  have,  for  metric  values, 

f        ii          .       7.5x11 
-=--orf  =  L        -=16.5  cm.,  focus  of  the  objective. 

7-5        5  5 

For  English  values 

-  =  -3-    or  f  =  - — —  =  6.5  in.,  focus  of  the  objective. 
3       16.5  16.5 

§  638.     Size  of  screen  image  for  moving   pictures. — As  the 

scenes  depicted  by  the  moving  picture  are  so  largely  of  human 
action,  and  thus  resemble  a  theater  play,  one  would  think  that  the 
standard  should  be  the  representation  of  people  in  their  natural 
size.  The  fact  is,  however,  that  in  most  picture  theaters  the  people 
represented  are  of  heroic  or  semi-heroic  size,  being  from  i^  to 
two  times  the  natural  size  of  ordinary  people. 

The  large  size  of  the  moving  picture  on  the  screen  has  come  about 
naturally,  as  the  details  of  movement  and  the  facial  expression  of 

§  636a-637a.  In  the  formula  here  given  it  is  assumed  that  the  objective  will 
always  be  at  its  principal  focal  distance  from  the  object  regardless  of  the  screen 
distance.  This  is  not  strictly  true,  but  as  the  screen  distance  is  so  great  rela- 
tively to  the  distance  of  the  objective  from  the  object,  the  slight  error  involved 
in  the  above  assumption  is  negligible.  If  the  screen  distance  and  the  principal 
focal  distance  were  more  nearly  the  same,  the  error  would  be  altogether  too 
great  to  be  neglected  (see  fig.  210). 

It  follows,  naturally  also,  from  this  formula  that,  if  any  three  of  the  elements 
are  given,  the  fourth  can  be  found.  Ordinarily,  it  is  the  proper  focus  of  the 
objective  to  use  that  is  unknown,  but  any  one  of  the  elements  might  be  desired, 
and  it  can  be  found  if  one  knows  three  of  them. 

As  it  is  the  focal  length  of  the  objective  that  is  most  often  required,  the 
following  may  be  of  assistance;  it  simply  states  in  words  what  the  formula 
shows : 

To  find  the  focal  length  of  the  objective  needed,  the  screen  distance  and  the 
size  of  the  screen  image  being  known :  Multiply  the  screen  distance  in  meters 
by  7.5,  and  divide  the  product  by  the  size  of  the  screen  image  in  meters  and  the 
result  will  give  the  focus  of  the  objective  in  centimeters.  For  English  measure : 
Multiply  the  screen  distance  in  feet  by  3  and  divide  the  product  by  the  size  of 
the  screen  image  in  feet,  and  the  result  will  give  the  focus  of  the  objective  in 
inches. 


CH.  XII]  ^SIZE  OF  SCREENS  469 

the  actors  could  not  be  seen  if  they  were  only  of  the  size  of  average 
human  beings.  On  the  theater  stage  the  action  is  made  more 
intelligible  by  the  spoken  words;  but  where  there  is  only  pan- 
tomine  one  must  see  the  details  of  the  action  and  the  facial  expres- 
sion to  make  the  play  fully  intelligible. 

To  enable  those  seated  in  the  extreme  rear  seats  to  see  the  action 
on  the  screen  without  getting  the  picture  too  large  for  those  on  the 
front  seats,  the  width  of  the  picture  should  be  between  Y(>  to  %  of 
the  distance  of  the  farthest  seat  to  the  screen.  The  width  of 
^6  is  on  the  whole  the  most  satisfactory  if  the  end  of  the  room  is 
large  enough  to  permit  a  screen  of  this  size. 

§  639.  The  size  of  the  screen  limited  by  the  room. — It  some- 
times happens  that  the  size  of  the  screen  which  can  be  used  is 
limited  by  the  size  of  the  wall  on  which  it  can  be  placed.  The  size 
of  the  screen  may  also  be  limited  by  the  height  of  the  ceiling  above, 
and  the  heads  of  the  spectators  below.  This  is  true  of  some  lecture 
halls,  and  of  many  of  the  moving  picture  theaters  which  are  re- 
modeled store  buildings.  If  the  screen  image  is  limited  in  size  by 
any  of  these  factors  thus  requiring  a  smaller  picture  than  that  hav- 
ing a  width  of  ^th  the  distance  from  the  screen  to  the  farthest  seat 
for  the  magic  lantern  or  ^6th  the  distance  for  a  moving  picture, 
it  is  necessary  to  use  an  objective  of  longer  focus  accordingly. 

If  the  width  is  limited  and  one  can  use  any  height  desired,  the 
calculation  is  made  exactly  as  in  the  previous  section. 

If  the  height  is  limited,  then  the  calculation  is  made  in  the  same 
way  except  that  the  height  of  the  object  instead  of  its  width  is 
taken;  that  is,  for  lantern  slides  the  extreme  opening  of  the  mat  is 
taken  as  7  cm.  (2^  in.),  or  for  moving  pictures  23.08  mm.  long, 
17.3  mm.  high,  2%2  in.  X  87/i28  in.  (see  §  57oa). 

For  example,  in  a  university  lecture  room  the  greatest  height  of 
the  screen  which  could  be  used  was  2.9  meters  (9.5  ft.),  and  the 
room  was  14.3  meters  (47  ft.)  long.  The  question  was:  What 
focus  of  objective  would  give  this  size  of  screen  image  with  the 
lantern  at  the  back  of  the  room? 


470  SIZE  OF  SCREENS  [Cn.  XII 

In  this  example 

f    is  unknown,  (i.  e.,  the  focus  of  the  objective). 

0  the  height  of  the  object  is  7  cm.  (2^  in.). 

d     the  distance  of  the  screen  is  14.3  meters  (47  ft.). 

1  the  size  of  the  image  is  2.9  meters  (9.5  ft.). 
Substituting  the  values  in  the  formula  we  have : 
For  metric  values : 

f        14  i  7  x  14  * 

-  =  — -  or  f  =  —   —2-  =  34.5  cm.,  focus  of  the  objective. 

7         2.9  2.9 

For  English  values : 

-  =  —  or  f  =  -^ =  13. 6  in.,  focus  of  the  objective. 

2-75       9-5  9-5 

An  objective  of  13.6  in.  or  34.5  cm.  would  have  to  be  specially 
constructed.  Those  on  the  market  and  easily  procurable  were  of 
30  cm.  (12  in.)  and  38  cm.  (15  in.).  The  shorter  focus  objective 
gave  considerably  too  large  a  screen  image  and  could  not  be  used, 
therefore  the  one  of  longer  focus  was  taken,  and  a  correspondingly 
long  focus  condensing  lens  used. 

Second  example.  In  another  lecture  room  the  lantern  must  be 
16.75  meters  from  the  screen  and  the  screen  could  not  exceed  3.35 
meters  in  width,  what  should  be  the  focus  of  the  objective  and  the 
second  element  of  the  condenser  to  meet  these  conditions? 

Applying  the  formula : 

-  =  — - —  whence  f  =  37.5  cm.  the  focus  of  the  objective  needed. 
7-5  3-35 

In  English  measure : 

—  =  —  whence  f  =  1 5  in.  That  is,  a  15  inch  objective  is  demanded. 
3  ii 

§  640.     Size  of  screen  and  screen  images  for  micro-projection. 

—Here  the  law  holds,  that  to  be  satisfactory,  the  details  to  b-e 
shown  must  be  large  enough  so  that  they  can  be  seen  with  ease. 
The  microscopic  specimens  vary  so  greatly  in  character  that  no 


CH.  XII]          TROUBLES  WITH  ROOMS  AND  SCREENS  471 

general  rule  can  be  given  for  the  size  of  screen  necessary.  For 
large  halls  the  screen  used  for  the  magic  lantern  usually  answers. 
In  small  rooms  for  special  demonstrations  it  is  advantageous  to 
have  a  movable  screen  on  a  stand  that  can  be  varied  in  distance  for 
different  conditions.  The  magnification  and  the  objective  neces- 
sary for  the  same  must  be  determined  in  each  case  by  the  lecturer 
before  the  lecture  or  demonstration  (see  §  400,  Ch.  IX.) 

§  641.    Troubles  with  Rooms  and  Screens : 

1.  Poor  image  on  the  screen.     This  may  be  due  to 

(a)  Insufficient  light  from  the  radiant; 

(b)  Too  much  light  in  the  room; 

(c)  A  poor  screen — dirty  or  thin; 

(d)  If  an  approximately  square    room   is   used,   then  the 
mirror  and  other  metallic  screens  will  appear  very  dark  and  un- 
satisfactory for  the  spectators  outside  of  an  angle  of  greater  than 
15  to  20  degrees  from  the  axis,  and  the  farther  outside  the  15  de- 
gree position  the  darker  will  appear  the  screen  image  (fig.  247). 

(e)     The  objective  and  second  element  of  the  condenser  may  be 
improperly  proportioned,  i.  e.,  focal  lengths  too  different  (§  89-90). 

2.  Oppressive  in  the  room.     Too  little  fresh  air. 

3.  Room  lights  shining  in  the  eyes  of  the  spectators.     Not 
properly  placed  or  shaded. 

4.  Distorted  image.     The  screen  and  the  axial  ray  from  the 
projection  apparatus  not  at  right  angles. 

5.  The  details  of  the  picture  not  visible  for  the  spectators  on  the 
back  seats.     The  objective  is  of  too  long  a  focus  and  it  does  not 
magnify  enough.     Use  a  shorter  focus  objective. 

6.  The  screen  picture  altogether  too  large.     Too  short  a  focus 
objective;   use  one  of  longer  focus,  and  adapt  the  condenser  to  it. 

7.  There  is  a  glare  in  the  room  from  the  ceiling  or  walls  or  both. 
The  paint  used  in  finishing  is  shiny,  not  dull  and  flat.     Use  more 
turpentine  and  less  oil  in  the  paint. 

8.  The  room  too  dark.     Use  more  room  lights  properly  placed 
and  shaded. 


472 


DO  AND  DO  NOT  WITH  ROOMS  AND  SCREENS     [Cn.  XII 


§  642.     Summary  of  Chapter 

Do 

1.  Use     a     room     properly 
equipped  for  projection  if  good 
results  are  expected  (§  602). 

2.  Use   light-absorbing   tints 
for  tinting  and  decorating  the 
projection  room  (§  604). 


3.  Make   all   paints   dull   or 
flat,  never  shiny,  for  a  projec- 
tion room. 

4.  Light  the  projection  room 
sufficiently,  so  that  the  specta- 
tors can  find  their  seats  without 
trouble  (§  605). 

5.  A  perfectly  darkened  room 
is    only    necessary    for    special 
projection  (§  608). 

6.  Lamps  for  general  lighting 
should  be  shaded  or  so  arranged 
that   their   light   cannot   shine 
directly  in  the  eyes  of  the  spec- 
tators or  upon  the  image  screen 
(§606). 

7.  Have  red  lights  near  all 
exits  (§  607). 

8.  Take   the   necessary   pre- 
cautions to  prevent  light  enter- 
ing the  room  at  the  edges  of  the 
window  shades  (§  611). 


XII: 


Do  NOT 


1.  Do  not  expect  good  pro- 
jection in  a  room  not  equipped 
for  it. 

2.  Do  not  use  light-reflecting 
colors   like   yellow,  white,  light 
red   or    green    for    decorating 
the  projection  room;    but  the 
dark,  rich,  light-absorbing  col- 
ors, dark  red,  brown,  etc. 

3 .  Do  not  use  paint  that  gives 
a  shiny  or  enamel  surface,  for 
this  will  produce  a  glare  by  the 
reflections.     Use  flat  paint. 

4.  Do  not  have  the  projection 
room  darker  than  necessary. 


5.  Do  not  attempt  the  most 
difficult   projection   unless   the 
room    can   be   made   perfectly 
black. 

6.  Do     not     use     unshaded 
lamps  for  the  general  lighting. 
Light   shining   directly   in   the 
eyes  of  the  spectators  is  very 
distressing. 

7.  Do  not  fail  to  have  red 
lights  by  the  exits. 

8.  Do  not  leave  the  window 
shades  without  protection  at  the 
margins. 


CH.  Xlli      DO  AND  DO  NOT  WITH  ROOMS  AND  SCREENS          473 


9.  If  possible  place  the  pro- 
jection apparatus  at  the  back  of 
the  room  (§612). 

i  o .  The  axial  ray  of  the  image 
beam  should  strike  the  screen  at 
right  angles  (§614). 

1 1 .  Incline  the  screen  if  neces- 
sary (§  614). 

12.  Light  the  black-board  by 
lights  behind  a  curved,  metal 
shield  (fig.  240). 

13.  If  a  translucent  screen  is 
used  the  objects  must  be  put 
into  the  apparatus  with  the  pic- 
ture facing  the  objective  (§516). 

14.  Use  a  good  screen  (§  621). 

15.  If  you  use  a  mirror  or 
metalliic  screen  remember  that 
it     does    not     reflect     equally 
throughout    the     180     degrees 
(§  629). 

1 6.  Make   the   screen  image 
large  enough  so  that  the  most 
distant   spectator  can  see   the 
details  (§633,  639). 


9.  Do  not  place  the  projec- 
tion apparatus  in  the  middle  of 
the  room  if  it  can  be  avoided. 

10.  Do  not  let  the  axial  ray 
strike  the  screen  obliquely. 

1 1 .  Do  not  incline  the  screen 
unless  necessary. 

12.  Do   not   try  to  use   the 
blackboard   without  lighting  it 
by  hidden  lights. 

13.  Do  not  forget  the  rules 
for  erect  images  when  using  a 
translucent  screen. 

14.  Do  not  use  a  dirty  screen. 
Wash  it  or  give  it  a  fresh  coat  of 
white  paint. 

15.  Do  not  use  a  mirror  or 
metal  faced  screen  in  a  square 
room ;  such  screens  are  not  good 
for  the  fine  details  necessary  in 
micro-projection  (§  360). 

1 6.  Do  not  have  the  screen 
image  too  small  nor  too  large. 


CHAPTER  XIII 

ELECTRIC     CURRENTS     AND     THEIR     MEASUREMENT; 
ARC    LAMPS,    THEIR    WIRING    AND    CONTROL; 
CANDLE-POWER   OF   ARC   LAMPS   FOR 
PROJECTION 

§  650.    Apparatus  and  Material  for  Chapter  XIII: 

Direct  and  alternating  current ;  Voltmeter,  ammeter  and  watt- 
meter for  direct  and  alternating  currents ;  Shunt  generator,  motor- 
generator  set,  mercury  arc  rectifier ;  Polarity  indicators ;  Arc  lamp ; 
Rheostat,  inductor,  transformer  and  other  ballast;  Carbons; 
Water-cell;  Insulated  wires,  flexible  cables,  insulators,  switches, 
separable  plugs,  caps,  etc.;  Fuses  and  circuit  breakers;  Wire,  iron 
plates,  etc.,  for  home-made  rheostats. 

§  651.  Historical  development  of  electric  lighting. — See  the 
Appendix. 

ELECTRIC  CURRENTS:     KINDS  AND  COMPARISON 

§  652.  Direct  current. — The  earliest  electric  currents  studied 
and  experimented  with  were  produced  by  the  voltaic  cell.  These 
currents  have  a  constant  polarity,  always  flowing  in  the  same 
direction. 

A  direct  electric  current  may  be  produced  by  a  voltaic  battery, 
or  by  a  dynamo  (§  652a). 

The  first  installations  of  electric  plants  were  all  for  direct  current, 
now  they  are  mostly  for  alternating  current  (see  below). 

The  principal  use  of  direct  current  at  present  is  for  trolley  cars, 
and  other  apparatus  where  variable  speed  motors  are  necessary; 
in  electrolysis  such  as  charging  storage  batteries  and  the  decomposi- 
tion of  chemical  compounds;  for  electric  lighting  in  some  of  the 
more  densely  populated  cities  and  for  projection  purposes. 

§  652a.  Generator,  Dynamo. — Generator  is  a  comprehensive  term  in- 
cluding all  means  of  producing  electric  currents  whether  these  means  be 
chemical  or  mechanical. 

Dynamo,  on  the  other  hand,  is  a  special  term  denoting  a  generator  by  which 
mechanical  is  transformed  into  electrical  energy;  for  example,  a  steam  engine 
may  give  motion  to  the  dynamo  and  thus  produce  an  electric  current.  In  a 
word,  a  dynamo  is  a  generator  of  electricity,  but  all  generators  of  electricity  are 
not  dynamos. 

474 


CH.  XIII]  ELECTRIC  CURRENTS  475 

§  653.  Alternating  current. — This  is  characterized  by  flowing 
first  in  one  direction  and  then  in  the  opposite  direction;  the  polarity 
is  therefore  constantly  changing.  (See  §  676). 

Alternating  current  is  pro- 
duced only  by  dynamos.  It  is 
used  especially  for  the  trans- 
mission of  power  to  great  dis- 
tances, incandescent  lighting, 
arc  lighting,  for  motors  and  for 

FIG.   252.      CONNECTIONS   OF    A       the  electric  furnace,  as  in  the 
VOLTMETER  TO  MEASURE  THE 

LINE  VOLTAGE.  manufacture    of    carborundum 

G    Dynamo.  and  graphite. 

A    Ar^Lamp.  Alternating  current  has  the 

R    Rheostat.  advantage  of  being  more  easily 

Note  that  the  terminals  of  the  volt-  produced,  as  the  dynamo  is  sim- 
meter  are  connected  to  the  two  points  r  .  .  . 

between  which  it  is  desired  to  measure  pier;  but  its  great  superiority 
the  potential  difference.  In  this  case  lies  in  the  fact  that  practically 
it  is  the  main  supply  (across  the  line). 

without  loss  it  can  be  stepped 

up  or  down  in  voltage  by  stationary  transformers.  This  makes  it 
possible  to  raise  it  to  a  very  high  voltage  (1000  to  100,000  volts) 
for  transmission  to  a  distance  over  wires  of  moderate  size.  It  is 
then  stepped  down  in  voltage  before  it  is  used.  In  this  process  of 
stepping  up  or  down  in  voltage  the  amperage  takes  the  reverse 
direction,  so  that  the  product  of  the  volts  by  the  amperes  is  a 
constant  quantity. 

The  disadvantages  of  alternating  current  for  the  arc  lamp  are: 

1 .  The  arc  is  not  as  bright  as  with  the  same  amperage  of  direct 
current. 

2.  The  light  is  intermittent. 

3.  The  alternating  current  arc  is  noisy. 

ELECTRIC  UNITS  AND  THEIR  MEASUREMENT 

§  654.  Electric  Units. — For  the  purposes  of  this  book  it  is 
necessary  to  refer  frequently  to  electric  units,  like  the  volt,  the 
ampere,  the  ohm  and  the  watt;  it  seems  proper  therefore  to  give 
a  brief  discussion  of  these  units. 


476  DIRECT  CURRENT  UNITS  [Cn.  XIII 

DIRECT  CURRENT  UNITS 

§  655.  The  Volt.— This  is  the  unit  of  electromotive  force,  that 
is  the  electric  force  or  pressure  necessary  to  produce  one  ampere  of 
current  in  a  circuit  with  a  resistance  of  one  ohm. 

The  difference  of  potential  between  the  two  poles  of  a  Weston 
standard  cadmium  cell  is  1.019  volts.  The  ordinary  battery  used 
for  ringing  door  bells  has  approximately  one  volt  pressure. 

Voltage  is  a  general  term  representing  the  pressure  in  volts  in  an 
electric  circuit. 

If  the  difference  of  pressure  between  the  two  given  points  is 
great,  then  the  voltage  is  said  to  be  high;  if  the  difference  is  slight, 
then  the  voltage  is  low.  For  example,  in  projection  one  might  use 
55  volts  for  the  arc  lamp,  or  220  volts,  or  500  volts.  Ordinarily 
neither  the  low  voltage  of  55  nor  the  high  voltage  of  500  or  220 
is  used,  but  an  intermediate  voltage  of  no. 

§  656.  The  Ampere. — This  is  the  unit  of  current.  It  is  the 
current  which  will  deposit  .001118  gram  of  metallic  silver  per 
second  from  a  15%  solution  of  silver  nitrate  in  water.  It  is  the 
current  which  one  volt  will  maintain  in  a  circuit  with  one  ohm 
resistance  (see  below) . 

Amperage  is  the  term  by  which  is  designated  the  amount   of 

current  in  amperes  flowing  at 
any  given  moment.  If  a  large 
amount  of  current  is  flowing  the 
amperage  is  said  to  be  high  or 
great,  if  a  small  amount,  then  it 
is  said  to  be  low  or  small.  For 

example,  in  projection,  the  am- 

FIG.  253.     CONNECTIONS  OF   A  VOLT-  /  .  ., 

METER  TO  MEASURE  THE  ARC          perage  needed  for  drawing  with 
VOLTAGE.  ^he  microscope  on  the  house  cir- 

cuit  (§  493)   is  small  (3-6  am- 
A     Arc  lamp.  peres) ,  while  for  opaque    pro- 

*  Ahrf  ?T!'  *  i,  Jection  (§  289),  and  for  moving 

Note  that  the  terminals  of  the  volt-       .  /«  \    •  t_n 

meter  are  connected  to  the  two  points  pictures   (§    693)   in  large  halls 

between  which  it  is  desired  to  measure  the  amOunt  of  amperage  needed 

the  potential  difference.     In  this  case  . 

it  is  the  two  carbons  (across  the  arc).  IS  great  (20  to  100  amperes). 


CH.  XIII]  DIRECT  CURRENT  UNITS  477 

§  657.  The  Ohm. — This  is  the  unit  of  resistance  to  the  flow  of 
an  electric  current.  It  is  represented  by  the  resistance,  at  zero 
centigrade,  of  a  column  of  mercury  106.3  centimeters  long,  of  uni- 
form cross  sectional  area,  and  weighing  14.4521  grams.  Such  a 
column  of  mercury  will  have  a  cross  sectional  area  of  one  square 
mm. 

Ohmage  is  a  term  analogous  with  voltage  and  amperage.  It  is 
used  to  designate  the  amount  of  resistance  in  ohms  of  an  electric 
circuit. 

A  conductor  may  have  little  resistance,  as  copper,  etc.,  or  it  may 
have  great  resistance  like  German  silver.  Naturally  then  copper 
wire  is  used  largely  for  electric  circuits,  and  German  silver  wire  for 
making  resistors  or  rheostats  (§  724a). 

§  658.  The  Watt. — This  is  the  unit  of  activity  and  is  the  rate 
at  which  work  can  be  done  by  a  current  of  one  ampere  under  a 
pressure  of  one  volt.  One  watt  means  the  doing  of  work  at  the 
rate  of  io7  ergs  per  second,  or  one  joule  per  second.  This  is  approx- 
imately equal  to  the  lifting  of  i  kilogram,  io  centimeters  every 
second. 

§  659.  Kilowatt. — A  kilowatt  is  1,000  watts.  This  term  is 
more  common  than  watt.  It  is  equal  to  1.34  horse  power. 

§  660.    The  watts  which  any  direct   current  represents  are 

obtained  by  multiplying  the  quantity  of  current  flowing  by  the 
pressure — that  is,  the  amperes  by  the  volts.  Thus,  if  there  were 
an  amperage  of  one  and  a  voltage  of  one,  there  would  be  an 
activity  of  one  watt.  If  the  voltage  were  io  and  the  amperage  100, 
or  the  voltage  100  and  the  amperage  io,  there  would  be  an  activity 
of  1,000  watts,  or  one  kilowatt. 

§  661.  Kilowatt-hour. — This  is  the  unit  of  electrical  energy  or 
work,  which  is  in  commercial  use  and  which  is  used  as  a  basis  for 
making  the  charges  to  consumers.  A  kilowatt-hour  is  the  work 
represented  by  one  kilowatt  when  acting  for  one  hour. 

In  order  to  find  the  amount  of  work  done  by  an  electric  current 
it  is  necessary  not  only  to  know  the  rate  at  which  the  work  is  being 
done  but  also  the  time  during  which  this  rate  is  continued.  Thus, 


478  ELECTRIC  MEASUREMENTS  [Cn.  XIII 

take  the  example  of  an  arc  lamp  which  uses  20  amperes  direct 
current  from  a  no  volt  line.  The  line  then  supplies  20  x  no  = 
2,200  watts  or  2.2  kilowatts.  If  this  arc  were  used  for  only  a  few 
minutes,  the  energy  supplied  would  be  comparatively  small,  but  if 
the  arc  were  used  all  day,  the  energy  supplied  and  hence  the  coal 
or  other  fuel  consumed  in  generating  this  power  would  be  compara- 
tively large.  In  order  to  measure  this  energy,  the  power  measured 
in  kilowatts  is  multiplied  by  the  time  the  power  is  used.  In  the 
above  example,  if  the  arc  were  run  for  eight  hours  the  electrical 
energy  used  would  be  2. 2x8  =  17.6  kilowatt-hours. 

ELECTRIC    MEASUREMENTS:     VOLTMETERS,    AMMETERS,    WATT- 
METERS FOR  DIRECT  CURRENT 

§  662.  Voltmeter  for  direct  current. — This  is  an  instrument  for 
measuring  in  volts  the  difference  of  potential  between  two  points 
of  an  electric  circuit. 

The  voltmeter  must  be  adapted  to  the  kind  of  current — direct 
or  alternating — and  for  the  pressure,  low  voltage  or  high  voltage. 
It  consists  of  a  delicate  galvanometer  of  exactly  the  same  type  as 
that  for  an  ammeter,  but  it  has  a  high  resistance  in  series  with  it. 
This  high  resistance  allows  but  a  small  current  to  flow  through  the 
galvanometer;  and  this  small  current  is  proportional  to  the  differ- 
ence of  pressure  or  voltage  between  the  binding  posts  of  the  volt- 
meter, and  causes  the  needle  of  the  voltmeter  to  be  deflected. 
Numbers  on  the  dial  indicate  the  voltage  for  different  amounts  of 
the  deflection. 

§  663.  Connection  of  the  voltmeter  with  the  circuit  to  be 
measured. — One  pole  of  the  voltmeter  is  positive  and  one  negative. 
To  connect  the  instrument  with  the  circuit  for  determining  the 
voltage  between  two  points,  the  positive  binding  post  of  the  volt- 
meter is  connected  by  a  wire  to  the  positive  point  in  the  circuit,  and 
the  negative  binding  post  with  the  negative  point  in  the  circuit 
(fig.  272).  This  gives  the  full  electric  pressure  between  the  two 
points  connected  with  the  voltmeter,  although  only  a  very  small 
current  flows  through  it  on  account  of  its  high  resistance.  The 


CH.  XIII] 


ELECTRIC  MEASUREMENTS 


479 


main  current  continues  to  flow  in  the  electric  circuit  between  the 
two  points  exactly  as  though  the  voltmeter  were  not  in  use. 

The  voltmeter  should  not  be  connected  in  series  with  the  line  as 
with  the  ammeter  (§  665).  While  no  particular  harm  might  be 
done,  the  high  resistance  of  the  voltmeter  w^ould  allow  only  a  small 
current  to  flow  in  the  line  and  if  one  were  using  an  arc  lamp  it 
would  go  out  from  the  insufficient  current. 

If  one  does  not  know  the  direction  of  the  flow  in  the  circuit  to  be 
tested,  the  voltmeter  can  be  correctly  connected  by  trial  as  follows : 
Connect  the  positive  binding  post  of  the  voltmeter  by  a  wire  to  one 
of  the  points,  and  the  negative  binding  post  by  a  wire  to  the  other 
point.  Turn  on  the  current,  and  if  the  connection  is  right  the 
needle  of  the  instrument  will  point  to  the  voltage ;  if  the  connection 
is  wrong  the  needle  will  tend  to  be  deflected  off  the  scale  below  the 
zero  point.  If  it  is  wrong,  turn  off  the  current  and  reverse  the 
position  of  the  wires  in  the  binding  posts. 

§    664.     Ammeter.— This  is 

an  instrument  to  show  the 
amount  of  current  flowing 
through  a  given  circuit  at  a 
given  instant.  It  consists  of  a 
galvanometer  of  the  particular 
type  adapted  to  the  current 
used,  that  is,  direct  or  alter- 
nating current.  It  is  also 
adapted  to  the  amount  of  cur- 
rent to  be  measured,  that  is 
small  currents  and  large  cur- 
rents, say  from  o  to  10,  o  to  25, 
o  to  50,  o  to  100,  etc. 

The  galvanometer  part  of  the 
ammeter  is  a  delicate  instru- 
ment so  that  the  whole  current 
used  in  projection  is  not  sent 


FIG.  254.     CONNECTING  AN  AMMETER 

IN  THE  LINE  TO  MEASURE  THE 

CURRENT  FLOWING. 

a    Ammeter,  A ,  with  external  shunt, 
5. 


b  Ammeter  with  self-contained 
shunt.  The  shunt  in  this  type  is  in- 
side of  the  instrument  case. 

Note,    the    ammeter    is    connected 
through  it,  but  a  definite    frac-    along  one  wire  so  that  the  entire  cur- 
,  .     .  rent  flows  through  the  instrument  and 

tional  part  goes  through  it  and  its  shunt. 


480  ELECTRIC  MEASUREMENTS  [Cn.  XIII 

the  main  part  of  the  current  goes  through  a  special  wire,  known  as 
a  shunt  (fig.  254). 

In  some  ammeters  the  galvanometer,  and  the  shunt  are  in  the 
same  box  (self-contained  ammeters) ,  in  others  the  shunt  is  outside 
(fig.  254). 

When  an  electric  current  flows  through  the  ammeter,  the 
galvanometer  needle  is  deflected,  the  amount  of  the  deflection 
measuring  the  amount  of  the  current.  With  the  ammeters  used 
in  projection,  the  galvanometer  has  been  calibrated  so  that  the 
needle  points  to  the  number  of  amperes  of  current  flowing  in  a  given 
case  (fig.  145). 

§  665.     Connection  of  the  ammeter  with  the  projection  circuit— 

If  one  is  to  use  an  ammeter  in  an  electric  circuit,  the  instrument  is 
connected  with  the  line  in  series,  that  is  along  one  wire.  Further- 
more, it  is  necessary  to  connect  the  positive  pole  of  the  ammeter 
with  the  positive  end  of  the  wire,  and  the  negative  end  with  the 
negative  pole.  In  most  cases  when  installing  a  projection  outfit 
the  direction  of  the  current  flow  is  not  known,  and  the  proper 
connection  of  the  apparatus  is  found  by  trial  (see  §  702  for  the 
direct  current  arc  lamp). 

To  install  an  ammeter  cut  one  of  the  wires,  and  insert  one  cut  end 
in  the  positive,  and  the  other  cut  end  in  the  negative  binding  post 
of  the  ammeter.  Then  the  arc  lamp  and  the  rheostat  are  wired  as 
shown  in  fig.  270. 

Now  close  the  switch  and  cause  the  arc  lamp  to  burn.  If  the 
ammeter  is  correctly  connected,  the  needle  will  point  to  the  number 
of  amperes  of  current  flowing.  If  the  connection  is  wrong,  then 
the  needle  will  tend  to  move  off  the  scale  below  the  zero  mark.  In 
case  the  connection  is  wrong,  open  the  switch  and  reverse  the  posi- 
tions of  the  wires  in  the  binding  posts  of  the  ammeter.  When  the 
current  is  turned  on  again  the  needle  will  be  deflected  until  it 
points  to  the  number  of  amperes. 

By  looking  at  fig.  273  it  will  be  readily  seen  why  one  of  the  cut 
ends  of  the  same  wire  will  be  positive  and  why  one  will  be  negative. 
That  is,  if  the  whole  circuit  is  considered  from  the  dynamo  back  to 
the  dynamo,  it  will  be  seen  that  starting  from  the  positive  pole  of 


CH.  XIII]  ELECTRIC  MEASUREMENTS  481 

the  dynamo,  any  point  in  the  circuit  toward  or  nearer  this  positive 
pole  will  be  positive  in  comparison  with  any  other  point  nearer  the 
negative  pole.  Then  if  the  circuit  is  cut  at  any  point  the  end  of 
the  wire  next  the  positive  pole  of  the  dynamo  will  be  positive,  and 
the  end  nearer  the  negative  pole  of  the  dynamo  will  be  negative. 
Now  if  the  cut  end  of  the  wire  nearer  the  positive  pole  of  the  dynamo 
is  inserted  in  the  negative  binding  post  of  the  ammeter,  and  the 
other  end  in  the  other  binding  post,  the  needle  tends  to  be  deflected 
in  the  wrong  direction.  If  the  two  ends  of  the  wire  are  correctly 
connected  with  the  ammeter,  the  needle  will  be  deflected  in  the 
right  direction,  and  indicate  the  amperage. 

§  666.  Ammeter  for  projection. — In  projection  the  ammeter  is 
usually  all  that  is  required,  for  the  voltage  on  a  given  line  is  nearly 
constant,  and  can  be  found  easily  by  inquiring  of  the  central  station. 
On  the  other  hand,  the  required  amount  of  current  for  different 
purposes  varies  greatly  and  the  factors  in  the  production  of  a  good 
image  are  so  many  that  an  ammeter  to  show  at  a  glance  what 
amount  of  current  is  flowing  is  of  the  highest  importance,  for  with 
a  given  amount  of  current  the  operator  knows  at  once  what  kind 
of  a  light  can  be  reasonably  expected  in  the  different  cases.  If  the 
screen  light  is  not  good  with  the  adequate  amperage  for  the  purpose 
then  he  can  look  to  the  other  possible  causes  of  failure  (see  §  61-96) . 

If  one  is  to  be  able  to  determine  for  himself  all  the  electric  fac- 
tors in  projection  work,  then  a  voltmeter  and  a  wattmeter  should  be 
added  to  his  apparatus. 

§  667.  Precautions  for  the  ammeter. — In  connecting  the  am- 
meter be  sure  not  to  connect  the  ammeter  directly  to  both  line 
wires.  As  the  ammeter  has  very  little  resistance,  putting  it  across 
the  line  would  have  practically  the  same  effect  as  connecting  the 
two  points  with  a  heavy  wire,  that  is  a  short  circuit  would  be 
formed  and  the  fuses  would  be  blown.  Besides  the  very  heavy 
current  which  would  flow  momentarily  might  be  sufficient  to 
seriously  damage  the  delicate  instrument. 

§  668.  Safe  rules  for  the  beginner  to  follow  when  connecting 
instruments  may  be  stated  as  follows: 


482  ELECTRIC  MEASUREMENTS  [Cn.  XIII 

For  the  voltmeter. — After  all  the  connections  for  the  circuit  are 
complete,  connect  the  two  terminals  of  the  voltmeter  to  any  two 
parts  of  the  line  between  which  it  is  desired  to  measure  the  differ- 
ence of  potential. 

For  the  ammeter. — After  all  the  connections  for  the  circuit  are 
complete,  and  after  the  arc  has  been  found  to  work  satisfactorily, 
cut  one  of  the  wires  and  insert  the  self -containing  ammeter  in 
between  the  two  cut  ends  just  as  if  it  were  a  short  piece  of  heavy 
wire.  If  there  is  an  outside  shunt  connect  the  ends  of  the  supply 
wire  to  the  large  binding  posts  of  the  shunt  and  the  wires  of  the 
ammeter  to  the  smaller  binding  posts  of  the  shunt. 

§  669.  The  Wattmeter. — This  is  an  instrument  for  measuring 
electrical  power  or  activity.  There  are  two  types  of  wattmeter — 
the  portable  or  indicating  wattmeter  and  the  integrating  or  supply 
wattmeter.  Both  work  on  the  same  principle,  but  the  method  of 
indication  is  different. 

The  wattmeter  has  two  sets  of  terminals  or  binding  posts.  One 
set  is  connected  with  the  line  in  series — along  one  wire — like  an 
ammeter,  while  the  other  set  is  connected  in  multiple — that  is,  to 
both  lead  wires — like  a  voltmeter.  In  fact  this  instrument  is  a 

sort  of  a  combination  voltmeter 
and  ammeter,  as  it  measures 
the  product  of  the  volts  times 
the  amperes. 

In  connecting  the  wattmeter 
FIG.  255.     WATTMETER  TO    MEASURE    preat  rare  must  be  taken  to  p-et 
THE  POWER  CONSUMED  AT  THE  ARC.     great  Care 

G    Dynamo.  the  se^s  °f  binding  posts  correct- 

W    Wattmeter.  ly  joined  with  the  line.     That  is 

R    Rheostat.  tne  binding  posts  for  the  current 

The   heavy  line  wire  passes  to  the    terminals  of  the  wattmeter  must 

wattmeter  and  from  it  to  the  upper  car-    be  connected  in  series  or  along 

bon.  P  rom  the  lower  carbon  the  heavy 

wire  passes  to  the  rheostat  and  back  one  wire  like  the  ammeter  (fig. 
to  the  dynamo.  The  fine  wire  passes  2?3)  while  the  voltage  binding 
from  the  upper  carbon  to  the  watt-  '^' 

meter,  and  from  the  wattmeter  to  the   posts  must  be  connected  in  para- 
lower  carbon.     With  this  connection  of   1M   or  across    the    Hne     Hke    a 
the  wires  the  power  consumed  at  the 
arc  is  measured.  voltmeter  (fig.  272). 


CH.  XIII]  ELECTRIC  MEASUREMENTS  483 

If  the  wattmeter  were  wrongly  connected,  the  instrument  could 
not  register  the  watts  on  the  one  hand  and  on  the  other  it  might  be 
injured. 

§  670.  Portable  wattmeter. — This  has  a  pointer  which  shows 
directly  in  watts,  the  power  consumed  at  a  given  instant. 

§  671.  Stationary  or  house 
wattmeter. — The  wattmeter  for 
the  electric  supply  looks  some- 
thing like  a  gas  meter  for  the  gas 
supply.  It  is  of  the  integrat- 

FIG.  256.    WATTMETER  TO  MEASURE  inS  tyPe>   is  permanently  con- 
THE  POWER  DELIVERED  BY  THE       nected  with  the  line,  and  con- 

DYNAMO-  tains  a  wheel,  the  speed  of  whose 

G      Dynamo.  .......  .        , 

W    Wattmeter.  rotation  is  directly  proportional 

A    Arc  lamp.  to  t]ie  pOwer  consumed.     This 
R     Rheostat.  . 

The  fine  wire  connects    the    watt-  wheel  turns   Pomters    over    the 

meter  with  the  line  where  the  power  is  dials    on    which    are    indicated 

kilowatt-hours.     The   numbers 

toward  which  the  pointers  are  directed  indicate  the  kilowatt- 
hours  which  have  been  used,  just  as  the  pointers  in  a  gas  meter 
indicate  the  number  of  cubic  feet  of  gas  which  have  been  used. 
For  example,  by  consulting  the  wattmeter  before  and  after  an 
exhibition  one  can  see  how  much  work,  measured  in  kilowatt-hours 
has  been  consumed  by  the  arc  light. 

Suppose  the  voltage  of  the  line  were  no,  and  the  voltage  between 
the  carbons  is  55  volts.  Suppose  the  amperage  is  20,  then  the 
watts  should  be  (volts  times  amperes)  55x20  =  noo  watts  at  any 
instant,  and  for  an  hour,  for  example,  it  would  be  noo  watt  hours, 
or  1. 100  kilowatt-hours. 


§  67 la.  With  both  direct  and  alternating  current,  when  a  rheostat  is  in 
the  circuit,  the  amperage  may  be  found  by  the  aid  of  the  stationary'  wattmeter, 
this  is  always  present  in  a  house  supply  of  electricity,  as  is  the  gas  meter  for  the 
gas  supply,  and  one  does  not  always  possess  an  ammeter. 

It  is  necessary  to  know  the  voltage  of  the  line.     This  is  usually  1 10  or  220. 

One  must  also  know  the  watts,  or  kilowatts  at  any  given  instant.  This  can 
be  found  by  the  wattmeter  as  follows:  Suppose  the  reading  is  1.87  kilowatt- 
hours.  As  this  number  was  obtained  by  multiplying  volts  x  amperes  x  time, 
and  the  time  is  one  hour,  then  the  kilow'atts  of  power  consumed  is  1.87.  The 


484  ALTERNATING  CURRENT  UNITS  [Cn.  XIII 

UNITS  AND  THEIR  MEASUREMENT  WITH  ALTERNATING  CURRENT 

With  alternating  current  there  is,  strictly  speaking,  no  such 
thing  as  voltage  and  amperage  as  the  electric  potential  is  varying 
from  instant  to  instant.  Consequently  a  kind  of  average  value  of 
the  electric  pressure  and  amount  of  current  is  used  instead. 

§  672.  Alternating  current  voltage. — When  alternating  current 
is  measured,  the  voltage  indicated  on  the  voltmeter  is  the  mean 
effective  voltage. 

In  order  that  this  average  effective  value  for  a  volt  shall  corres- 
pond as  nearly  as  possible  to  the  analogous  value  with  direct  cur- 
rent, the  value  taken  is  the  square  root  of  the  average  of  the  squares 
of  the  instantaneous  values  of  the  potential  difference  during  an 
entire  cycle.  Or  briefly,  it  is  the  root  mean  squares  of  the  instan- 
taneous pressure. 

§  673.  Alternating  current  amperage. — The  number  of  amperes 
indicated  on  an  ammeter  when  using  alternating  current  represents 
the  mean  effective  amperage.  The  average  effective  value  of  the 
ampere  is,  as  with  the  volt,  the  square  root  of  the  average  of  the 
squares  of  the  instantaneous  values  of  the  current  during  an  entire 
cycle. 


voltage  with  a  rheostat  is  the  line  voltage.  Now  as  the  kilowatts  are  the  pro- 
duct of  voltage  by  amperage  divided  by  i  ,000  and  both  the  voltage  and  the 
kilowatts  are  known  the  amperage  can  be  found  by  multiplying  the  kilowatts 
by  1,000  to  reduce  them  to  watts,  and  dividing  the  watts  by  the  voltage  =  1 10. 
1870  -T-  no  =  17  amps.  With  alternating  current,  if  an  inductor  (choke-coil) 
is  used  for  regulating  the  current,  the  wattmeter  can  also  be  utilized  for  deter- 
mining the  amperage  at  the  arc,  for  by  experiment  it  is  known  that  no  matter 
what  the  line  voltage  is,  the  voltage  across  the  arc  is  usually  about  34  volts. 
The  fall  of  potential  across  the  inductor  does  not  count.  The  wattmeter  only 
records  the  power  consumed  by  the  lamp.  The  amperage,  assuming  the  same 
number  of  watts  as  in  the  above  example,  would  be  found  this:  1870  -5-  34  = 
55  amperes.  That  is,  with  an  inductor  in  place  of  a  rheostat  one  could  use 
several  times  the  amount  of  current  and  use  only  the  same  number  of  kilowatts 
of  power.  As  it  is  the  power  consumed  that  must  be  paid  for,  one  can  appre- 
ciate the  saving  by  using  an  inductor  or  choke-coil  rather  than  a  rheostat. 

The  two  cases  just  given  are  the  only  ones  in  which  the  wattmeter  can  be 
used  to  find  the  amperage.  If  a  current-saver,  transformer,  rectifier,  or  other 
similar  device  is  used  in  the  circuit  the  amperage  in  the  arc  cannot  be  deter- 
mined by  the  wattmeter,  one  must  use  an  ammeter  of  the  proper  type  for  the 
current. 


CH.  XIII]  ALTERNATING  CURRENT  UNITS  485 

§  674.  Watts  with  alternating  current. — With  alternating  as 
with'  direct  current,  the  instantaneous  watts  are  equal  to  the  pro- 
duct of  the  instantaneous  volts  by  the  instantaneous  amperes. 

As  the  voltage  and  amperage  with  alternating  current  vary 
from  instant  to  instant  over  the  entire  cycle,  it  follows  that  the 
instantaneous  watts  must  also  vary  from  instant  to  instant.  To 
obtain  the  average  watts  over  an  entire  cycle,  the  arithmetical 
mean  of  the  instantaneous  watts  is  taken.  This  average  of  the 
watts  may  be  anything  between  zero  and  the  product  of  the  mean 
effective  volts  times  the  mean  effective  amperes,  depending  on  the 
character  of  the  circuit,  i.  e.,  whether  the  circuit  contains  resistance 
only  or  whether  it  contains  both  resistance  and  inductance. 

§  675.  Power  factor. — When  alternating  current  is  used  with 
inductance  in  the  circuit  as  described  in  §  736  (where  an  inductor 
or  choke-coil  is  put  in  series  with  the  arc)  the  power  transformed 
into  heat  or  work,  and  hence  which  must  be  supplied  to  the  dynamo 
by  coal  or  other  fuel  is  less  than  the  product  of  the  mean  effective 
volts  by  the  mean  effective  amperes.  This  is  because  most  of  the 
energy  required  to  magnetize  the  iron  core  of  the  inductor  when  the 
current  is  increasing  is  returned  to  the  line  when  the  current  is 
decreasing.  In  the  case  mentioned  the  line  voltage  was  no;  at 
the  arc  the  voltage  was  34,  and  55  amperes  were  drawn.  The  power 
consumption  at  the  arc,  which  is  unable  to  return  any  absorbed 
energy  to  the  line,  is  the  product  of  the  volts  by  the  amperes,  i.  e., 
34x55  =  1,870  watts.  In  this  case  the  power  factor  is  unity.  In 
the  case  of  the  entire  circuit,  however,  by  multiplying  the  line 
voltage  by  the  amperage,  i.  e.,  1 10  x  55  we  get  6050.  A  wattmeter 
would  register  only  the  1870  watts  consumed  at  the  arc.  The 
power  factor  is  the  value  by  which  we  must  multiply  the  product  of 
volts  x  amperes  in  order  to  get  watts.  Thus,  if  we  multiply  6050 
by  .3 1  we  get  1870.  The  power  factor  is  of  course  obtained  in  prac- 
tice by  dividing  the  watts  by  the  product  of  volts  by  amperes,  i.  e., 
P.  F.  =  Watts  -i-  Volts  x  Amperes;  and  Watts  =  Volts  x  Amperes  x 
Power  factor.  Nothing  comparable  to  this  effect  is  possible  with 
direct  current,  that  is,  with  direct  current  the  power  factor  is  always 
unitv. 


486  DYNAMO  FOR  ARC  LAMPS  [Cn.  XIII 

§  676.  Cycle. — With  alternating  current  where  the  current 
flows  first  in  one  direction  and  then  in  another  with  a  change  in 
polarity  for  each  reversal,  a  cycle  includes  a  change  in  polarity  to 
the  opposite,  and  back  to  the  starting  point.  That  is,  a  cycle 
includes  flow  in  two  directions  and  consequently  includes  two 
polarities;  and  this  is  repeated  over  and  over  again. 

§  677.  Frequency. — The  number  of  cycles  per  second  with  an 
alternating  current  is  called  its  frequency.  The  frequencies  in 
most  common  use  are:  25  cycles,  60  cycles  and  135  cycles  per 
second.  The  60  cycle  frequency  is  most  generally  used  for  lighting 
circuits  and  the  25  cycle  frequency  is  mostly  employed  for  long 
distance  transmission,  and  frequently  for  motors.  The  130  or  135 
cycle  frequency  is  now  uncommon. 

SPECIAL  DYNAMO  FOR  ARC  LAMPS 

§  678.  The  characteristics  of  the  arc  are  that  the  potential 
difference  between  the  electrodes  is  dependent  upon  the  arc  length 
but  not  upon  the  current  (see  §  743).  It  is  required  to  supply  this 
arc  with  a  constant  current  regardless  of  the  differences  in  arc 
length.  This  may  be  done  with  a  constant  potential  supply  by 
using  a  rheostat  in  series  with  the  arc,  or  it  may  be  done  by  using  a 
constant  current  generator.  Since  the  early  days  of  arc  lighting, 
street  arcs  have  been  connected  in  series  and  are  supplied  by  a 
direct  current  dynamo  of  this  type,  no  resistance  being  used.  These 
dynamos  have  an  automatic  controlling  device  which  increases  the 
voltage  when  the  current  falls  slightly  below  the  rated  value  (6.6 
amperes)  and  which  decreases  the  voltage  should  the  current  rise 
slightly  above  this  value.  For  street  lighting  this  regulation  must 
be  very  close,  but  for  projection  purposes  the  regulation  need  be 
only  approximate.  There  are  some  types  of  dynamos  which  have 
the  proper  characteristics  to  be  connected  directly  to  an  arc  lamp 
without  intervening  resistance.  The  characteristics  of  such  a 
dynamo  must  be  that  a  slight  momentary  increase  in  current 
caused  by  a  lowering  in  the  potential  difference  at  the  arc  will  be 
met  by  a  decrease  in  the  voltage  generated,  and  conversely  a 


CH.  XIII]  DYNAMO  FOR  ARC  LAMPS  487 

decrease  in  current  will  be  met  by  an  increase  in  the   voltage 
generated. 

§  679.  Shunt  generator. — The  connections  for  a  shunt  genera- 
tor or  dynamo  are  shown  diagrammatically  in  fig.  257.  A  is  the 
revolving  armature  from  which  the  current  is  drawn.  N  and  S  are 
the  poles  of  the  field  magnet  and  F  is  the  field  coil  which  keeps  it 
strongly  magnetized.  The  stronger  the  magnetization  of  this  field 
magnet  the  higher  the  voltage  furnished  by  the  machine.  As 
usually  operated  the  field  rheostat  R  must  be  continually  adjusted 
so  that  the  right  current  is  supplied  to  the  field  coil  F  to  keep  the 
machine  at  the  desired  voltage. 

§  680.  Adaptability  of  a  shunt  generator  for  direct  connection 
to  an  arc  lamp. — If  instead  of  continually  adjusting  the  rehostat  R 
so  that  the  dynamo  will  supply  a  constant  potential,  the  machine 
is  left  to  itself  it  will  be  found  that  when  no  current  is  supplied,  i.  e., 
the  dynamo  is  running  on  no  load,  the  potential  difference  between 
the  terminals  a  and  b  is  greatest  and  consequently  the  current 
flowing  in  the  field  coil  F  is  greatest.  If  now  current  is  drawn  from 
the  dynamo  the  potential  difference  between  a  and  b  will  drop 
slightly.  This  will  result  in  a  decrease  in  the  current  flowing  in  the 
field  coil  F,  a  decrease  in  the  magnetization  of  the  field  magnets  and 
hence  a  decrease  in  the  voltage  generated.  The  result  is  in  the 
direction  desired,  namely,  that  an  increase  in  the  current  will  be 
met  by  a  decrease  in  the  voltage. 

Whether  or  not  a  shunt  generator  connected  directly  to  an  arc  will 
work  satisfactorily,  or  whether  the  arc  will  be  unstable  and  want  to 
either  "run  away"  or  "die  out"  will  depend  upon  the  details  of  the 
design  of  the  dynamo ;  that  is,  the  voltage  at  no  load,  the  resistance 
of  the  shunt  field  coils  and  the  resistance  of  the  armature  and  also 
on  the  resistance  of  the  wiring  to  the  arc.  Some  dynamos  have 
been  designed  which  will  work  satisfactorily  when  connected 
directly  to  the  arc  without  any  intervening  resistance.  Such 
dynamos  may  be  run  directly  by  some  form  of  engine  or  they  may 
be  part  of  a  motor-generator  set  in  which  high  voltage,  direct 
current  or  alternating  current  is  used  to  furnish  the  power.  (See 
also  §  682,  684). 


488 


DYNAMO  FOR  ARC  LAMPS 


[Cn.  XIII 


FIG.  257.  SHUNT  GENERATOR  CONNECTED  DIRECTLY  TO  THE  ARC  LAMP 
WITHOUT  INTERVENING  RESISTANCE. 

N  S     Poles  of  field  magnet. 

A     Armature  rotating  between  the  poles  of  the  field  magnet. 

a  and  b     Terminals  of  the  Dynamo. 

F  Field  coil;  current  through  this  coil  magnetizes  the  iron  of  the  field 
magnets. 

R  Adjustable  field  rheostat  controlling  the  current  flowing  through  the 
field  coil. 

L     Arc  lamp. 

+  and  —  indicating  the  polarity  of  the  wires  connected  to  the  arc. 

This  will  maintain  a  uniform  current  in  the  arc  regardless  of  its  length  in 
case  the  dynamo  is  properly  designed  and  proportioned  for  the  purpose. 


CH.  XIII] 


CURRENT  RECTIFIERS 


CURRENT  RECTIFIERS 


489 


§  681  .  While  alternating  current  is  more  cheaply  generated  and 
transmitted,  especially  if  the  distance  is  great,  the  available  light 
given  by  the  alternating  arc  is  much  inferior  to  that  given  by  a 
direct  current,  as  can  be  seen  by  consulting  the  table  of  available 
candle-powers  for  different  amperages  (§756).  On  this  account 
and  from  the  noiseless  character  of  the  direct  current  arc,  efforts 
have  been  made  to  utilize  alternating  current  to  get  direct  current. 

Up  to  the  present  time  two  methods  of  doing  this  for  projection 
purposes  have  proven  themselves  successful  : 


FIG.  258.     MOTOR-GENERATOR  SET. 
(Cut  loaned  by  the  General  Electric  Co.). 

The  alternating  current  motor  is  at  the  left,  the  direct  current  generator  is 
at  the  right.  The  two  armatures  are  mounted  on  the  same  shaft. 

§  682.  Motor-generator  sets. — This  is  an  indirect  way  of 
getting  direct  current  from  alternating.  It  consists  of  an  alternat- 
ing current  motor  and  a  direct  current  dynamo  attached  to  the 
same  shaft.  The  alternating  current  is  not  converted  into  direct 
current  but  is  used  to  furnish  mechanical  power  which  drives  the 
direct  current  dynamo  just  as  it  could  be  driven  by  a  water-wheel, 
a  gas  or  other  engine. 

The  efficiency  of  a  motor-generator  is  about  60%. 

If  the  dynamo  is  specially  designed  for  the  purpose,  the  arc  lamp 
can  be  connected  directly  to  it  without  using  a  rheostat  so  that 
there  is  no  loss  from  this  cause  as  must  be  the  case  when  the  rheo- 
stat is  used.  (See  above  §  680). 


4QO  CURRENT  RECTIFIERS  [Cn.  XIII 

§  683.  Mercury  arc  rectifier. — This  is  a  method  of  securing 
direct  current  from  alternating.  It  is  a  utilization  of  the  mercury 
arc,  and  gives  an  efficiency  of  about  70%.  The  current  is  slightly 


FIG.  259  FIG.  260 

FIG.  259.     MERCURY  ARC  RECTIFIER,  FRONT  VIEW. 

(Cut  loaned  by  the  General  Electric  Co.}. 

This  size  is  designed  for  30  amperes.  It  requires  14.5  amperes  alternating 
current  at  220  volts  or  29  amperes  at  1 10  volts,  and  delivers  30  amperes  direct 
current  at  62  volts.  (See  tests  §  754).  It  consumes  2600  watts  alternating 
current  and  delivers  1860  watts  direct  current  which  gives  8,600  candle-power 
with  the  projection  arc. 

FIG.  260.     MERCURY  ARC  RECTIFIER,  REAR  VIEW. 

(Cut  loaned  by  the  General  Electric  Co.}. 

This  gives  a  good  view  of  the  rectifier  bulb  and  the  inductor  directly  below 
the  rectifier  bulb  which  serves  to  limit  the  current  in  the  arc  by  acting  upon  the 
alternating  current  primary.  The  iron  case  on  the  floor  contains  a  com- 
pensating reactance  which  serves  to  smooth  out  the  fluctuations  on  the  rectified 
current. 


CH.  XIII]  CURRENT  RECTIFIERS 

I AVWWVWW ' 


491 


© 


/).C.3upp/y 


*     *  J    IB**  € 


FIG.  261.     MERCURY  ARC  RECTIFIER,  DIAGRAM  OF  CONNECTIONS. 
(Cut  loaned  by  the  General  Electric  Co.). 

The  alternating  current  supply  comes  in  at  the  upper  part  of  the  transformer. 
This  supplies  alternating  current  at  220  volts  (for  a  no  volt  arc)  between  the 
points  G  and  H.  The  arrows  indicate  the  direction  of  flow  of  the  current  dur- 
ing one-half  of  the  cycle  and  the  arrows  enclosed  in  circles  indicate  the  flow  of 
current  during  the  other  half  of  the  cycle.  Taking  the  time  when  H  is  the 
positive  pole  of  the  transformer,  the  current  flows  dowrn  this  wire  and  over  to 
the  point  A.  Here  the  current  flows  through  the  tube  to  the  cathode  B, 
through  the  battery  J  (or  the  arc  lamp  situated  at  J)  to  D.  It  then  flows  to 
the  right  through  E  and  up  to  G. 

When  the  current  is  reversed,  current  cannot  follow  this  path  because 
between  A  and  B  the  rectifier  tube  acts  as  a  valve,  as  the  mercury  arc  allows 
current  to  flow  towards  B  but  never  away  from  it,  hence  the  current  must  flow 
from  G  to  A l  to  B  through  /  to  D,  through  the  coil  F  to  the  left  and  up  to  the 
point  H. 

The  function  of  the  coils  E  and  Fis  to  act  as  an  auto-transformer,  for  without 
them  current  could  flow  directly  from  G  to  H  without  passing  through  the 
rectifier  tube.  In  actual  practice  both  coils  E  and  F  are  wound  on  the  same 
iron  core. 


492  CURRENT  RECTIFIERS  [Cn.  XIII 

The  small  electrode  in  the  bottom  of  the  tube,  at  C  is  used  in  starting  the 
tube.  In  starting,  the  tube  is  first  rocked  making  and  breaking  a  mercury 
contact.  A  small  amount  of  current  flows  through  between  C  and  B  and  starts 
the  arc  going,  after  which  it  will  continue  to  burn  as  long  as  B  is  the  cathode, 
but  if  the  arc  is  extinguished  even  for  an  instant,  it  will  go  out  and  the  tube 
must  be  tilted  again  before  it  will  work. 

pulsating,  but  the  current  is  always  in  one  direction  and  the  pulsa- 
tions are  so  slight  that  the  crater  of  the  positive  carbon  remains 
almost  as  constant  as  with  the  direct  current  furnished  by  a  motor- 
generator  set. 

Both  the  motor-generator  set  and  the  mercury  arc  rectifier  are 
necessarily  expensive.  For  a  small  plant  to  be  used  much  of  the 
time  for  the  arc  lamp,  and  where  power  is  needed  for  other  pur- 
poses, like  the  lighting  of  the  house,  pumping  water,  running 
machinery,  etc.,  etc.,  it  would  be  cheaper  to  install  one  of  the 
modern  forms  of  engines.  The  cost  of  running  these  is  relatively 
very  little,  much  less  than  for  the  current  supplied  to  the  rectifier 
or  for  the  motor-generator  set.  It  is  also  very  easy  to  care  for  the 
modern  engine  used  with  the  generator. 

By  adapting  the  generator  set  for  low  voltages  (60  volts)  it  is 
possible  to  connect  the  arc  lamp  directly  without  a  rheostat,  thus 
saving  the  energy  wasted  by  heating  the  rheostat.  A  rheostat 
may  also  be  used  but  if  so  it  is  called  upon  to  give  very  slight  reduc- 
tion in  voltage,  and  therefore  uses  up  but  little  energy. 

PROJECTION  WITH  135  CYCLE  AND  25  CYCLE  CURRENT 

§  684.  In  most  places  where  alternating  current  is  used  for 
lighting,  the  supply  has  a  frequency  of  60  cycles  per  second,  and 
in  this  chapter  it  has  generally  been  assumed  that  the  alternating 
current  has  this  frequency.  There  are,  however,  places  in  which 
the  supply  has  a  frequency  of  135  cycles  per  second  and  there  are 
others,  especially  small  towns  in  the  neighborhood  of  large  hydro- 
electric plants,  in  which  the  supply  has  a  frequency  of  25  cycles. 
The  authors  of  this  book  have  had  practically  no  experience  with 
other  frequencies  than  60  cycles.  We  have  reason  to  believe  how- 
ever, that  with  135  cycle  current  the  arc  will  give  as  good  results  as 
with  60  cycles  and  will  perhaps  have  less  tendency  to  show  a  flicker, 


CH.  XIII] 


CURRENT  RECTIFIERS 


493 


especially  when  used  with  moving  picture  projection.  When  25 
cycle  current  is  used  directly  (is  used  raw)  to  supply  the  arc,  the 
result  is  very  bad.  The  screen  shows  a  violent  flicker.  The 
general  appearance  is  much  the  same  as  when  a  pan  of  mercury  is 
jarred  rapidly,  the  surface  appears  covered  with  ripples.  This 
effect  is  naturally  very  trying  to  the  eyes. 


FlG.  262.    OSCILLOGRAMS  OF  THE  ALTERNATING  CURRENT  SUPPLY  AND  THE 
DIRECT  CURRENT  DELIVERED. 

(Cut  loaned  by  the  General  Electric  Co.;   made  from  the  original  photograph}. 

Curve  A  The  direct  current  delivered. 
B  The  direct  current  zero  line. 
C  The  alternating  current  voltage  curve  and  its  corresponding  zero 

line. 

The  height  of  the  curve  A  above  its  zero  line  B  represents  the  instantaneous 
value  of  the  direct  current.  Note  that  while  there  are  slight  fluctuations  in  the 
current,  i.  e.,  it  is  slightly  pulsating,  the  current  is  always  in  the  same  direction 
and  that  these  fluctuations  amount  to  only  about  30%  of  the  average  value. 
Note  also  that  the  maximum  current  value  corresponds  to  a  maximum  positive 
value  or  to  a  maximum  negative  value  of  the  alternating  current  voltage  as 
shown  in  curve  C  given  just  below. 


494 


CURRENT  RECTIFIERS 


CH.  XIII] 


In  order  to  get  good  projection  when  this  current  supply  only  is 
available,  a  motor-generator  set  can  of  course  be  used,  that  is,  the 
25  cycle  current  is  used  as  power  to  drive  a  direct  current  dynamo 
(§  682).  The  25  cycle  current  can  be  changed  to  direct  current  by 
the  use  of  a  rectifier  (§  683).  Such  current  would  of  course  be 
pulsating  although  always  in  the  same  direction.  As  the  authors 
have  never  seen  an  arc  supplied  from  a  rectifier  on  2  5  cycle  current 
we  can  make  no  recommendation  except  to  examine  one  of  these 
machines  in  actual  operation.  If  the  arc  should  prove  sufficiently 


B 


FlG.  263.    OSCILLOGRAMS  OF  THE  POTENTIAL  DIFFERENCE  BETWEEN  THE 
ANODE  AND  CATHODE,  IN  RELATION  TO  THE  IMPRESSED  ELECTROMOTIVE 

FORCE. 

(Cut  loaned  by  the  General  Electric  Co.;  made  from  the  original  photograph}. 

Curve  A     Potential  difference  between  anode  and  cathode. 

Note  that  during  half  of  the  wave  this  difference  is  equal  to  the  full  impressed 
(line)  voltage  while  during  the  other  half  wave  the  potential  difference  increases 
until  the  voltage  has  reached  the  constant  value  of  14  volts.  When  this  occurs 
current  is  caused  to  flow  through  the  arc  and  is  used  on  the  direct  current  side 
of  the  rectifier. 

Curve  B  Impressed  electromotive  force,  i.  e.,  instantaneous  value  of  the 
line  voltage. 


CH.  XIII] 


CURRENT  RECTIFIERS 


495 


free  from  flicker  the  rectifier  would  doubtless  answer  perfectly  in 
all  other  particulars.  There  is  no  doubt  about  the  motor-genera- 
tor; it  will  give  perfect  direct  current  for  projection. 

§  685.     Need  of  apparatus  designed  for  the  frequency  used. — 

All  alternating  current  apparatus  is  designed  to  work  with  one 
frequency  only,  that  is  a  transformer,  for  example,  if  designed  for 
use  on  60  cycle  current  will  not  work  satisfactorily  on  either  135 
or  25  cycles.  Hence,  in  ordering  apparatus  for  alternating  current 
it  is  necessary  to  ascertain  and  specify  the  frequency  as  well  as  the 
voltage  and  other  particulars  of  the  supply.  This  information 
can  be  furnished  by  the  power  company. 


FlG.  264.       OSCILLOGRAMS  OF  1HE  ANODE  CURRENTS. 

(Cut  loaned  by  the  General  Electric  Co.;  made  from  the  original  photograph). 

Curve  A     Portion  of  the  current  furnished  by  one  anode. 

Curve  B     Portion  of  the  current  furnished  by  the  other  anode. 

Note  that  from  a  single  anode,  current  flows  in  one  direction  only,  the  mer- 
cury arc  acting  as  a  valve  which  prevents  the  current  from  flowing  in  the 
opposite  direction.  When  current  ceases  in  one  anode  the  other  anode  com- 
mences to  furnish  the  current. 


496 


WIRING  FOR  AN  ELECTRIC  CURRENT 


[Cn.  XIII 


WIRING  FOR  AN  ELECTRIC  CIRCUIT  FROM  THE  DYNAMO  BACK  TO 

THE  DYNAMO 

§  686.  For  the  purposes  of  projection  by  the  aid  of  an  arc  lamp, 
the  electric  current  required,  whether  it  be  direct  current  or 
alternating  current,  is  practically  always  furnished  by  a  dynamo. 
To  make  the  electricity  available  there  is  a  conductor  of  some  kind, 
usually  a  copper  wire  extending  from  one  pole  of  the  dynamo  to  the 
arc  lamp  or  lamps,  and  from  them  back  to  the  other  pole  of  the 
dynamo.  Such  a  loop  of  wire  from  pole  to  pole  of  the  dynamo 
forms  an  electric  circuit,  regardless  of  the  length  of  the  wire.  With 
direct  current,  any  part  of  the  wire  nearer  the  positive  pole  of  the 
dynamo  is  positive  to  any  part  of  the  wire  nearer  the  negative  pole 
of  the  dynamo,  hence  the  wire  extending  out  from  the  positive  pole 
of  the  dynamo  is  often  designated  the  positive  wire,  and  the  wire 
received  into  the  negative  pole  of  the  dynamo  is  called  the  negative 
wire.  It  will  be  seen  from  fig.  275,  280  that  the  circuit  is  a  loop  of 
wire  with  the  two  ends  connected  with  the  two  poles  of  the  dynamo. 

With  alternating  current,  as  stated  above,  there  is  no  constant 
polarity,  hence  it  is  not  correct  to  speak  of  negative  and  positive 
wires  or  positive  and  negative  poles  of  the  alternating  current 
dynamo. 

§  687.     Amperage  for  different  purposes.  —  As  the  quantity  of 

electricity  needed  for  different 
purposes  varies,  the  capacity  of 
the  generator  or  dynamo  must 
vary.  Also  the  carrying  capac- 
ity of  conducting  wires  is  in  gen- 
eral proportional  to  their  size, 
hence  for  large  currents  it  is 
necessary  to  have  larger  wires 

circuit  making  the  path  back  to  the  than  for  small  currents  (see  the 
dynamo   (G)   shorter  than  the  course    ,    11     •,    -\         g   ,      \ 
through  the  arc  lamp  (A)  and  the  rheo-    table  below  §  694)- 

If  a  wire  is  put  across  the  points  <>  §  688.      Short  circuit.—  By  a 

and  c  the  electricity  will  take  that  path  short  circuit  js  meant  the  short- 

mstead  of  the  longer  path  through  the 

arc.  emng  of  the  distance  which  the 


CH.  XIII]  WIRING  FOR  AN  ELECTRIC  CURRENT  497 

current  must  travel  to  get  back  to  the  dynamo.  In  figure  265  if  a 
wire  were  put  across  the  circuit  at  the  points  s.  c.  instead  of  the 
current  extending  entirely  around  the  circuit,  it  would  take  the 
shorter  course.  Short  circuits  are  undesirable  for  two  reasons : 
(i)  the  current  is  not  available  where  wanted;  (2)  it  may  be 
dangerous. 

§  689.  Ground. — With  many  electric  circuits  such  as  with 
street  railway  circuits,  one  terminal  of  the  dynamo  is  permanently 
connected  with  the  earth.  If  now  the  wire  connected  to  the  other 


FIG.  266.    AN  ELECTRIC  CIRCUIT  WITH  A  SINGLE  GROUND. 

C  D     The  two  poles  of  the  dynamo. 
G     Generator  (dynamo). 

BT     A  conductor  extending  from  the  electric  circuit  to  the  ground  (g1). 
If  all  the  rest  of  the  circuit  is  insulated  this  will  do  no  harm,  but  see  fig.  267. 
g1     The  earth  into  which  the  conductor,  B  ,  extends. 
A     Arc  lamp. 
R     Rheostat. 

terminal  of  the  dynamo  should  also  become  connected  with  the 
earth,  as  through  a  water  or  a  gas  pipe,  current  would  wholly  or  in 
part  take  that  path  back  to  the  dynamo. 

When  any  part  of  the  circuit  is  connected  with  the  earth  it  is 
called  a  "ground." 

In  case  the  dynamo  and  circuit  are  entirely  insulated  from  the 
earth,  a  single  ground  will  result  in  no  flow  of  current  outside  the 
wire.  If,  however,  two  points  in  a  circuit  are  connected  to  the 
earth  the  effect  will  be  the  same  as  if  the  two  points  of  the  circuit 
were  connected  to  each  other,  by  an  additional  wire  (fig.  266,  267). 

§  690.  Insulation  of  wires. — To  avoid  short  circuits  and  the 
consequent  danger  to  men  and  animals  and  also  the  danger  from 


498  REGULATIONS  FOR  WIRING  [Cn.  XIII 

fire  by  the  wires  coming  in  contact  with  inflammable  material,  the 
wires  are  carefully  insulated  so  that  the  current  is  kept  in  the  circuit 
and  not  allowed  to  escape  by  taking  short  cuts  or  by  going  to  the 
ground.  Two  things  are  necessary  for  this:  (i)  The  naked  wires 
must  in  no  case  touch  each  other  at  any  point,  for  that  would  make 
a  short  circuit.  (2)  The  naked  wires  must  not  touch  anything 
which  is  a  conductor. 

The  wires  are  insulated  by  covering  them  with  a  coating  of 
rubber,   asbestos,   silk,    etc.,  that   is,  some   substance  which  will 


FIG.  267.     AN  ELECTRIC  CIRCUIT  WITH  A  DOUBLE  GROUND. 
C  D    The  two  poles  of  the  dynamo. 
G     Generator  (dynamo). 
Bl     A  conductor  extending  from  the  circuit  near  the  pole  C  to  the  ground 

<£')• 

B2     A  conductor  near  the  pole  D  extending  to  the  ground  (g2). 

In  this  case  the  current  will  short-circuit,  passing  from  the  point  B1  to  g1  and 
from  g1  to  ga,  B2  and  back  to  the  dynamo  at  the  pole  D  instead  of  passing 
through  the  arc  lamp  (A )  and  the  rheostat  (R) .  The  single  ground  is  dangerous 
only  in  that  there  is  liable  to  be  formed  a  second  ground  from  some  other  part 
of  the  circuit. 

g1,  g2     The  earth  into  which  the  conductors,  B1,  Bs  extend. 

A     Arc  lamp. 

R     Rheostat. 

not  serve  as  a  conductor.  Where  the  wire  must  be  uncovered,  as 
at  switches,  etc.,  some  solid  substance  like  porcelain,  slate,  hard 
rubber,  glass  or  some  other  non-conducting  substance  is  used,  for 
the  naked  wires  to  rest  against. 

REGULATIONS  FOR  WIRING;   PRECAUTIONS 
§  691.     National  Electric  Code. — To  make  the  wiring  and  con- 
nections of  electric  apparatus  good  and  safe  in  every  respect,  the 
electrical  engineers,  architects  and  fire  underwriters  have  formu- 


CH.  XIII]  REGULATIONS  FOR  WIRING  499 

lated  definite  rules  for  wiring,  insulation  and  the  character  and 
construction  of  fittings,  the  installation  of  apparatus  and  of  light- 
ing plants,  etc.  This  national  code  of  rules,  with  all  authorized 
modifications  found  desirable  from  time  to  time,  is  published  in 
pamphlet  form  by  the  National  Board  of  Fire  Underwriters  for  the 
guidance  of  those  having  electric  wiring  to  do  and  apparatus  to 
install.  This  board  also  publishes  a  list  of  electric  apparatus  and 
fittings  which  conform  to  this  code.  The  two  pamphlets  can  be 
secured  by  any  one  interested  by  sending  five  cents  in  stamps  to 
cover  postage,  to  the  National  Board  of  Fire  Underwriters,  135 
William  St.,  New  York  City,  N.  Y. 

General  precautions :  In  wiring  or  changing  wires  and  in  work- 
ing about  the  arc  lamp,  rheostat,  etc.,  the  current  should  always  be 
turned  off  at  a  switch  which  will  render  all  the  wires  and  apparatus 
to  be  changed  in  any  way  entirely  without  voltage  ("dead"),  so 
that  no  matter  what  is  done  there  is  no  danger  of  receiving  a  shock 
or  of  short-circuiting. 

If  "live  wires"  must  be  worked  with,  use  the  asbestos-patch 
gloves,  and  wrap  the  naked  wires  in  asbestos  paper  so  that  it  will 
be  impossible  to  bring  naked  wires  in  contact.  Remember  also 
that  a  concrete  floor,  if  at  all  moist,  makes  an  excellent  "ground" 
for  the  wires,  and  if  a  person  stands  on  the  moist  floor  with  the 
wires  in  his  hands  the  current  is  liable  to  pass  through  his  body  to 
the  ground.  It  is  safer  to  use  a  dry  board  or  rubber  mat  on  the 
concrete  floor  to  stand  on,  or  to  wear  rubbers. 

§  692.  Municipal  regulations  for  wiring,  etc. — In  addition  to 
the  regulations  of  the  National  Board  of  Fire  Underwriters,  it 
frequently  happens  that  there  are  special  regulations  by  the 
municipality  concerning  the  number  and  character  of  the  general 
lights  in  a  theater,  etc.,  and  also  the  source  of  the  electricity  for  the 
arc  lamp  and  for  the  general  lights.  There  may  also  be  special 
regulations  for  the  number  and  color  of  exit  lights  and  the  source 
of  the  current  for  supplying  them.  It  is  necessary  then  to  know, 
not  only  the  latest  regulations  of  the  National  Fire  Underwriters, 
but  the  regulations  of  the  city  or  state  where  the  electric  plant  is 
installed. 


5oo 


INSTALLATION  OF  ARC  LAMPS 


[CH.  XIII 


INSTALLATION  OF  AN  ARC  LAMP  FOR  PROJECTION 

§  693.  In  the  first  place  it  is  necessary  to  know  the  maximum 
amperage  to  be  used  for  the  projection.  The  wiring,  the  fuses  and 
the  ballast  (rheostat,  inductor,  etc.)  must  be  adapted  to  this 
maximum  current. 

If  the  installation  is  adequate  for  the  highest  current  that  may 
need  to  be  used,  it  will  of  course  be  adequate  for  any  smaller 
current. 

For  drawing,  and  much  of  the  ordinary  magic  lantern  work,  the 
current  varies  from  5  to  15  amperes,  and  if  the  installation  were 
for  such  work  alone,  wiring  and  accessory  apparatus  which  is  safe 
for  15  amperes  would  suffice.  If,  on  the  other  hand,  the  line  were 
to  be  used  for  large  halls  also,  and  especially  for  opaque  projection 
(Ch.  VII),  then  the  wiring  and  accessory  apparatus  would  reed  to 
have  a  maximum  capacity  of  50  to  60  amperes.  For  moving  pic- 
tures, the  line  should  safely  carry  a  maximum  of  75  amperes,  or 
finally  if  kinemacolor  moving  pictures  are  to  be  shown  in  a  large 
hall,  the  wiring  and  accessory  apparatus  must  be  adapted  for  an 
amperage  of  100  to  200. 

The  size  of  solid  wires  for  different  currents  is  given  in  the  follow- 
ing table : 

§  694.  Table  of  allowable  carrying  capacity  of  single  copper 
wires  of  98%  conductivity.* 

AMERICAN  INSTITUTE  OF  ELECTRICAL  ENGINEERS 


No.  Brown  and 
Sharp  Gauge 

Diameter  in 

Millimeters 

Diameter  in 
inches 

Circular 
Mils 

With   Rubber 
Insulation 
Amperes 

With  other 
Insulation 
Amperes 

No.    i 

7.248 

.289 

83,690 

107 

156 

No.    2 

6-543 

•257 

66,370 

90 

131 

No.    3 

5.826 

.229 

52,630 

76 

110 

No.    4 

5.189 

.'2O4 

41,740 

65 

92 

No.    5 

4.620 

.182 

33,ioo 

54 

77 

No.    6 

4-II5 

.162 

26,250 

46 

65 

No.    8 

3-264 

.128 

16,510 

33 

46 

No.  10 

2.588 

.102 

10,380 

24 

32 

No.  12 

2.053 

.081 

6,530 

17 

23 

No.  14 

1.627 

.064 

4,107 

12 

16 

No.  1  6 

1.291 

.051 

2^,583 

life 

**    8^ 

No.  1  8 

1.024 

.040 

1,624 

3 

5 

*From  the  1913  National  Electrical  Code,  §  18,  pp  32-33. 


CH.  XIII] 


INSTALLATION  OF  ARC  LAMPS 


501 


§  694a.  The  carrying  capacity  of  the  different  wires  in  this  table  is  the 
amperage  which  can  "be  safely  and  continuously  carried  by  the  wires  without 
injury  to  the  insulation  or  to  the  wire.  The  rubber  covered  wire  is  capable  of 
carrying  as  great  an  amperage  as  the  wires  with  more  resistant  insulation,  but 
the  amperage  given,  is  that  which  experience  has  shown  can  be  carried  without 
undue  injury  to  the  rubber  insulation,  and  with  entire  safety  in  continuous  use. 

Furthermore,  it  should  be  said  that  the  carrying  capacity  given  in  the  table 
is  by  no  means  the  maximum  capacity  which  the  wire  could  carry.  For  exam- 
ple, one  might  send  a  current  of  20  amperes  through  a  No.  18  wire,  but  this 
would  soon  injure  the  insulation  from  the  overheating.  By  following  the 
Electrical  Code,  one  is  on  the  safe  side. 

§  695.  Table  of  allowable  carrying  capacity  of  flexible 
cables  and  cords  composed  of  several  small  wires. 


B  &  S  Gauge 
No.  of  Wire 

Number  of 
wires 

Rubber  Insulation 
Amperes 

No.  1  8 
No.  1  8 
No.  1  8 

,1 

61 

25 
50 
120 

No.  1  6 
No.  1  6 
No.  1  6 

7 
19 
61 

35 
70 
170 

No.  14 
No.  14 

61 
9i 

235 
320 

ESTIMATED  CARRYING  CAPACITY 


No. 
No. 

32 
32 

40              5 
80             10 

No. 
No. 

30 
30 

15 
30 

3 
6 

1913  National  Electrical  Code,  §  94,  p.  186-187. 


§  695a.  This  estimate  is  based  upon  the  law  that  "The  conductivity  of  a 
wire  is  directly  proportional  to  its  sectional  area."  Thus,  No.  30  wire  has  a 
diameter  of  .01003  m-  and  an  area  in  circular  mils  of  010.03  x  010.03  =  100.6. 
The  area  in  circular  mils  of  No.  18  wire  is  1624  The  allowable  carrying  capa- 
city of  No.  1 8  wire  is  three  amperes  when  there  is  rubber  insulation  (see  table 
above) .  Assuming  the  same  proportional  carrying  capacity  for  the  No.  30  wire 

then  its  capacity  would  be  l624  =  IOQ-6,  whence  1624 X  =301.8  andX  =  .18 

3  X 

amp.  If  one  small  wire  can  carry  .18  ampere,  15  should  carry  .18x15  =2.7 
amperes  or  in  round  numbers,  3  amperes.  If  both  cords  are  united  into  one 
conductor  there  would  be  30  small  wires  with  the  capacity  of  .18  x  30  =  5.4 
amps,  or  6  amperes  in  round  numbers. 

For  No.  32  wire  in  the  same  way:  Thus,  No.  32  wire  has  a  diameter  of 
•°°795  in.  The  circular  mils  =  7.95  x  7.95  =  63.21  for  each  wire. 


502  INSTALLATION  OF  ARC  LAMPS  [Cn.  XIII 

§  696.  Selection  of  material  for  installing  the  arc  lamp. — After 
determining  the  maximum  amount  of  current  needed  for  the  arc 
lamp,  then  the  wire  of  proper  size  and  quality  and  insulation  to 
conform  with  the  National  Electrical  Code  should  be  obtained. 
The  simplest  way  to  do  this  is  to  go  to  some  reliable  dealer  in  elec- 
trical supplies  and  get  the  standard  material. 

Standard  switches,  etc.,  are  all  marked  plainly  so  that  there  is  no 
difficulty  in  selecting  the  correct  sizes.  In  America,  wire  is  more 
often  designated  by  some  standard  wire  gauge,  e.  g.,  that  of  Brown 
&  Sharp,  than  by  the  actual  diameter  in  millimeters  or  inches.  In 
the  above  table  the  sizes  in  millimeters  and  inches  corresponding 
with  the  B  &  S  gauge  numbers  are  given,  also  the  area  measured  in 
circular  mils. 

One  must  not  forget  that  everything  that  is  used  wears  out,  and 
when  any  piece  of  apparatus  or  the  wire  becomes  deteriorated  by 
use  it  should  be  replaced. 


WIRING  FOR  THE  ARC  LAMP,  THE  RHEOSTAT  OR  OTHER  BALANCING 
DEVICE,  AND  THE  LAMP  SWITCH 

§  697.  Connection  with  the  electric  supply. — It  is  assumed  that 
the  electric  supply  has  been  properly  installed  by  an  electric  com- 
pany, or  from  a  private  dynamo,  to  within  a  short  distance  of  the 
arc  lamp.  This  supply  will  be  in  a  proper  outlet  box,  with  fuses 
and  switches  in  accordance  with  the  National  Electrical  Code.  In 
case  the  outlet  box  is  on  the  wall  close  to  the  arc  lamp,  the  simplest 
and  most  convenient  connection  between  the  lamp  switch  and  the 
supply  in  the  outlet  box  is  by  means  of  a  separable  attachment  of 
the  proper  capacity  for  the  maximum  current.  (See  the  table  of 
flexible  cables,  §  695.)  If  the  current  is  direct,  then  it  is  a  conve- 
nience to  have  this  attachment  irreversible,  or  polarized  so  that 

For  No.  1 8  wire,  as  before,  the  circular  mils  are  1624  and  the  relative  carrying 

capacity  is  assumed  to  be  —   —  =  — — ,  whence  X  =  .116  amperes.     If  there 

3  X 

are  40  wires  in  each  cord  then  each  cord  should  carry  .  1 16  x  40  =  4.64  amperes, 
or  in  round  numbers  5  amperes.  If  the  double  cord  were  used  for  each  conduc- 
tor to  the  lamp,  then  in  like  manner  twice  as  much  could  be  carried,  as  there 
are  80  wires:  .116  x  80  =  9.28  amperes  or  10  amperes  in  round  numbers. 


CH.  XIII]  INSTALLATION  OF  ARC  LAMPS  503 

one  cannot  make  a  wrong  connection  (fig.  2 68 A).  Such  an  attach- 
ment would  also  serve  for  alternating  current,  but  is  unnecessary, 
as  it  makes  no  difference  which  way  the  attachment  is  connected. 

The  conductor  from  the  electric  supply  in  the  outlet  box  to  the 
lamp  switch,  if  the  distance  is  small,  not  over  2  to  3  meters  (6  to  10 
ft.),  is  most  conveniently  made  of  flexible  cable  of  the  proper 
carrying  capacity  (see  the  table  of  carrying  capacity  of  flexible 


FIG.  268.     SEPARABLE  WALL  RECEPTACLES,  POLARIZED  (A)  AND 

NON-POLARIZED  (B). 

(Cuts  loaned  by  H.  Hubbell,  Inc.). 

With  direct  current,  a  polarized  attachment  insures  the  same  polarity  with- 
out attention  on  the  part  of  the  operator;  with  the  non-polarized  form  there 
is  liability  of  reversing  the  polarity  unless  the  connections  are  specially  marked, 
and  care  is  taken  in  putting  the  separable  cap  in  position.  Either  form  can  be 
used  with  alternating  current  also. 

electric  cables).     The  two  wires  or  cables  are  often  enclosed  in  a 
common  sheath. 

§  698.  In  connecting  the  two  wires  to  the  attachment  cap,  the 
insulation  is  removed  for  a  short  distance  (i  to  2  cm.  ^  in.),  the 
wires  scraped  clean,  twisted  all  together,  and  then  turned  to  a  loop 
to  surround  the  set  screw.  Great  care  must  be  taken  to  avoid 
leaving  any  of  the  strands  free;  this  would  lessen  the  carrying 
capacity,  but  more  important  still,  they  might  become  displaced 
and  make  a  short  circuit  (§688). 


504  INSTALLATION  OF  ARC  LAMPS  [Cn.  XIII 

The  wire  is  fixed  firmly  under  the  set  screw,  and  if  the  current  is 
to  be  large,  30  amperes  and  more,  the  wire  should  be  soldered  to  its 
connection  after  the  screw  is  firmly  set  down. 

§  699.  Connecting  the  conductors  to  the  switch. — This  is  done 
exactly  as  for  the  attachment  cap. 

In  case  direct  current  is  used  it  is  important  to  know  which  is  the 
positive  and  which  the  negative  wire.  This  should  be  determined 
before  clamping  the  wire  to  the  switch.  The  best  method  is  by  the 


FIG.  269.     SEPARABLE  ATTACHMENTS,  POLARIZED  (A)  AND  NON-POLARIZED 

(BC). 
(Cuts  loaned  by  H.  Hubbell.  Inc.}. 

The  attachments  A  and  B  are  for  the  ordinary  bulb  socket. 

A  is  polarized  so  that  the  same  polarity  of  the  wires  is  insured,  for  the  connec- 
tion cannot  be  reversed. 

B  is  non-polarized  and  the  polarity  may  be  reversed  every  time  the  connec- 
tion is  made. 

C  is  for  receiving  an  incandescent  lamp ;  connection  is  made  with  the  supply 
by  inserting  the  prongs  into  an  attachment  plug  which  has  been  screwed  into  a 
lamp  socket. 

use  of  the  arc  lamp  (§  702),  after  the  arc  lamp  and  rheostat  have 
been  properly  connected. 

§  700.  Wiring  the  arc  lamp,  including  the  rheostat  or  other 
balancing  device. — From  one  pole  of  the  switch  (fig.  270),  a  wire 
of  the  proper  size  and  insulation  is  carried  directly  to  the 
negative  binding  post  of  the  lamp,  i.  e.,  to  the  post  for  the  lower 
carbon.  From  the  other  pole  of  the  switch  a  suitable  wire  is 
carried  to  one  binding  post  of  the  rheostat.  From  the  other  bind- 


CH.  XIII] 


INSTALLATION  OF  ARC  LAMPS 


505 


FIG.  270.     WIRING  OF  THE  ARC  LAMP  FOR  PROJECTION. 
For  full  explanation,  see  fig.  3  and  fig.  40. 

ing  post  of  the  rheostat  a  suitable  wire  is  carried  to  the  positive 
binding  post  of  the  arc  lamp,  that  is  to  the  binding  post  for  the 
upper  carbon.  This  puts  the  rheostat,  or  other  balancing  device 
in  one  wire,  or  in  series,  not  in  parallel,  or  across  both  the  wires  of 
the  circuit. 

In  securing  the  ends  of  the  wires  to  the  binding  posts,  scrape 
them,  and  twist  the  strands,  then  make  a  loop  and  put  under  the 
binding  screw  of  the  switch  as  described  for  the  attachment  cap. 
Usually  for  the  rheostat,  and  the  arc  lamp,  the  wires  are  twisted 
and  kept  straight,  then  inserted  into  a  hole,  and  a  set  screw  turned 
down  upon  them. 

If  flexible  cord  or  cables  are  used  for  these  connections,  the  wires 
on  the  end,  after  being  scraped  clean  should  be  twisted  and 
soldered,  then  none  of  the  strands  will  escape  to  lessen  the  carrying 
capacity,  or  possibly  to  make  a  short  circuit. 


506  POLARITY  TESTS  [Cn.  XIII 

DETERMINING  THE  POLARITY  WITH  DIRECT  CURRENT 

§  701.  General  statement  and  precautions. — With  direct 
current  it  is  necessary,  in  most  cases,  to  install  the  apparatus,  like 
the  ammeter,  the  voltmeter,  the  lamp,  etc.,  in  a  very  definite  man- 
ner so  that  the  current  extends  through  the  instrument  in  a  given 
direction.  That  is,  the  positive  end  of  the  wire  must  be  attached 
to  the  positive  binding  post.  But  when  ready  to  install  any  piece 
of  apparatus  with  direct  current  one  rarely  knows  which  is  the 
positive  and  which  the  negative  wire.  It  is  necessary  to  find  out 
by  experiment. 

Precautions  in  making  polarity  tests. — If  possible,  have  a  rheo- 
stat in  the  circuit  before  making  the  tests.  One  of  the  small 
rheostats  for  use  with  the  small  current  arc  lamp  can  be  very  easily 
introduced  into  the  circuit  (see  fig.  188,  270  for  wiring).  If  an 
adjustable  rheostat  is  already  in  the  circuit,  set  it  for  the  least 
current. 

In  making  the  tests  never  allow  two  naked  wires  to  come  in 
contact  for  that  would  complete  the  circuit  and  might  burn  out  a 
fuse  or  do  something  worse. 

Never  use  a  piece  of  metal,  or  a  metal  dish  for  holding  the  testing 
materials.  Always  use  glass,  porcelain  or  wood  or  some  other 
non-conducting  material.  The  tests  are  perfectly  definite  and  safe 
if  applied  with  due  care. 

Remember  also  that  when  repair  work  on  the  line  is  done,  the 
polarity  of  the  supply  wires  may  be  changed.  This  would  of  course 
change  the  polarity  of  the  arc  lamp  and  a  good  light  could  not  be 
obtained.  One  must  be  on  the  lookout  for  every  possible  trouble 
and  have  the  knowledge  and  the  resourcefulness  to  make  the  neces- 
sary modifications. 

DETERMINING  THE  POLARITY  WITH  AN  ARC  LAMP,  WITH  A 
VOLTMETER  OR  AN  AMMETER 

§  702.  (A)  If  an  arc  lamp  and  rheostat  are  available  the 
simplest  test  is  to  connect  the  arc  lamp,  large  or  small,  and  rheostat 
as  directed  above  (§  700).  With  proper  carbons  in  place  turn  on 


CH.  XIII] 


POLARITY  TESTS 


507 


the  current  and  strike  the  arc.  After  the  lamp  has  burned  a 
minute  or  two  open  the  switch  or  pull  the  separable  plug  apart  and 
watch  the  ends  of  the  carbons.  The  one  that  remains  red-hot  the 
longer  is  the  positive  one,  and  the  wire  leading  to  it  is  the  positive 


B 


FIG.  271. 


SIDE  AND  FRONT  VIEWS  OF  THE  RIGHT-ANGLE  CARBON  ARC  WITH 
CORRECT  AND  INCORRECT  POLARITY. 


A  The  upper  figures  show  the  correct  polarity,  that  is,  with  the  positive 
crater  on  the  upper  carbon. 

B  The  lower  figures  show  reversed  polarity,  that  is,  with  the  lower  carbon 
positive  and  hence  the  large  crater  on  it. 

The  photographs  were  made  with  a  color  screen  in  order  to  bring  out  the  posi- 
tive and  the  negative  craters  with  the  greatest  clearness.  The  exposure  for 
the  craters  was  instantaneous,  then  there  was  an  additional  exposure  of  90 
seconds  without  a  color  screen,  and  with  an  illumination  from  a  mazda  lamp 
to  bring  out  the  carbons  and  give  the  appearance  seen  by  the  human  eye  (see 
also  fig.  292-293). 


508  POLARITY  TESTS  [Cn.  XIII 

wire.  The  method  in  practice  is  to  watch  the  burning  carbons 
through  smoked  glass  or  smoky  mica.  The  positive  one  is  markedly 
brighter  than  the  negative  one  (fig.  271). 

If  the  upper  carbon  is  positive  the  lamp  is  correctly  installed,  if 
the  lower  carbon  is  positive  then  it  is  improperly  installed  for 
ordinary  projection.  If  the  positive  wire  goes  to  the  lower  carbon, 
turn  off  the  light  by  opening  the  switch  or  pulling  the  separable 
plug  apart.  Now  reverse  the  position  of  the  wires  in  the  binding 
posts  of  the  lamp,  and  this  will  bring  the  positive  wire  in  connection 
with  the  upper  carbon,  and  the  negative  wire  in  connection  with  the 
lower  carbon  (fig.  2,  270). 

If  a  non-polarized  separable  plug  is  used  (fig.  268  B),  the  simplest 
way  to  reverse  the  polarity  is  to  pull  the  cap  off,  turn  it  half  way 
round  and  insert  it  again.  When  found  to  be  in  the  correct  posi- 
tion mark  the  socket,  the  plug  and  the  cap  in  some  way  so  that  the 
connections  can  be  made  at  some  future  time  with  certainty. 
There  are  polarized  plugs  (fig.  2  68 A)  in  which  the  connections  are 
so  arranged  that  the  attachment  plug  can  be  inserted  only  in  one 
way,  thus  avoiding  the  change  of  polarity  when  once  the  wiring  is 
correctly  installed. 

When  the  polarity  is  found  to  be  correct  it  is  advantageous  for 
future  work  to  mark  the  insulation  of  the  positive  wire  near  the 
switch  with  red  paint.  The  positive  side  of  the  table  switch 
should  also  be  marked  with  a  +  sign  or  with  P.  using  black  or  red 
paint.  In  like  manner  the  insulation  of  the  wire  near  where  it  is 
connected  with  the  binding  post  of  the  arc  lamp  should  be  marked 
red,  and  a  +  or  P.  should  be  put  alongside  the  binding  post  for  the 
upper  carbon  unless  it  is  so  evident  that  no  mistake  is  likely  to 
occur. 

(B)  Testing  the  polarity  with  a  direct  current  voltmeter — 
To  do  this  connect  the  voltmeter  with  both  wires  (fig.  272). 
Turn  on  the  current  by  closing  the  switch  and  if  the  positive  wire 
is  connected  with  the  positive  binding  post  the  voltmeter  will 
record  the  voltage  in  the  line.  If  the  wires  are  wrongly  connected 
then  the  hand  will  try  to  move  off  the  dial  face  below  zero.  If  the 
hand  does  not  register  the  voltage,  open  the  switch,  and  reverse  the 


CH.  XIII] 


POLARITY  TESTS 


509 


position  of  the  wires  in  the  binding  posts  of  the  voltmeter.  Turn 
on  the  current  and  the  voltmeter  will  register.  It  is  well  to  mark 
the  insulation  of  the  positive  wire  with  red,  or  in  some  other  way. 


FIG.  272.    VOLTMETER  FOR  TESTING  POLARITY. 

G  Dynamo  for  direct  current.  The  positive  pole  is  above  and  the  negative 
pole  is  below,  as  indicated  by  the  arrows. 

Vm  The  terminals  of  the  voltmeter  are  correctly  connected  across  the  line 
(in  multiple)  or  to  both  wires  and  the  hand  indicates  the  voltage  on  the  dial. 
If  the  terminals  were  wrongly  connected  the  hand  would  not  register. 

A     Arc  lamp. 

R    Rheostat. 

The  arrows  indicate  the  direction  of  the  current. 

The  +  and  —  signs  indicate  that  any  point  in  the  circuit  nearer  the  positive 
pole  of  the  dynamo  is  positive  to  any  point  nearer  the  negative  pole. 


FIG.  273.    AMMETER  FOR  TESTING  POLARITY. 

G  Dynamo  for  direct  current.  The  positive  pole  is  above  and  the  negative 
pole  below. 

Am  Direct  current  ammeter.  The  terminals  a +, — b  are  connected  along 
one  wire  (in  series).  If  the  positive  pole  of  the  ammeter  is  connected  to  the 
circuit  next  the  positive  pole  of  the  dynamo,  and  the  negative  terminal  in  the 
wire  toward  the  negative  pole  of  the  dynamo,  as  here  shown,  the  hand  will 
register  when  there  is  current  flowing.  If  the  connections  are  reversed  the 
hand  will  not  register  when  the  current  is  flowing. 

a-f ,  — b     The  positive  and  the  negative  terminals  of  the  ammeter. 

A     Arc  lamp. 

c-\-     The  positive  carbon. 

c —  The  negative  carbon  (the  minus  sign  is  put  parallel  with  the  carbon  to 
show  the  direction  of  the  current). 

R     Rheostat. 

The  +  and  —  signs  and  the  arrows  are  as  with  the  voltmeter  (fig.  272). 


510  POLARITY  TESTS  [Cn.  XIII 

(C)  Testing  the  polarity  with  a  direct  current  ammeter. — 
The  circuit  should  be  connected  with  a  rheostat  and  an  arc  lamp 
or  one  or  more  incandescent  lamps  in  series  (along  one  wire)  then 
the  switch  is  opened  and  the  ammeter  is  inserted  in  one  wire  (in 
series),  (fig.  273).  Now  turn  on  the  current  and  light  the  lamp 
(§  30).  If  the  wires  are  correctly  connected  the  ammeter  will 
indicate  the  amount  of  current  flowing;  if  it  is  wrongly  connected 
then  the  hand  will  try  to  move  off  the  dial  below  zero.  That  is,  the 
positive  wire  has  been  inserted  in  the  negative  binding  post  of  the 
ammeter,  and  the  negative  wire  in  the  positive  binding  post. 
Open  the  switch,  and  reverse  the  position  of  the  wires  in  the  binding 
posts ;  turn  on  the  current  and  the  hand  will  register  the  amperage. 
The  positive  wire  can  then  be  marked  red  or  in  some  other  way. 

CHEMICAL  POLARITY  INDICATORS 

§  703.  Litmus,  iodized  starch,  salt  solution  and  potato  indica- 
tors.— (A)  Litmus  indicator. — Take  some  blue  litmus  or  other 
acid-alkaline  testing  paper,  about  10  cm.  (4  in.)  long  and  place  it 
on  a  pane  of  glass  or  a  porcelain  plate.  Moisten  it  well.  Separate 
the  ends  of  the  wires  as  indicated  in  the  testing  lamp  (fig.  21). 
Put  the  two  ends  about  10  centimeters  (4  in.)  apart  on  the  mois- 
tened litmus  paper.  Turn  on  the  current.  The  positive  wire  will 
turn  the  blue  litmus  paper  red  when  the  current  flows.  Turn 
off  the  current  and  mark  the  positive  conductor  red,  or  white. 

(B)  Iodized  starch  polarity  indicator. — Make  some  starch  paste 
by  mixing  15  grams  (]4  oz.)  of  dry  starch  (corn  starch,  laundry 
starch  or  wheat  flour)  with  300  cc.  (10  oz.)  of  cold  water.  Add  ^ 
gram  (7  or  8  grains)  of  iodide  of  potassium.  Now  heat  the  mixture 
with  constant  stirring  until  the  starch  is  cooked.  Put  some  of  the 
iodized  paste  in  a  glass  or  porcelain  dish  and  insert  the  separated 
wires  to  be  tested  in  the  paste.  Turn  on  the  current  and  the  starch 
at  the  positive  pole  will  be  turned  blue.  Turn  off  the  current  and 
mark  the  positive  wire  in  some  way.  (The  iodized  starch  test  is 

§  702a.  If  one  uses  a  voltmeter  or  an  ammeter  of  the  new,  soft-core  type 
(Eclipse  Volt — and  Ammeters)  which  register  both  alternating  and  direct  cur- 
rent, one  cannot  determine  polarity  with  them,  for  they  register  whichever 
way  they  are  connected  with  the  circuit. 


CH.  XIII] 


POLARITY  TESTS 


the  one  commonly  employed  for  weak  currents  for  it  is  very 
sensitive;  it  is,  however,  equally  good  for  large  currents). 

(C)  Salt  and  water  polarity  indicator. — Make  a  >^%  solution 
of  common  salt  (NaCl)  in  water.     Place  the  solution  in  a  glass  or 
porcelain  dish  about  10  cm.  (4  in.)  across.     Insert  the  two  separ- 
ated wires  to  be  tested  in  the  liquid  and  turn  on  the  current.     When 
the  current  is  on,  many  small  bubbles  will  appear  at  the  negative 
pole.     In  making  this  test  remember  the  precautions  (§  701). 

(D)  Raw  potato  polarity  indicator. — Cut  an  ordinary  uncooked 
potato  in  half.     Insert  the  wires  into  the  potato  having  the  wires 
as  far  apart   as   possible.     Turn  on  the   current.     The  potato 
around  the  positive  pole  will  turn  greenish.     If  the  potato  is  quite 


FIG.  274.     THREE-WIRE  ARC  LAMP  OF  THE  BAUSCH  &  LOME  OPTICAL  COMPANY 
For  a  full  explanation  see  fig.  14.5  and  §  704. 


512  WIRING  FOR  ALTERNATING  CURRENT         [Cn.  XIII 

moist,  many  small  bubbles  will  appear  around  the  negative  pole. 
But  the  greenish  color  given  at  the  positive  pole  is  the  most  certain. 
Turn  off  the  current  and  mark  the  positive  wire  red. 

With  the  other  chemical  tests  (A,  B,  C)  the  indications  are  in  no 
way  dependent  on  the  metal  forming  the  wire,  but  with  the  potato 
test  the  poles  entering  the  potato  must  be  copper  or  contain 
copper. 

§  704.  Wiring  the  three-wire  automatic  lamp  of  the  Bausch  & 
Lomb  Optical  Company. — This  lamp  is  regulated  wholly  by  elec- 
tricity, there  being  no  clock-work.  In  wiring  the  lamp  one  pro- 
ceeds exactly  as  described  above  (§  693-700),  except  that  a  wire 
is  carried  from  the  positive  side  of  the  switch  to  the  middle  binding 
post  of  the  lamp  directly.  Another  wire  from  the  same  point  is 
carried  down  to  the  resistor  or  rheostat,  and  from  the  rheostat  a 
wire  to  the  positive  or  upper  binding  post  of  the  lamp.  From  the 
negative  pole  of  the  switch  a  wire  is  carried  directly  to  the  lower 
or  negative  binding  post  of  the  lamp.  This  wiring  gives  the  full 
voltage  of  the  line  for  the  electric  mechanism  governing  the  lamp 
(see  fig.  145). 

WIRING  FOR  ALTERNATING  CURRENT 

§  705.  This  is  precisely  as  for  direct  current,  and  one  does  not 
have  any  trouble  about  the  polarity.  It  makes  no  difference 
which  supply  wire  is  connected  with  the  upper  carbon  and  which 
with  the  lower. 

§  706.  Insertion  of  the  rheostat,  inductor  or  other  balancing 
device. — It  makes  no  difference  in  which  of  the  lead  wires  the 
rheostat,  etc.,  are  inserted.  Just  as  with  direct  current,  however, 
the  balancing  device  must  be  inserted  along  one  wire  (fig.  281), 
otherwise  the  current  would  not  traverse  the  entire  circuit. 

§  707.  Position  of  the  rheostat,  etc. — The  balancing  effect  of 
the  rheostat  is  the  same  no  matter  where  it  is  installed  in  the  special 
circuit  for  the  arc  lamp.  For  convenience  it  is  frequently  put  on 
or  near  the  projection  table.  This  is  especially  necessary  if  the 
rheostat  is  adjustable.  With  a  fixed  rheostat  it  is  sometimes  safer 


CH.  XIII]         WIRING  FOR  ALTERNATING  CURRENT  513 

to  put  it  near  the  supply  intake,  especially  if  that  is  at  a  consider- 
able distance  from  the  lantern  or  other  projection  apparatus,  then 
in  case  of  a  short  circuit  in  working  about  the  lamp  or  table  switch, 
an  excessive  current  could  not  flow,  and  there  would  be  much  less 
danger  from  fire  or  the  burning  out  of  fuses.  (See  also  §  708). 

§  708.     Wiring  when  the  arc  lamp  is  far  from  the  supply.— 

When  the  supply  is  at  a  considerable  distance  from  the  arc  lamp 
the  flexible  wire  connection  is  sometimes  used  for  temporary  work, 
but  is  not  suitable  for  permanent  installation. 

Instead  of  a  conduit,  well  insulated  wires  are  sometimes  used 
from  the  general  supply  box  to  the  neighborhood  of  the  arc  lamp. 
The  wires  must  be  secured  by  porcelain  or  other  non-conducting 
supports  every  meter  (3  or  4  feet)  which  will  separate  them  from 
the  wall  i  to  2  cm.  (>£  in.)  and  from  each  other  5  to  7  cm.  (2^"  in.) 
and  hold  them  in  place.  Where  the  wires  pass  through  partitions, 
each  wire  should  have  its  own  porcelain  tube  so  that  is  does  not 
come  in  contact  with  the  partition.  The  safe  rule  in  wiring  is  to 
treat  the  rubber  covered  wires  as  if  they  were  naked.  At  the  end 
it  is  desirable  to  have  a  metal  box  for  the  special  fuse  block  and 
switch.  An  attachment  fixture  is  also  very  convenient  (fig.  270). 

For  the  position  of  the  rheostat,  etc.,  see  §  707. 

§  709.  Wiring  an  arc  lamp  for  large  currents. — Arc  lamps  for 
opaque  projection  (Ch.  VII)  and  for  moving  pictures  (Ch.  XI) 
require  large  amperages,  and  frequently  the  lamps  become  very 
hot,  especially  if  the  lamp-house  is  not  large  and  well  ventilated. 
For  lamps  requiring  the  large  currents  it  is  best  to  use  flexible 
cables  of  higher  capacity  than  is  needed  outside  the  lamp-house. 
The  wire  should  also  be  insulated  with  some  fire-proof  material  like 
woven  asbestos. 

The  ordinary,  rubber  insulation  will  answer  for  low  amperages 
especially  when  the  lamp-house  is  well  ventilated.  An  excellent 
wiring  material  is  the  flexible  cord  used  for  heating  apparatus. 
This  has  rubber  insulation,  and  also  woven  asbestos,  and  the 
outside  is  covered  with  woven  cotton  to  protect  the  asbestos.  Of 
course  a  flexible  cord  of  the  proper  carrying  capacity  should  be 
selected. 


5U  SWITCHES,  FUSES,  CIRCUIT  BREAKERS  [Cn.  XIII 

If  it  is  difficult  to  get  double  cord  of  the  right  size,  then  each  of 
the  wires  to  the  lamp  can  be  composed  of  the  double  cable.  This 
is  easily  done  by  removing  the  insulation  at  each  end  of  the  double 
cord  and  twisting  both  the  wires  together.  (See  the  tables  §  694, 
695,  for  the  carrying  capacity  of  flexible  cord  and  cables). 

§  710.  Wiring  the  arc  lamp  with  a  three-wire  supply. — Only 
two  wires  go  to  the  arc  lamp,  then  if  one  must  connect  the  arc  lamp 
for  projection  to  a  three-wire  supply  system  it  is  necessary  to 
remember  that  the  middle  (neutral)  wire  and  either  outer  wire  will 
give  no  volts  the  same  as  the  two-wire  no  volt  circuit. 

If  connection  is  made  with  the  two  outer  wires  then  220  volts 
will  be  used  in  the  arc  lamp.  In  this  case  a  rheostat  for  a  220  volt 
circuit  must  be  employed,  or  two  no  volt  rheostats  in  series  (fig. 
287). 

Naturally  one  would  connect  'with  the  middle  or  neutral  and  an 
outside  wire  and  employ  the  usual  no  volt  rheostat  but  for  the 
fact  that  such  an  arrangement  would  badly  unbalance  the  work  of 
the  line,  and  might  cause  trouble  if  the  electric  circuit  was  running 
nearly  on  full  load.  It  is  therefore  safer  to  connect  with  the  out- 
side wires  and  use  the  requisite  amount  of  ballast. 

SWITCHES,  CIRCUIT  BREAKERS  AND  FUSES;    THEIR  CHARACTER, 

INSTALLATION  AND  USE 

§  711.  A  switch  is  a  device  by  means  of  which  a  gap  (fig.  275 
and  276)  can  be  made  in  an  electric  circuit  thus  stopping  the  flow 

of  current. 

A  switch  should  be   so  con- 

structed  that  when  it  is  opened 

\         it  makes  a  gap  in  all  the  wires 
B      of  the  circuit.    For  example,  in 
J        a  two -wire   circuit,   the   switch 
should  make  a  gap  in  both  wires, 

FIG.  275.    CIRCUIT  WITH  A  BREAK  and  in  a  three-wire  circuit,  a  gap 

OR  GAP.  •        ir    .,  .  TJ-         1 

Unless  the  metallic  circuit,  from  the  m  a"   three   ™*-    .  If  SUch  a 

dynamo,  G,   back  to  the  dynamo,    is  switch  IS  used  the  line  beyond 

complete,  no  current  will  flow.     A  gap  the  switch  ig  "dead,  "and  no  CUT- 
in  the  circuit  (B)  prevents  the  flow  of 

current.  rent  can  be  drawn  from  it. 


CH.  XIII]          SWITCHES,  FUSES,  CIRCUIT  BREAKERS 


SIS 


FIG.  276.    SNAP  AND  KNIFE  SWITCHES  SHOWING  OPEN  AND  CLOSED  CIRCUIT. 

A     Snap  switch  with  circuit  closed  (current  on). 

B     Knife  switch  with  circuit  closed  (current  on). 

C    Snap  switch  with  circuit  open  (current  off). 

D     Knife  switch  with  circuit  open  (current  off). 

A  W    Wires  from  the  switch  to  the  arc  lamp. 

Base     The  insulating  support  of  the  knife  switch. 

H    Handle  of  the  switch  blades. 

L  W    Supply  wires  for  the  electric  current  to  the  switch. 

There  are  two  main  forms  of  switches:  The  knife  switch  like 
that  shown  in  fig.  276  B,  D,  and  the  snap  switch,  which  rotates 
(fig.  276  A,  C).  Any  switch  to  be  installed  should  conform  in  its 
construction  with  the  National  Electric  Code  and  be  plainly  marked 
with  its  capacity — voltage  and  amperage — and  the  maker's  name. 

§  712.  Installation  of  a  switch. — The  non-combustible,  non- 
conducting base  should  be  fastened  to  some  support,  and  then  the 
wires  of  the  line  cut  and  scraped  and  connected  firmly  in  the  bind- 
ing posts  or  under  the  binding  screws.  If  the  current  is  over  30 
amperes  the  wires  should  also  be  soldered  to  the  switch  after  the 
screws  are  well  set  down.  A  switch  at  the  supply  for  the  building 


SWITCHES,  FUSES,  CIRCUIT  BREAKERS 


[Cn.  XIII 


or  special  plant  should  be  enclosed  in  a  metal  box  where  it  can  be 
easily  got  at,  but  not  where  the  naked  metal  parts  might  inad- 
vertently become  short-circuited. 

It  is  necessary  also  to  put  the  switch  in  such  a  position  that 
when  it  is  opened  it  will  not  close  of  itself  by  gravity.  If  the 
switch  is  in  a  vertical  position  it  must  be  placed  with  the  hinge 
below,  so  that  gravity  will  tend  to  open  it,  never  to  close  it  (fig. 
277). 

If  the  switch  is  horizontal,  then  the  hinge  should  be  tight  enough 
so  that  the  blades  will  remain  in  any  position  in  which  they  are 
placed.  For  a  double-pole,  double-throw  switch  for  two  lamps  see 
fig.  162. 

A  knife  switch  has  an  appreciable  amount  of  naked  metal 
exposed.  It  therefore  makes  a  short  circuit  easily  possible.  For 
use  with  projection  apparatus,  especially  if  high  amperages  are  to 
be  used  as  with  opaque  projection  and  with  moving  pictures  it  is 


Base 


— Hg— 


SB — 


AW 


FIG.  277.     OPEN  KNIFE  SWITCH  IN  A  VERTICAL 

POSITION,  WITH  THE  HANDLE  BELOW  so  THAT 

THERE  IS  NO  DANGER  OF  THE  SWITCH 

CLOSING  BY  GRAVITY. 

L  W  Line  wires  from  the  electric  supply  (fig. 
270)  to  the  switch. 

A  W  Arc  lamp  wires  from  the  switch  to 
the  arc  lamp.  A  rheostat  is  inserted  in  one  of 
them  (fig.  270). 

S  C  Spring  clamps  pressing  against  the  switch 
blades  when  the  switch  is  closed,  thus  making 
good  metallic  contact. 

Base  The  insulating  base  of  the  switch.  It 
is  held  in  position  by  two  or  more  screws. 

Hg     Hinges  of  the  switch  blades. 

5  B  Switch  blades.  When  the  switch  is 
closed  these  blades  make  a  continuous  circuit, 
and  when  the  switch  is  open  the  circuit  is 
broken. 

Cb  Cross-bar  of  insulating  material  to 
which  the  switch  blades  and  the  handle  are 
attached. 

H  Handle  for  opening  and  closing  the  switch. 
It  is  of  insulating  material 


CH.  XIII] 


SWITCHES,  FUSES,  CIRCUIT  BREAKERS 


517 


advantageous  to  enclose  the  switch  in  a  metal  box  with  a  slit 
allowing  the  handle  to  project  and  move  so  that  the  switch  can  be 
opened  and  closed.  As  only  the  handle  is  exposed  with  this  arrang- 
ment  the  operator  is  safe  when  manipulating  the  switch  in  the  dark 
(fig.  278).  See  also  §  714. 

§  713.    End  of  the  switch  to  connect  with  the  supply  wires. — 

Sometimes  the  supply  wires  are  connected  with  the  hinge  end  of  the 
switch  as  in  fig.  2.  This  has  the  disadvantage  that  the  switch  is 
then  energized  up  to  the  break  at  the  handle,  when  the  main  supply 
is  on.  As  the  switch  is  liable  to  get  out  of  order  and  need  screwing 
up  occasionally  it  is  better  to  insert  the  lead  wires  in  the  opposite  or 


FIG.  278.     ENCLOSED  SWITCH  IN  A  HORIZONTAL  POSITION. 

Commencing  at  the  right: 

L  W    Supply  or  line  wires  from  the  outlet  box  (fig.  270)  to  the  table  switch. 

k     Key  for  locking  the  metal  cover  when  it  is  closed. 

H  Handle  of  the  knife  switch.  It  projects  through  the  slot  (s)  in  the  cover. 
In  the  position  shown  the  switch  is  open. 

sb  Switch  box.  This  is  a  sheet  iron  box  enclosing  the  switch  so  that  noth- 
ing can  come  in  contact  with  the  naked  metal  of  the  switch.  Only  the  switch 
handle  projects  beyond  the  box.  The  enclosing  box  is  represented  as  trans- 
parent in  order  to  show  the  switch  and  its  connecting  wires  within.  The  bot- 
tom of  the  enclosing  box  is  covered  with  asbestos  board  and  the  switch  base 
rests  on  the  asbestos,  not  on  the  metal  of  the  box. 

hg  Hinge  of  the  metal  cover.  By  turning  the  cover  over  to  the  left  the 
entire  switch  is  exposed. 

A  W    Wires  from  the  switch  to  the  arc  lamp. 


Si8  SWITCHES,  FUSES,  CIRCUIT  BREAKERS  [Cn.  XIII 

jaw  end  of  the  switch  as  in  fig.  277,  then  when  the  switch  is  open 
the  hinges  and  blades  are  ''dead"  and  can  be  put  in  order  with 

safety. 

§  714.  Snap  Switches. — These  are  sometimes  used  for  turning 
on  and  off  the  current  at  the  operating  table.  They  are  mounted 
on  insulating  material  like  porcelain,  and  are  enclosed  by  a  metal 
covering  which  is  lined  with  insulating  material.  The  key  or 
button  for  turning  on  and  off  the  current  is  also  of  insulating 
material.  This  form  of  a  switch  around  the  work  table  is  con- 
venient, and  avoids  any  danger  of  accidentally  short-circuiting  the 
line.  It  should  turn  on  the  current  and  turn  it  off  with  a  snap.  It 
is  also  desirable  that  there  should  be  a  sign  indicating  when  the 
current  is  on  or  off,  as  one  cannot  see  directly  as  with  the  knife 
switch.  If  such  a  switch  is  used,  make  sure  that  it  is  of  the  right 
capacity  for  the  maximum  current  and  that  it  conforms  in  every 
way  with  the  standard  requirements.  It  will  be  plainly  marked  so 
that  after  it  is  installed  one  can  see  at  any  time  the  current  and 
voltage  for  which  it  is  designed.  Snap  switches  are  better  adapted 
for  small  currents,  than  for  large  ones.  Knife  switches  are  to  be 
used  on  lines  with  large  currents. 

FUSES  AND  CIRCUIT  BREAKERS 

§  715.  Fuses  and  circuit  breakers  are  devices  for  opening  or 
breaking  the  circuit  whenever  the  current  in  any  particular  situa- 
tion becomes  too  great.  For  example,  if  a  part  of  the  line  should 
be  short-circuited. 

The  devices  used  are  of  two  kinds ; .  fuses,  and  magnetic  cut-outs 
or  circuit  breakers. 

§  716.  Circuit  breakers. — The  circuit  breaker  is  a  device  by 
which  a  magnetic  trip  releases  a  catch  which  allows  a  large  switch 
to  open,  thus  breaking  the  circuit. 

The  great  advantage  of  a  circuit  breaker  is  that  nothing  is 
burned  out  or  melted.  It  is  only  necessary  to  close  the  switch 
again  and  the  current  will  be  on.  It  acts  instantly  whenever  the 
current  rises  above  the  amperage  for  which  it  is  adjusted. 


CH.  XIII]  SWITCHES,  FUSES,  CIRCUIT  BREAKERS  519 

§  717.  Fuses. — A  fuse  is  a  wire  of  low  melting  point  forming 
part  of  the  circuit.  If  the  current  becomes  too  great  this  fuse  is 
melted,  thus  making  a  gap  in  the  line.  The  fuse  is  then  said  to 
burn  out  or  to  "blow."  If  the  current  becomes  much  too  great  as 
in  a  short  circuit  the  fuse  will  "blow"  instantly,  if  however,  the 
current  is  only  slightly  larger  than  the  fuse  is  designed  for — as  for 
example,  when  striking  the  arc  in  an  arc  lamp — the  fuse  will  not 
"blow"  instantly,  and  if  the  overload  is  only  for  a  short  time  it  will 
not  melt  at  all.  If  the  overload  continues  for  some  time,  however, 
the  fuse  will  get  hotter  and  hotter  until  its  melting  point  is  reached, 
when  it  will  melt  and  open  the  circuit.  This  property  of  the  fuse  is 
of  great  advantage  when  using  arc  lamps,  for  the  temporary  over- 
load in  lighting  the  arc  lamp  is  unavoidable. 

§  718.  Location  and  installation  of  fuses. — Like  the  switch,  the 
fuses  should  be  placed  in  the  path  of  all  the  wires  of  a  circuit — i.  e., 
with  a  two-wire  system  two  fuses,  and  with  a  three-wire  system, 
three  fuses,  etc.  The  wiring  of  a  fuse  block  is  the  same  as  for  a 
switch  (§  712). 

There  is  always  a  switch  in  the  supply  box  from  the  electric 
lighting  system  or  from  the  private  dynamo.  In  this  box  are  also 
fuses  to  open  the  circuit  in  case  of  accidental  short-circuiting.  The 
fuse  block,  whether  for  cartridge  fuses  or  for  plug  fuses  should  be 
selected  with  care  to  make  sure  that  it  is  of  the  right  capacity 
for  the  maximum  current  and  conforms  to  the  standard  code.  The 
fuses  are  plainly  marked,  so  there  need  be  no  mistake. 

One  should  not  use  fuses  of  higher  capacity  than  the  line  was 
designed  for,  for  fear  of  fire  or  other  accident. 

If  the  supply  box  is  some  distance  from  the  arc  lamp,  many 
careful  operators  have  fuses  as  well  as  a  switch  at  the  supplemen- 
tary supply  box  in  the  operating  room,  when  a  conduit  or  fixed 
wires  are  carried  from  the  main  supply  to  the  operating  room.  The 
fuse  nearest  the  arc  lamp  is  preferably  of  somewhat  less  capacity 
than  the  ones  farther  away,  then  if  a  fuse  is  blown  it  will  be  the 
handiest  one  to  renew. 

§  719.  Fuses  and  the  wattmeter. — If  but  a  single  meter  is  used 
to  measure  the  current  for  arc  lights,  house  lights,  heating  appara- 


520  SWITCHES,  FUSES,  CIRCUIT  BREAKERS  [Cn.  XIII 

tus,  etc.,  then  each  group  should  be  separately  fused  after  the 
wattmeter,  for  then  if  one  part  of  the  line  is  cut  out  the  rest  can  go 
on  drawing  current.  For  example,  if  the  arc  lamp  were  misman- 
aged it  ought  not  to  be  possible  to  blow  out  the  fuse  for  the  house 
lights,  and  the  reverse. 

§  720.  Location  of  fuse  blocks. — The  general  rule  is  that  there 
must  be  a  fuse  block  wherever  there  is  a  change  in  the  size  of  the 
wire  used.  These  fuse  blocks  must  be  in  cabinets  in  plain  sight  and 
readily  accessible.  Usually  also,  with  every  fuse  block  there  is  a 
knife  switch. 

§  721.  Capacity  of  fuses. — The  rated  capacity  of  fuses  should 
not  exceed  the  allowable  carrying  capacity  of  the  conducting  wire 
(see  tables  §  694,  695),  and  circuit  breakers  should  not  be  set  more 
than  30%  above  that  allowable  capacity. 

The  allowable  capacities  for  incandescent  lamp  lines  are  as 
follows : 

55  volts  or  less 12  amperes 

55-125  volts 6  amperes 

125-250  volts 3  amperes 

For  electric  lighting  each  special  circuit  or  line  should  not  be  used 
for  a  current  greater  than  will  give  a  power  of  660  watts.  This 
would  mean  for  example,  that  if  one  wished  to  use  60  watt  lamps 
there  could  be  only  n  of  the  lamps  on  a  single  line.  If  40  watt 
lamps  were  used  then  there  might  be  as  many  as  16  lamps  on  a  line, 
etc. 

In  using  flat-irons  and  other  heating  devices  on  an  electric  lamp 
circuit,  care  must  be  exercised  not  to  turn  on  any  lights  on  that 
branch  of  the  circuit. 

Likewise  in  using  the  small  arc  lamp  for  drawing  with  the  micro- 
scope, ultra-microscopy,  etc.,  where  from  four  to  six  amperes  of 
current  are  needed,  one  should  not  use  incandescent  lights  on 
that  line  at  the  same  time,  for  the  current  would  exceed  the  allow- 
able amount  and  probably  blow  a  fuse. 

§  722.  Replacement  of  fuses. — As  fuses  are  liable  to  blow  out 
it  is  well  to  have  a  supply  on  hand,  then  the  burnt  out  ones  can  be 


CH.  XIII]  RHEOSTATS  AND  OTHER  BALLAST  521 

quickly  replaced.  To  replace  a  fuse,  open  the  nearest  switch  which 
will  turn  off  the  current  from  the  line.  Take  out  both  fuses,  and 
examine  them  ;  only  one  is  likely  to  have  melted.  It  is  usually  easy 
to  tell  which.  Discard  that  one,  then  insert  two  good  fuses  of  the 
proper  capacity,  close  the  switch,  and  the  current  will  be  available 
again. 

If  the  lights  on  a  particular  line  go  out  from  the  blowing  of  a  fuse, 
and  one  is  not  sure  which  branch  it  is  in  the  fuse  box,  the  one  is 
easily  found  by  using  the  testing  lamp  (fig.  21)  beyond  the  fuses. 
The  lamp  will  light  on  all  the  lines  with  perfect  fuses  when  put 
across  the  blades  of  the  special  line  switch,  or  when  put  in  contact 
with  any  naked  metal  part  across  the  line.  The  line  with  a  burned 
out  fuse  will  not  light  the  testing  lamp,  when  it  is  applied  beyond 
the  fuse. 

RESISTORS  OR  RHEOSTATS:  INSTALLATION  AND  USE 

§  723.  Resistor  or  rheostat.  —  A  rheostat  is  a  conductor  having 
considerable  resistance  ;  it  is  placed  in  an  electric  circuit  to  regulate 
the  amount  of  current.  In  passing  through  the  rheostat  much  heat 
is  developed  by  the  energy  consumed  in  overcoming  the  resistance. 
This  energy  consumption  is  a  dead  loss. 

The  conductor  used  is  ordinarily  in  the  form  of  wire  or  strips  of 
metal  such  as  German  silver,  iron  or  nickel. 

§  724.  Amount  of  resistance  needed.  —  Electricians  have 
worked  out  with  much  accuracy  the  resistance  of  different  metals 
and  by  consulting  the  tables  furnished  in  books  on  electrical 
engineering  one  can  find  how  great  a  length  of  a  given  size  iron 
or  German  silver  wire  is  necessary  to  afford  the  proper  resistance 
for  any  given  constant  voltage,  as  no  or'  2  20.  See  § 


§  724a.  Ohm's  Law  and  its  application  to  projection  apparatus.  —  While  the 
units,  volt,  ampere  and  ohm  (§  654-657)  might  be  worth  defining,  still  it  would 
lead  to  no  very  practical  results  unless  there  was  a  definite  relation  between  the 
electric  quantities  for  which  these  units  stand. 

It  has  been  found  by  experiment  that  there  is  a  very  definite  relationship, 
known  as  Ohm's  Law.  (For  a  history  of  the  djio^very  of  this  law  by  Ohm,  see 
Dr.  Shedd  in  the  Popular  Science  Monthly  forTDec.,  1913). 

Briefly  stated  Ohm's  law  is:  "The  current  in  a  given  circuit  is  directly  pro- 
portional to  the  electromotive  force,  and  inversely  as  the  resistance:" 
Nichols,  p.  294. 


522  RHEOSTATS  AND  OTHER  BALLAST  [CH.  XIII 

As  stated  by  Norris  it  is:  "The  electromotive  force  consumed  in  the 
resistance  of  a  conductor,  is  proportional  to  the  current."  P.  8. 

Using  the  terms  now  employed  in  place  of  electromotive  force  (voltage), 
resistance  (ohmage),  and  current  (amperage),  the  law  can  be  stated  thus: 

(1)  The  voltage  in  a  conductor  is  equal  to  the  amperage  multiplied  by  the 
ohmage:     V  =  A  O. 

(2)  The  amperage  is  equal  to  the  voltage  divided  by  the  ohmage:    A  = 

(3)  The  ohmage  is  equal  to  the  voltage  divided  by  the  amperage:     O  =  — - 

A 

As  V  =  A  O  — —    =  i  •     From  this  form  is  derived  the  very  simple  dia- 

'    A  O 

gram  used  practically  in  getting  the  formula  for  the  value  of  any  single  quantity 
if  two  are  known.     The  formula  for  the  unknown  quantity  is  found  thus: 

Cover  the  letter  representing  the  unknown 
V  quantity,  and  the  remaining  letters  will  indicate 

the  value  of  the  unknown  quantity. 

A         °  Examples: 

FIG.  279.       DIAGRAM  OF          i-     If  the  voltage  and  amperage  are  known, 
OHM'S  LAW  FOR  SOLV-        what  is  the  ohmage? 

ING  PROBLEMS   (§  724a).      Cove?  the^  9  and  there  remain  V/A  and  this  is 
„  _  v  ,,  equal  to  O,  i.  e.,  O   =  V/A.     Suppose  the  volt- 

A   —  A        g  age  is  no  and  the  amperage  is  20,  what  is  the 

n  -  ^nperage'  ohmage?     Applying  the  formula,  O    =    110/20, 

or  5.5  ohms. 

2.  If  the  voltage  and  the  ohmage  are  known  what  is  the  amperage?     Here 
if  A  is  covered  there  is  left  V/O,  whence  the  amperage  equals  the  voltage  divided 
by  the  ohmage.     If  the  voltage  is  220  and  the  ohmage  is  5.5  as  before,  what  is 
the  amperage?     A  =220/5.5  =40  amperes.     This  example  also  illustrates  the 
fact  that  if  the  ohmage  remains  constant  the  amperage  will  increase  in  direct 
proportion  to  the  voltage.     (See  Dr.  Nichols'  definition  above). 

3.  If  the  amperage  and  ohmage  are  known  what  is  the  voltage?     Here  the 
unknown  quantity  is  represented  by  V.     If  this  is  covered  there  will  be  left 
A  O,  whence  V  =  A  O.     If  the  amperage  is  15,  and  the  ohmage  8  then  the 
voltage  must  be  15  x  8  =  120,  i.  e.,  V  =  120  volts. 

As  a  further  example  suppose  one  wished  to  make  a  water-cooled  rheostat 
(fig.  283)  and  he  had  some  wire  which  had  ah  ohmage  or  resistance  of  0.25  ohm 
per  meter,  how  much  wire  would  be  needed  with  a  voltage  of  no  and  an 
amperage  of  15?  Here  voltage  and  amperage  are  known.  From  the  formula 
it  is  seen  that  ohmage  equals  voltage  divided  by  amperage :  whence  110/15  = 
7.33  ohms,  the  total  resistance  required. 

Now  as  55  is  the  voltage  required  by  the  arc  with  the  direct  current  arc 

lamp,  the  lamp  itself  must  offer  a  resistance  of  .    ~       for  3-66  ohms. 

A  =  15 

As  the  total  ohmage  needed  is  7.33,  the  rheostat  must  possess  the  difference 
between  7.33  and  3.66  or  3.67  ohms. 

If  each  meter  of  the  wire  to  be  used  offers  a  resistance  of  0.25  ohm,  it  will 

require  for  3.67  ohms,  3>67  =  14.68  meters  of  the  wire  for  the  rheostat.     (For 
the  wattage  of  the  current  see  §  660). 


CH.  XIII]  RHEOSTATS  AND  OTHER  BALLAST  523 

§  725.  Getting  rid  of  the  heat  developed. — As  much  heat  is 
developed  in  the  rheostat,  it  is  necessary  to  so  arrange  the  coils  of 
wire,  etc.,  forming  it,  that  the  heat  can  easily  escape,  otherwise  the 
wire  might  get  so  hot  that  it  would  melt.  Provision  for  carrying 
away  the  heat  then  is  of  prime  importance.  For  example,  a  large 
iron  telegraph  wire  would  get  red  hot  in  the  air  if  it  were  used  for 
100  amperes,  while  a  much  smaller  wire  if  immersed  in  water  would 
carry  the  current  easily  on  account  of  the  rapid  dissipation  of  the 
heat  in  the  water. 

Ordinarily  the  resistance  wire  is  in  coils,  and  these  are  hung  on 
non-conductors  in  such  a  way  that  there  is  free  circulation  of  air 
around  and  through  the  coils  to  carry  off  the  heat. 

Sometimes  the  wire  or  strips  of  metal  serving  for  the  resistance 
are  imbedded  in  porcelain,  and  a  considerable  surface  of  the  porce- 
lain being  exposed  to  the  air,  the  heat  readily  escapes.  This  is 
often  the  method  with  the  rheostats  used  for  dimming  the  lights  in 
theaters  (theater  dimmers).  (See  fig.  183,  186  for  a  theater 
dimmer  used  as  a  rheostat). 

In  fig.  198  is  a  small  rheostat  with  the  metal  in  a  helical  coil  and 
wound  around  a  porcelain  core.  This  rheostat  is  for  the  small  arc 
lamp  to  be  used  on  the  house  lighting  system,  and  restricts  the 
current  to  4-6  amperes. 

§  726.  Fixed  rheostat. — This  is  a  rheostat  in  which  the  entire 
amount  of  resistance  wire  must  be  traversed  whenever  the  current 
is  on,  the  amperage  of  the  current  is  then  practically  constant. 
For  example  in  using  the  arc  lamp  if  the  rheostat  is  designed  for  1 5 
amperes,  that  current  must  always  be  used.  The  fixed  rheostat  is 

best  adapted  for  any  place  where 
many  use  the  same  apparatus 
(fig.  280). 

§  727.  Adjustable  rheostat— 
The  adjustment  consists  of  an 
arrangement  by  which  a  greater 

or  less  length  of  the  resistance 
FIG.  280.     CIRCUIT  WITH  DYNAMO  (G)       •  1      •     i     1    1  •      ,1 

ARC  LAMP  (4),  AND  FIXED  RHEO-       W11"6  can  be  included  in  the  cir- 
STAT  (R).  cuit  at  will.     The  more  resis- 


524  RHEOSTATS  AND  OTHER  BALLAST  [Cn.  XIII 

tance  in  the  circuit  the  less  will  be  the  amperage,  and  the  less  resis- 
tance the  higher  the  amperage. 

In  some  forms  it  is  possible  to  have  a  great  range  of  current,  say 
from  5  to  45  amperes  (fig.  281) ;  in  other  forms  the  range  may  be 
limited,  say  from  15-25  amperes. 

For  the  projection  microscope  and  the  magic  lantern  it  is  desir- 
able to  have  a  rheostat  giving  a  range  of  amperage  from  5  to  25 


FIG.  281.  THE  USE  OF  AN  ADJUSTABLE  RHEOSTAT  AS  BALLAST  FOR  AN  ARC 

LAMP 

G     Generator  (dynamo). 

A     Arc  lamp  with  right-angle  carbons. 

AR     Adjustable  rheostat. 

5  If  the  movable  contact-arm  is  at  5,  the  resistance  allows  but  5  amperes  to 
flow. 

25  If  the  contact-arm  is  at  25  then  only  half  of  the  resistance  is  in  the  cir- 
cuit and  25  amperes  of  current  can  flow. 

45  If  the  contact-arm  is  opposite  45,  only  a  small  amount  of  resistance  is  in 
the  circuit  and  forty-five  amperes  of  current  is  allowed  to  flow. 

The  arrow  indicates  the  direction  to  turn  the  contact-arm  to  increase  the 
current. 


amperes.  Such  a  rheostat  is  not  difficult  to  construct,  nor  is  it 
expensive.  The  theater  dimmer  shown  in  fig.  183  is  of  this  range. 
But  an  adjustable  rheostat  requires  judgment  for  its  proper  use; 
for  apparatus  to  be  used  by  everybody  it  is  better  to  have  a  fixed 
rheostat  (§  726). 

§  728.  Installing  the  rheostat. — It  is  usually  placed  close  to  the 
arc  lamp,  i.  e.,  inside  the  lamp  switch,  so  that  when  the  lamp  switch 
is  open  the  current  is  entirely  off  the  arc  lamp  and  its  rheostat. 


CH.  XIII]  RHEOSTATS  AND  OTHER  BALLAST  525 

In  wiring  the  rheostat,  it  is  to  be  placed  in  one  wire,  (in  series)  as 
all  the  current  must  pass  through  it  (fig.  1 88,  281).  It  makes  no 
difference  whether  it  is  placed  in  the  wire  going  to  the  upper  carbon 
or  coming  from  the  lower  carbon. 

§  729.  Calibration  of  a  rheostat. — The  maker  of  a  rheostat 
should  mark  plainly  upon  it  its  capacity  if  it  is  of  the  fixed  form. 
If  it  is  adjustable,  then  the  range  of  the  rheostat  should  be  given. 

Furthermore,  the  lower  range  should  be  plainly  marked  at  the 
lowest  step  and  the  highest  range  at  the  highest  step.  The  user  of 
a  rheostat  like  that  in  fig.  145  could  not  tell  easily  which  way  to 
turn  the  knob  to  increase  or  diminish  the  current  unless  the  maker 
indicates  the  amperage  at  the  two  ends  of  the  steps.  In  case  there 
is  no  indication  a  person  can  determine  it  for  himself  if  he  has  an 
ammeter. 

Insert  the  ammeter  in  one  wire  of  the  line  (fig.  273).  Turn  the 
knob  of  the  rheostat  to  the  middle  step,  insert  proper  carbons  in  the 
arc  lamp,  and  turn  on  the  current.  When  the  lamp  is  burning 
properly  note  the  reading  on  the  ammeter.  Turn  the  knob  toward 
one  side  and  the  ammeter  will  indicate  whether  there  is  more  or  less 
current.  One  can  in  this  way  find  the  amount  of  current  delivered 
for  the  different  positions.  It  is  well  to  mark  on  the  rheostat  face 
with  white  paint  the  amperages  corresponding  to  these  positions. 
It  is  also  a  help  to  have  an  arrow  pointing  from  the  lowest  to  the 
highest  amperage  (fig.  182,  281). 

§  730.  Home-made  rheostats. —  While  it  is  altogether  false 
economy  to  use  anything  but  the  best  in  the  form  of  a  rheostat  it 
is  worth  while  knowing  how  one  could  be  made  in  case  of  urgent 
need. 

§  73 1 .  Barrel  or  bucket  type  of  salt  water  rheostat. — A  wooden 
bucket  or  barrel  is  used.  In  the  bottom  is  placed  a  large  plate  of 
iron,  and  one  end  of  the  supply  wire  is  firmly  fixed  to  this.  The 
other  end  of  the  wire  is  fixed  to  another  mass  of  iron.  The  barrel 
or  bucket  is  then  filled  nearly  full  of  water,  and  enough  common 
salt  added  to  make  about  a  >£%  solution.  The  water  should  be 
well  stirred  to  evenly  distribute  the  salt.  The  upper  iron  and 


526 


RHEOSTATS  AND  OTHER  BALLAST 


[Cn.  XIII 


wire  are  then  covered  by  a  burlaps  sac  so  that  there  can  not  be  a 
metallic  contact  between  the  masses  of  metal.  This  upper  wire 
and  its  iron  are  then  immersed  in  the  barrel.  If  now  the  arc  lamp 
is  fitted  with  carbons,  and  the  switch  closed  the  arc  will  form  as 
usual,  the  salt  water  and  the  iron  plates  serving  as  a  rheostat. 


W2 


FIG.  282.     SALT  WATER  RHEOSTAT. 

Wlt  W?  Conductors.  One  end  of  conductor  Wa  is  connected  to  an  iron 
plate  P2  in  the  bottom  of  the  dish.  The  other  end  is  connected  to  the  plate  Pt 
which  is  suspended  by  a  string  wound  around  the  clamp.  The  burlaps  sack  5, 
prevents  contact  of  PI  and  P2  with  resulting  short  circuit  should  the  upper 
plate  be  let  down  too  far.  It  is  safer  still  to  have  both  plates  covered,  and 
the  container  must  be  of  wood,  glass  or  stoneware,  i.e.  some  non-conductor. 

The  jar  contains  a  l/4%  solution  of  salt.  The  resistance  is  regulated  by 
raising  or  lowering  the  plate  PT.  If  more  current  is  required,  lower  the  upper 
plate,  if  less  current,  raise  Pz  so  that  the  two  plates  will  be  farther  apart. 


CH.  XIII] 


RHEOSTATS  AND  OTHER  BALLAST 


527 


If  one  wishes  a  greater  amperage  the  upper  wire  is  lowered  in  the 
barrel  and  if  less  current  is  desired  the  upper  iron  is  lifted  higher 
in  the  barrel  (fig.  282).  Of  course  there  must  be  some  means  of 
holding  the  upper  wire  in  position  when  it  is  at  the  right  height 
in  the  barrel. 


FIG.  283.    WATER  COOLED  RHEOSTAT. 

Wlt  W2     Conductors. 

R  Rheostat  composed  of  the  proper  length  of  small  naked  wire  wound 
around  a  frame  of  wood.  The  two  ends  of  this  resistance  wire  are  soldered  to 
the  cut  ends  of  the  supply  wire  W3  Wy  The  rheostat  is  then  immersed  in 
running  water  and  the  containing  vessel  of  wood,  glass  or  stoneware  is  placed  in 
a  sink. 


528  RHEOSTATS  AND  OTHER  BALLAST  [Cn.  XIII 

In  no  case  should  one  use  naked  wires  for  this  rheostat,  but  the 
rubber,  water-proof  insulated  copper  wires  required  by  the  National 
Electric  Code.  The  ends  of  the  wires  must  be  scraped  and  fastened 
to  the  plates  of  iron.  This  is  rather  a  poor  make-shift  for  a  rheo- 
stat. The  water  soon  heats  up,  and  as  it  heats  the  resistance 
becomes  less  so  that  more  current  flows.  Then  to  counterbalance 
this,  fresh  cold  water  can  be  added  or  the  upper  plate  lifted  to  make 
the  distance  between  the  iron  plates  greater.  Furthermore,  increas- 
ing the  amount  of  salt  lessens  the  resistance.  If  there  is  too  much 
salt  there  will  be  too  much  current,  if  too  little  one  cannot  get 
enough  current  without  bringing  the  iron  plates  very  close  together, 
and  this  is  not  safe. 

§  732.  Home-made  water  cooled  rheostat. — A  home-made 
rheostat  can  be  constructed  of  small,  naked  wire  of  the  proper 
length  as  shown  by  calculation  or  by  the  electrical  tables.  The 
wire  is  wound  around  a  wooden  frame  in  a  single  layer,  care  being 
taken  that  the  different  turns  do  not  touch  one  another.  The  cut 
ends  of  one  of  the  heavy  insulated  supply  wires  are  then  soldered 
to  the  two  ends  of  the  coil.  The  coil  with  the  soldered  junctions  is 
then  immersed  in  a  glass  or  porcelain  dish  containing  pure  water, 
no  salt  being  used  (fig.  283).  If  the  current  is  to  be  on  for  some 
time  it  is  a  great  advantage  to  have  the  vessel  containing  the  rheo- 
stat stand  in  a  sink  or  in  some  place  where  water  can  drain  away, 
and  then  to  keep  a  stream  of  cold  water  flowing  into  the  vessel  to 
keep  the  wire  cool. 

This  general  scheme  is  used  in  making  tests  of  the  gigantic 
generators  used  in  large  power  plants.  For  such  tests  the  wire  used 
is  naked  telegraph  wire  of  the  right  resistance  and  length  laid  out 

§  371a.  With  such  a  bucket  rheostat,  12  liters  (12  quarts)  of  #%  salt 
solution  were  used,  and  the  distance  between  the  iron  discs  could  be  as  great 
as  15  cm.  (6  in.).  With  the  discs  15  cm.  apart  and  the  solution  at  23°  centi- 
grade a  current  of  10  amperes  flowed.  After  an  hour,  when  the  temperature 
had  risen  to  43°  C.,  12  amperes  of  current  flowed.  With  the  discs  nearly  in 
contact  20  amperes  were  given. 

In  this  experiment  the  iron  discs  were  18  cm.  (7  in.)  in  diameter.  By  in- 
creasing the  size  of  the  iron  discs  the  current  could  be  increased,  and  by 
diminishing  it  the  current  could  be  diminished.  Iron  (tin)  funnels  are  some- 
times used  instead  of  discs.  It  is  safer  to  have  both  discs  covered  with  the 
burlaps,  and  the  conducting  wires  soldered  to  the  discs  or  funnels. 


CH.  XIII] 
W, 


RHEOSTATS  AND  OTHER  BALLAST 


FlG.  284.    A.DJ  USTABLE  RHEOSTAT 
MADE  OF  SHEETS  OF  TIN. 

A,  B,  C,  D  Clip-connectors  to  hold 
the  ends  of  the  wires. 

Permanent  connectors  c  n  1-2  are 
used  to  join  the  further  ends  of  the  tin 
strips  1-2  and  3-4  and  a  connector  (c  n) 
is  used  between  B  and  C. 

J,  J'  Movable  adjusters  to  include  more  or  less  of  the  resistance  in  the 
circuit  and  thus  increase  or  diminish  the  amperage. 

This  rheostat  is  composed  of  four  sheets  of  tin  cut  as  shown  in  fig.  285.  It  is, 
therefore,  four  rheostats  in  series  (see  fig.  287).  As  here  connected  all  four 
sheets  are  used.  By  putting  supply  wire  W2  from  A  to  C  or  from  D  to  B  only 
two  of  the  sheets  would  be  used.  Then  by  means  of  the  adjusters  /  and  /'  the 
amount  of  resistance  can  be  increased  or  diminished  at  will. 

The  small  diagram  at  the  left  shows  how  the  pairs  of  strips  of  each  side  are 
connected  with  each  other  at  the  far  end. 

At  the  near  end  of  the  frame  the  arched  wire  connects  the  two  pairs  of  plates 
of  both  sides  at  B  and  C. 


530 


RHEOSTATS  AND  OTHER  BALLAST 


[Cn.  XIII 


FIG.  285.  To  SHOW  THE  TIN  PLATE  CUT 
IN  INCOMPLETE  STRIPS  FOR  THE 
RHEOSTAT. 


straight  in  the  bottom  of  a 
river  or  creek.  The  flowing 
water  keeps  the  resistance 
wire  cool. 

§  733.  Home-made  rheo- 
stat of  tin  strips. — A  good 
adjustable  rheostat  for  experi- 
mental purposes  can  be  cheap- 
ly made  by  cutting  tinned 
sheet  iron  into  strips  as  shown 
in  figure  284,  285,  and  nail- 
ing these  strips  to  a  wooden 
frame.  One  end  of  the  con- 
ductor is  fastened  to  one  end 
of  the  sheet,  and  the  other  to 


Cut  in  this  wav  the  tin  plate  is  like  a    A 1  ..     -  , , 

continuous  flat  wire.  the  other  end  of  the  sheet. 

To  make  this  an  adjustable 

rheostat,  a  "jumper"  of  heavy  copper  wire  or  of  sheet  copper 
is  put  across  from  one  sheet  to  the  other  as  shown.  By  this 
means  the  current  can  be  sent  through  as  much  or  as  little  of 
the  resistance  as  desired,  thus  giving  a  great  range  in  the 
amperage.  As  the  surface  is  very  great  in  the  thin  sheet  iron,  the 
air  currents  carry  off  the  heat  developed  so  that  this  rheostat  does 
not  become  unduly  heated.  It  is  a  very  common  form  around 
physical  laboratories,  but  is  bulky  and  not  very  well  adapted  to  a 
magic  lantern  or  a  moving  picture  installation.  Furthermore,  such 
a  rheostat  does  not  fulfill  the  requirements  of  the  National  Elec- 
trical Code,  as  there  is  too  much 
combustible  material  in  connec- 
tion with  it,  and  the  resistance 
is  not  boxed  in. 

§  734.  Rheostats  in  series. — 
If  one  has  two  rheostats,  less 
current  will  be  allowed  to  flow 
if  they  are  connected  to  the  line 
in  series,  that  is,  so  that  all  the 


FIG.  286. 


AN  ELECTRIC  CIRCUIT  AND 
GENERATOR. 


current  must  flow  through  both 


Generator. 
Arc  Lamp. 
Rheostat. 


CH.  XIII] 


RHEOSTATS  AND  OTHER  BALLAST 


rheostats.     According  to  Ohm's 
law  (§  724a),  the  amount  of  cur- 
rent varies  inversely  as  the  resis- 
tance, then  if  two  equal  rheo- 
stats were    used   only  half   as 
much    current    would    flow    as       FlG  28?     RHEOSTATS  ix  SERIES. 
when  one  rheostat  is  used.   Also       G    Dynamo, 
if  the  voltage  is  increased  the      A    Arc  lamp. 

.,,    .  -At.          RI>  R*     Rheostats  in  series,  all   the 

amperage  will  increase  in  the  current  must  pass  through  both  of  them 
same  ratio  provided  the  resis-  (compare  fig.  288). 

mi  The  two  rheostats  R,  and  Ra  are  con- 

tance  remains  constant.      Then  nected  in  s&des  to  get  a  smaller  current 

if  one  has  two  rheostats,   each    than  can  be  obtained  by  the  use  of  one 

t  ^        •  1  ,  r  alone, 

of  the  right  capacity  for  an  arc 

lamp  with  a  no  volt  circuit,  the  two  in  series  would  give  approxi- 
mately the  correct  number  of  amperes  on  a  220  volt  circuit.  The 
amperage  would  be  somewhat  higher  on  the  220  volt  circuit  because 
when  used  singly  on  a  1 10  volt  circuit  each  is  somewhat  reinforced 
by  the  resistance  of  the  arc  lamp.  When  both  are  used  for  one 
lamp  on  a  220  volt  circuit  there  is  not  twice  the  resistance,  hence 
the  amperage  will  be  somewhat  greater  than  with  one  rhostat  on 
a  no  volt  circuit. 


§  735.     Rheostats  in  parallel. 

parallel  as  shown  in  fig.  288,  two 


FIG.  288.   Two  RHEOSTATS  IN  PAR- 
ALLEL, GIVING  Two  PATHS 
FOR  THE  CURRENT. 

G     Dynamo. 

.4     Arc  lamp. 

RI:  R,     Rheostats  in  parallel. 

With  two  or  more  paths  for  the  cur- 
rent, the  total  amperage  will  be  the 
sum  of  the  amperages  going  over  each 
path  (§  735). 


—If  two  rheostats  are  inserted  in 
paths  are  furnished  for  the  cur- 
rent. The  amperage  given  by 
both  will  be  the  sum  of  that  given 
by  each  separately,  for  example, 
if  one  had  two  fixed  rheostats, 
each  one  giving  five  amperes  of 
current,  if  they  were  connected 
with  the  line  in  parallel,  10  am- 
peres would  be  allowed  to  flow. 
On  the  other  hand  if  they  were 
connected  in  series  (fig.  287)  so 
that  all  the  current  had  to  flow 
through  both  of  them  then  only 
2>^  amperes  of  current  would  be 
available.  (See  §  724  a). 


532  RHEOSTATS  AND  OTHER  BALLAST  [Cn.  XIII 

§  736.  Reactors,  inductors,  choke-coils,  economy-coils,  com- 
pensator-coils, etc. — When  alternating  current  is  used  the  wasteful 
method  of  current  control  by  means  of  a  resistor  or  rheostat  where 
so  much  electrical  energy  is  transformed  into  heat  should  be 
avoided  whenever  possible. 

In  place  of  a  rheostat  such  as  is  described  above  (§  723  +  )  an 
inductor  is  used.  This  consists  of  a  soft-iron  core  around  which  is 
wound  a  coil  of  insulated  wire.  The  alternating  current  passes 
through  this  coil;  this  alternately  magnetizes  and  demagnetizes 
the  soft-iron  core  and  limits  the  flow  of  the  current.  But  the 
energy  is  not  dissipated,  for  the  energy  used  in  magnetizing  the 
core  is  given  up  again  when  the  core  is  demagnetized.  It  is  true 
that  a  small  amount  of  the  energy  is  wasted  in  heating  the  appar- 
atus, but  the  amount  is  so  small  (5%  to  8%)  as  compared  with  that 
lost  in  a  rheostat  that  it  is  negligible. 

Variable  amperage  can  be  obtained  with  an  inductor  by  having 
the  soft-iron  core  movable  so  that  a  greater  or  less  amount  of  it 
will  be  within  the  coil. 

The  more  of  the  soft-iron  core  within  the  coil  the  greater  will  be 
the  inductance  and  hence  the  less  the  amperage;  and  conversely, 
the  less  of  the  soft-iron  core  within  the  coil  the  less  will  be  the 
inductance  and  the  greater  the  amperage.  In  fig.  197  the  core 
is  only  partly  inserted  in  the  coil  and  a  medium  amount  of  current 
is  therefore  allowed  to  flow. 


1 


§  737.     Wiring  the  inductor  and  transformer. — The  inductor  is 

inserted  along  one  wire  (in  series) 
exactly  as  the  rheostat  is  inserted 
(fig.  289).  With  a  special  arc 
lamp  transformer  the  line  is  con- 
nected to  the  primary  of  the  trans- 
former and  the  arc  lamp  is  con- 
FIG.  289.  INDUCTOR  IN  SERIES  WITH  ,  ,  ^  ..  .  , 

AN  ARC  LAMP.  nected  to  the  secondary  without 

G    Dynamo.  the  use  of  resistance  (fig.  290). 

=*=     Alternating  current  circuit. 

A     Arc  lamp  with  right-angle  car-        §     738.       Comparison     of     the 

bTlnductortoserveasballastwith  amount  of  energy  used  with  an 
alternating  current.  inductor  and  with  a  rheostat. — (A) 


CH.  XIII]  RHEOSTATS  AND  OTHER  BALLAST  533 

With  an  inductor. — Let  the  line  voltage  be  no  and  the  amper- 
age 55  as  shown  by  the  ammeter;  the  voltage  across  the  arc  will 
be  34  volts.  The  power  consumption  will  be  volts  times  amperes, 
that  is,  in  this  case,  34  x  55  =  1870  watts  or  1.87  kilowatts.  As 
the  inductor  does  not  absorb  an  appreciable  amount  of  energy, 
the  1.87  kilowatts  represents  the  energy  needed  to  produce  the 
arc  light. 

(B)  With  a  rheostat. — If  now  a  rheostat  is  used,  the  watt- 
meter will  record  not  only  the  energy  required  to  maintain  the  arc 
light,  but  also  the  energy  wasted  in  heating  the  rheostat. 

For  example,  suppose  as  above  that  the  line  voltage  is  no,  the 
amperage  55,  and  the  voltage  across  the  arc  is  34.  Then  as  before 
the  arc  light  requires  34x55  =  1870  watts  or  1.87  kilowatts. 

But  the  difference  between  the  34  volts  at  the  arc  and  the  no 
volts  in  the  line  (76  volts)  is  used  in  heating  the  rheostat. 

The  energy  used  in  heating  the  rheostat  is  then  76x55  =  4180 
watts  or  4.18  kilowatts.  Both  this  wasted  energy  as  well  as  the 
actual  energy  used  in  the  arc  will  be  recorded  on  the  wattmeter 
and  the  user  of  the  arc  lamp  will  have  to  pay  for  1.87  +  4.18  or  6. 05 
kilowatts  to  run  his  lamp  instead  of  the  1.87  kilowatts  when  the 
inductor  is  used.  That  is  it  will  cost  more  than  three  times  as 
much  to  run  the  arc  lamp  with  a  rheostat  as  with  an  inductor  or 
choke-coil. 

STATIONARY  TRANSFORMER  FOR  ALTERNATING  CURRENT 

§  739.  Transformer. — A  transformer  is  a  device  for  changing 
the  voltage  of  an  alternating  electric  current.  This  change  may 
be  an  increase  in  the  voltage — step-up  transformer,  or  a  decrease 
in  the  voltage — step-down  transformer.  The  device  consists  in  a 
soft -iron  ring  wound  with  coils  of  insulated  wire.  In  the  simplest 

§  738a  There  is  no  simple  method  of  economizing  with  direct  current 
comparable  with  the  use  of  an  inductor  with  alternating  current.  Sometimes 
when  one  must  draw  on  a  current  at  220  volts  pressure  there  is  used  a  motor 
generator  set.  The  motor  is  driven  by  the  220  volts  current  and  the  genera- 
tor produces  current  at  60  to  70  volts  pressure.  At  this  voltage  only  a 
limited  amount  of  resistance  is  necessary  (§  747),  and  there  is  some  saving, 
but  not  so  much  as  by  using  an  inductor  with  alternating  current. 


534  RHEOSTATS  AND  OTHER  BALLAST  [Cn.  XIII 

case  there  are  two  coils  (fig.  291).     If  an  alternating  current  supply 

is  connected  with  the  primary  coil  an  alternating  current  can  be 

drawn  from  the  secondary  coil. 

The  voltage  and  amperage 
which  can  be  drawn  from  the 
secondary  coil  will  depend  upon 
the  electric  supply  and  upon  the 
relative  number  of  turns  of  wire 

FIG.  290.    USE  OF  A  SPECIAL  TRANS-  m  tne  primary  and  in  the  second- 
FORMER  WITH  AN  ARC  LAMP.          ary  coiis.     if  the  number  of  turns 

G    Dynamo.  is  the  same    in   both,   then    the 

=t=     Alternating  current  circuit.  i ,  j 

T    Transform^.  voltage    and    amperage    remain 

A    Arc  lamp.  practically   the   same    as  if   the 
The  primary  of  the  transformer  is        ••,  ^       . , 

connected  to  the  dynamo  while  the  colls  were  not  present.      In  other 

secondary  is   connected   to   the  arc  words  the  circuit  is  in  every  way 

'aThe  transformer  has  sufficient  "re-  almost  as  if  the  wire  were  contin- 
actance"  to  serve  as  a  ballast  for  the  uous.  If  the  transformer  were 
arc  as  well  as  to  act  as  a  step-down  £  ,  , -u  i±  j 

transformer.  perfect  the  voltage  and  amperage 

would  be  exactly  the  same  as  if  it 

were  not  present.  In  practice  they  are  a  little  less,  but  a  good 
transformer  gives  an  efficiency  of  95%  to  98%. 

If  the  secondary  coil  has  a  different  number  of  turns  from  the 
primary  coil  then  the  voltage  will  vary  directly  as  the  ratio  of  the 
number  of  turns  in  the  two  coils,  and  the  amperage  will  vary 
inversely  as  that  ratio.  That  is,  assuming  that  there  is  no  loss  in 
the  transformer,  the  watts  delivered  will  remain  constant  as  the 
product  of  volts  x  amperes  remains  the  same. 

For  example,  suppose  the  secondary  coil  has  %'th  as  many  turns 
as  the  primary  coil,  then  the  number  of  volts  across  the  secondary 
will  be  J4ih  the  number  across  the  primary  and  the  number  of 
amperes  delivered  by  the  secondary  will  be  four  times  the  number 
drawn  by  the  primary.  If  now  the  primary  is  connected  to  a  220 
volt  line  there  will  be  a  potential  difference  of  one-fourth  that 
number  or  55  volts  across  the  terminals  of  the  secondary  coil. 
Suppose  the  secondary  coil  supplies  60  amperes,  as  might  be  the 
case  with  an  arc  lamp,  then  the  primary  coil  would  draw  one-fourth 


CH.  XIII]  THE  ELECTRIC  ARC  535 

of  this  number,  or  1 5  amperes  from  the  line.     The  watts  in  the  two 
cases  are  theoretically  exactly  the  same. 

The  watts  for  the  primary  are  220x15  =3300. 

The  watts  for  the  secondary  are  55  x  60  =  3300. 

(1)  Volts  secondary  _     Turns  secondary 

Volts  primary  Turns  primary 

(2)  Amperes  primary          Turns  secondary 
Amperes  secondary          Turns  primary 


Secondary 


FIG.  291.     DIAGRAM  OF  A  TRANSFORMER. 

Two  coils  of  a  wire,  Primary  and  Secondary,  are  wound  on  an  iron  ring.  An 
alternating  current  in  the  primary  sets  up  an  alternating  magnetic  flux  in  the 
iron  ring,  which  in  turn  sets  up  an  alternating  electric  potential  in  the  secondary 
coil. 

THE  ELECTRIC  ARC 

§  740.  The  construction  of  an  electric  arc  is  very  simple.  Two 
electrodes  are  taken  which  may  be  made  of  any  conducting  material. 
One  electrode  is  connected  directly  to  one  of  the  wires  of  a  direct 
current  supply  of  over  40  volts,  the  other  electrode  is  connected 
through  a  rheostat  to  the  other  wire  (fig.  280).  When  the  two 
electrodes  are  brought  in  contact  an  electric  current  will  flow 
between  them.  If  now,  the  electrodes  are  slightly  separated,  the 
current  will  not  be  immediately  interrupted,  but  will  flow  through 
the  air  gap  between  the  electrodes. 


536  THE  ELECTRIC  ARC  [Cn.  XIII 

The  exact  nature  of  the  resulting  phenomenon  will  depend  upon 
the  material  of  which  the  electrodes  are  made,  upon  the  voltage  of 
the  current  supply  and  the  resistance  of  the  rheostat,  and  the  kind 
of  gas  surrounding  the  electrodes. 

§  741.  Arc  lamp. — Any  arrangement  for  holding  the  electrodes 
and  feeding  them  together  as  they  wear  away  may  be  called  an  arc 
lamp. 

It  consists  of  three  essential  elements: — (i)  A  clamp  for  holding 
the  positive  electrode;  (2)  A  clamp  for  holding  the  negative  elec- 
trode; (3)  A  mechanism  for  moving  the  holders  and  therefore  the 
electrodes  nearer  together  or  separating  them  farther  apart. 

The  electrode  holders  must  be  insulated  so  that  the  current  must 
flow  through  the  electrodes  and  not  follow  any  short  circuits  (fig. 
270). 

For  the  hand-feed  and  the  automatic  types  of  arc  lamps  see 
Chapter  I,  §  9-11. 

§  742.     With  direct  current,  the  arc  is  made  up  of  three  parts. 

1.  The  arc  stream;    a  highly  heated,  incandescent  gas  which 
conducts  the  current  between  the  electrodes. 

2.  The  positive  crater;    where  the  current  leaves  the  positive 
electrode  to  enter  the  arc  stream. 

3 .  The  negative  crater ;  where  the  current  leaves  the  arc  stream 
to  enter  the  negative  electrode  (fig.  292). 

§  743.  Electrical  behavior  of  the  direct  current  arc. — Measure- 
ment of  the  voltage  drop  in  various  parts  of  the  carbon  arc  reveals 
the  fact  that  the  potential  difference  between  the  two  electrodes 
(§  7 43 a)  is  made  up  of  three  parts.  Starting  from  the  positive 
side,  the  potential  difference  between  the  positive  electrode  and  the 
arc  stream  is  about  3  2  volts.  The  potential  difference  between  the 
arc  stream  and  the  negative  electrode  is  about  9  volts,  thus  the 
potential  difference  between  the  electrodes  with  the  shortest  possi- 
ble arc  is  about  41  volts  (§  743b). 

As  the  arc  is  lengthened  there  is  an  additional  drop  in  potential 
in  the  arc  stream  which  depends  mainly  on  the  length,  but  partly  on 
the  cross  section  of  the  arc  stream.  As  the  arc  length  is  changed, 


CH.  XIII] 


THE  ELECTRIC  ARC 


537 


'  Positive  Crater 

Arc  Stream 
Negative  Crater 


FIG.  292. 


THE  VERTICAL  CARBON  ARC  WITH  20  AMPERES  OF  DIRECT 
CURRENT. 


a  Vertical  carbons  with  the  positive  carbon  above  and  the  negative  carbon 
below.  This  shows  that  the  large  crater  is  on  the  positive  carbon  and  the  small 
crater  on  the  negative  carbon.  Between  the  two  craters  extends  the  arc  stream 
of  hot  gases. 

This  photograph  was  made  with  an  exposure  of  i/ioo  second,  the  aperture 
being  F,  22.  A  color  screen  was  used  to  cut  out  most  of  the  violet,  so  that  the 
arc  stream  would  not  obscure  the  craters.  A  subsequent  exposure  of  90  seconds 
was  made  without  a  color  screen  and  with  an  aperture  of  F/8.  The  illumina- 
tion during  this  exposure  was  by  means  of  a  40  watt,  mazda  lamp. 

b  Vertical  carbons  with  a  20  ampere  direct  current.  No  color  screen. 
Exposure  i/ioo  sec.;  opening  F/22. 

This  shows  the  size  of  the  two  craters;  it  also  shows  the  conical  arc  stream 
almost  as  light  as  the  craters.  This  is  because  the  violet  light  which  has 
relatively  little  effect  in  illumination  has  a  great  effect  on  the  photographic 
plate. 

This  picture  shows  how  the  carbons,  the  craters  and  the  arc  stream  appear 
in  an  instantaneous  view  to  the  photographic  plate,  while  the  one  at  the  left 
(a)  gives  much  more  nearly  the  appearance  to  the  human  eye  with  an  instan- 
taneous view. 


533 


THE  ELECTRIC  ARC 


[Cn.  XIII 


the  change  in  voltage  is  almost  entirely  due  to  the  change  in  the 
length  of  the  arc  stream. 

When  the  arc  is  of  medium  length,  as  for  use  in  projection,  the 
potential  difference  between  the  two  carbons  averages  about  55 
volts.  This  would  mean  that  there  is  a  drop  of  32  volts  between 
the  positive  carbon  and  the  upper  end  of  the  arc  stream,  a  drop  of 
14  volts  between  the  upper  and  lower  ends  of  the  arc  stream,  and  9 
volts  between  the  lower  end  of  the  arc  stream  and  the  negative 
carbon. 

If  the  electrodes  are  made  of  other  substances  than  carbon,  the 
potential  drop  is  differently  distributed.  Thus  in  the  ''Luminous" 


FIG.  293. 


SIDE  VIEW  OF  THE  RIGHT-  ANGLE  CARBON  ARC  WITH  10  and  WITH 
20  AMPERES  OF  DIRECT  CURRENT 


A  1  6  ampere  arc,  B  20  ampere  arc.  The  size  of  the  positive  crater  is 
markedly  larger  with  the  higher  amperage. 

The  lower  pictures  were  made  by  an  instantaneous  exposure. 

The  upper  pictures  were  made  by  a  double  exposure,  that  is,  an  instantaneous 
exposure  with  the  current  on,  to  show  the  craters  and  the  arc  stream,  and  then 
an  additional  exposure  of  90  seconds  with  the  current  off  to  bring  out  the  car- 
bons. For  the  second  exposure  a  40  watt,  mazda  lamp  was  used  for  illuminat- 
ing the  carbons. 


CH.  XIII]  THE  ELECTRIC  ARC  539 

arc  which  consists  of  a  copper  positive  electrode  and  a  negative 
electrode  made  of  a  mixture  of  iron  and  titanium  oxides,  the  lowest 
arc  voltage  is  about  30  volts.  The  lowest  arc  potential  between 
electrodes  of  other  substances  than  carbon  are,  magnetite  30; 
platinum  27;  iron  26;  nickel  26;  copper  23;  silver  15;  zinc  16; 
cadmium  16;  mercury  13. 

The  potential  differences  in  the  arc  lamp  are  practically  constant 
no  matter  what  current  is  flowing,  but  there  is  a  small  change  with 
change  in  current.  This  is  generally  such  that  the  greater  the 
current  the  less  the  potential  difference,  and  may  be  explained  as 
follows : 

Suppose  a  current  of  10  amperes  to  be  flowing  between  the  two 
electrodes  of  an  arc  lamp.  This  will  be  carried  by  a  small  cone 
shaped  mass  of  conducting  gas  (fig.  293.  A).  If  the  current  is 
increased  to  20  amperes  the  extra  heat  developed  is  sufficient  to 
bring  more  air  to  a  high  enough  temperature  to  conduct  current, 
and  the  cone  of  conducting  gas  increases  in  diameter  (fig.  293  B). 

A  large  cone  of  conducting  gas  will  be  losing  heat  at  a  relatively 
less  rate  than  will  a  small  cone,  hence  its  temperature  will  be  higher 
and  its  resistance  will  be  less.  As  a  result  of  the  increased  con- 
ductivity of  the  hot  gases  of  the  arc  stream,  the  greater  the  current 
the  lower  will  be  the  potential  difference  between  the  electrodes. 
There  is  also  a  slight  lowering  of  the  contact  potential  difference 
between  the  electrodes  and  the  arc  stream  as  well  as  a  lessening  of 
the  potential  drop  in  the  arc  stream. 

THE  USE  OF  BALLAST 

§  744.  The  need  of  a  ballast  in  series  with  the  arc  to  control  the 
current. — On  account  of  the  peculiar  electrical  behavior  of  the  arc 
lamp  it  is  necessary  to  use  a  ballast  such  as  rheostat,  or  an  inductor 
in  series  with  the  arc,  or  else  to  use  an  especially  designed  generator. 

§  743a.  While  the  two  electrodes  of  an  arc  lamp  may  be  of  any  conducting 
material,  with  projection  arc  lamps  the  electrodes  are  always  made  of  carbon 
and  are  generally  referred  to  simply  as  carbons. 

§  743b.  These  figures  are  approximations  and  vary  slightly  with  arc 
length  and  current  but  are  general  averages  for  the  usual  arc  lengths  employed : 
3  to  10  mm. 

See  Mrs.  Ayrton,  The  Electric  Arc. 


540  USE  OF  BALLAST  WITH  ARC  LAMPS  [Cn.  XIII 

With  a  metallic  wire,  the  resistance  is  nearly  constant,  and  the 
potential  difference  is  greater  the  greater  the  current  flowing.  Any 
change  in  resistance  is  due  to  the  rise  of  temperature  when  a  current 
is  flowing.  The  higher  the  temperature,  the  greater  the  resistance. 
An  arc,  on  the  other  hand,  has  no  definite  resistance,  but  its  resist- 
ance varies  with  the  current  flowing.  This  variation  is  such  that 


A  B 

FIG.  294.     FACE  AND  LATERAL  VIEWS  OF  THE  RIGHT-ANGLE  CARBON  ARC 
WITH  10  AND  WITH  2O  AMPERES  OF  DIRECT  CURRENT. 

A     With  10  amperes,  B  with  20  amperes  of  direct  current. 

The  size  of  the  crater  in  the  two  cases  is  very  strikingly  brought  out. 

The  middle  figures  had  an  additional  exposure  to  bring  out  the  carbons  (see 
fig.  292-293),  while  the  lateral  views  above  and  the  front  views  below  had  only 
an  instantaneous  exposure. 

The  positive  crater  above  and  the  negative  crater  below  are  clearly  brought 
out  in  all  the  pictures  (see  fig.  292). 


CH.  XIII]  USE  OF  BALLAST  WITH  ARC  LAMPS  541 

the  potential  difference  across  the  arc  remains  nearly  the  same 
regardless  of  how  much  current  is  flowing. 

The  commercial  electric  supply  is  designed  to  furnish  current  for 
incandescent  lamps,  and  is  maintained  at  a  nearly  constant  voltage 
no  matter  how  much  current  is  used.  The  arc  lamp,  on  the  other 
hand,  is  to  be  supplied  by  a  constant  current.  If  one  were  to 
attempt  to  connect  an  arc  directly  to  the  terminals  of  the  supply 


FIG.  295.     LATERAL  AND  FACE  VIEW  OF  THE  RIGHT-ANGLE  CARBON  ARC  WITH 
20  AMPERES  OF  DIRECT  CURRENT. 

No  color  screen  was  used  with  the  lateral  view  so  that  the  arc  stream  would 
show.  In  the  front  view  a  color  screen  was  used  to  bring  out  clearly  the  large 
positive  crater  above  and  the  small  negative  crater  below. 

This  figure  is  for  comparison  with  the  alternating  current  arc  in  fig.  296. 

To  bring  out  the  carbons,  an  additional  exposure  was  made  as  for  fig.  292- 
293- 

line  without  an  intermediate  rheostat,  as  soon  as  the  two  electrodes 
were  brought  in  contact  an  extremely  large  current  would  flow. 
Theoretically,  this  current  would  be  infinite,  but  practically  the 
flow  is  limited  by  the  very  small  resistance  of  the  supply  wires  and 
the  capacity  of  the  dynamo.  In  a  modern  installation  the  current 
would  be  immediately  interrupted  by  the  circuit  breakers  and  burn- 


542 


USE  OF  BALLAST  WITH  ARC  LAMPS 


[Cn.  XIII 


ing  out  of  the  fuses  before  any  serious  damage  could  result.  Even 
after  the  arc  is  burning,  if  one  were  to  remove  the  resistance  by 
short-circuiting  it,  the  current  would  increase  to  an  enormous 
value. 

§  745.     Example  with  110  volt  supply,  using  a  rheostat. — If  we 

assume  that  the  arc  is  of  such  a  length  that  the  potential  difference 
between  the  electrodes  is  10  volts,  and  that  this  potential  difference 


FIG.  296.     LATERAL  AND  FACE  VIEWS  OF  THE  RIGHT-ANGLE  CARBON  ARC 
WITH  25  AMPERES  OF  ALTERNATING  CURRENT. 

By  comparing  this  picture  with  fig.  295  it  will  be  seen  that  in  this  both 
craters  are  of  the  same  size;  and  that,  although  25  amperes  of  current  are 
flowing,  the  crater  on  the  upper  carbon  from  which  the  light  is  derived  is  much 
smaller  than  with  the  direct  current.  The  sizes  of  the  upper  crater  give  a  good 
idea  of  the  amount  of  illumination  furnished  in  the  two  cases. 

An  additional  exposure  was  made  to  bring  out  the  carbons  as  in  fig.  292-293. 

remains  practically  the  same  if  the  current  is  diminished  or  in- 
creased, and  if  the  supply  is  no  volts,  and  that  this  voltage  is 
practically  independent  of  the  current  used,  it  is  evident  that 
between  one  of  the  electrodes  and  one  of  the  supply  wires  there  must 
be  a  potential  drop  of  60  volts.  By  using  a  rheostat  at  this  point 
the  current  is  controlled.  Thus  suppose  that  the  rheostat  has  a 
resistance  of  6  ohms,  then  according  to  Ohm's  law  (§  7243,),  as  the 
potential  difference  across  its  terminals  is  60  volts,  the  current  will 


CH.  XIII] 


USE  OF  BALLAST  WITH  ARC  LAMPS 


543 


be  10  amperes,  V/O  =  A.  Now  suppose  the  arc  length  were 
changed  say  by  bringing  the  electrodes  in  contact.  In  this  case 
there  would  be  the  full  line  voltage,  no  volts  across  the  rheostat 
and  the  current  would  be  no/6  =  18.3  amperes.  Suppose  the  arc 
length  were  increased  until  the  potential  at  the  arc  was  60  volts. 
The  potential  across  the  rheostat  would  then  be  no  —  60  =  50 
volts.  The  current  would  then  be  50/6  =  8.2  amperes.  In  this 
example  the  conditions  are  what  is  known  as  stable,  that  is,  as  the 
arc  length  is  decreased  the  current  is  increased,  but  does  not  reach 
an  infinite  value,  and  as  the  arc  length  is  increased  the  current 
decreases  but  it  does  not  become  zero. 


FIG.  297. 


LATERAL  AND  FACE  VIEWS  OF  AN  INCLINED  CARBON  ARC  WITH  20 
AMPERES  OF  DIRECT  CURRENT. 


This  picture  shows  that  with  the  inclined  carbons  in  proper  position,  the 
positive  crater  on  the  upper  carbon  faces  toward  the  condenser.  It  is  evident 
also  that  as  the  carbon  burns  away  the  crater  will  get  farther  and  farther  above 
the  principal  axis  of  the  projection  apparatus. 

An  additional  exposure  was  made  to  bring  out  the  carbons  as  with  fig.  292- 
293- 


544  USE  OF  BALLAST  WITH  ARC  LAMPS  [CH.  XIII 

§  746.  Line  voltage  exactly  equal  to  arc  voltage. — It  would 
appear  that  it  might  be  desirable  to  use  a  line  voltage  of  exactly 
what  is  required  by  the  arc  and  omit  the  rheostat.  Suppose  in  the 
above  example  that  this  were  done  by  using  a  line  voltage  of  50 
volts.  Now  as  the  arc  voltage  is  constantly  varying  owing  to  slight 
irregularities  in  the  carbons,  to  the  wearing  away  of  the  carbons  and 
to  other  causes,  it  is  evident  that  for  an  instant  the  arc  voltage 
might  drop  below  50  volts  or  it  might  rise  above  50  volts.  If  the 
arc  voltage  should  rise  above  50  volts,  the  arc  would  immediately  go 
out  as  the  supply  is  but  50  volts,  and  if  the  arc  voltage  should  drop 
slightly  below  this  value,  the  current  would  rapidly  increase.  The 
result  would  be  that  the  arc  would  either  go  out  or  else  would  act 
like  a  short  circuit.  In  this  example  the  conditions  are  unstable ; 
that  is,  no  definite  current  can  be  maintained. 

§  747.  Intermediate  voltage. — In  practice  an  intermediate 
voltage  is  sometimes  used,  that  is,  dynamos  to  be  used  for  projector 
arcs  are  sometimes  designed  for  about  70  volts.  Here  the  arc  is 
sufficiently  stable  for  practical  purposes  but  requires  more  atten- 
tion than  with  the  higher  supply  voltage.  Taking  the  above 
example.  The  arc  voltage  at  50  volts  leaves  20  volts  across  the 
rheostat.  To  give  10  amperes  requires  20/10  =  2  ohms  resistance. 
If  now  the  electrodes  are  brought  in  contact  to  start  the  arc  the 
current  will  be  limited  only  by  the  resistance  in  the  rheostat  and 
the  current  will  be  70/2  =35  amperes.  If  the  arc  gets  long  enough 
to  take  60  volts,  the  difference  to  be  taken  up  in  the  rheostat  is  but 
10  volts,  and  the  current  will  drop  off  to  10/2  =  5  amperes.  This, 
therefore,  means  that  with  the  smaller  margin  between  the  line 
voltage  and  the  arc  voltage,  the  arc  becomes  less  stable. 

§  748.  Ballast  with  alternating  current. — With  alternating 
current,  an  inductor  (choke-coil)  is  often  used  instead  of  a  rheostat. 
This  behaves  as  a  ballast  in  a  somewhat  similar  way  to  the  rheostat 
but  to  explain  the  exact  process  of  regulation  would  require  a  more 
exhaustive  discussion  of  alternating  currents  than  is  justified  in 
this  book,  but  see  §  736. 


CH.  XIII] 


USE  OF  BALLAST  WITH  ARC  LAMPS 


545 


546  LIGHT  FROM  THE  ARC  [Cn.  XIII 

THE  LIGHT  PRODUCTION  OF  THE  ARC 

§  749.  Cause  of  light  from  the  arc. — The  light  production  from 
the  carbon  arc  is  due  entirely  to  the  high  temperature  to  which  the 
tips  of  the  carbons  are  raised,  i.  e.,  they  become  white  hot.  The 
practical  problem  in  projection  with  the  arc  deals  with  the  best 
method  of  producing  this  white  heat  and  of  utilizing  it. 

When  the  electric  current  passes  between  the  two  electrodes  the 
heating  effect  in  the  different  parts  is  proportional  to  the  power 
consumed  in  them. 

The  current  being  the  same  in  all  parts,  the  heating  effect  must 
be  in  proportion  to  the  potential  drop  (or  voltage  consumed)  in  the 
different  parts. 

Counting  the  total  drop  55  volts,  it  is  divided  into: 

+  crater  drop  =  32  volts  =  58% 

—  crater  drop  =    9  volts  =  17% 

arc  stream        =  14  volts  =  25% 

Total,  55  volts  100% 

We  see  from  this  that  the  heating  effect  will  occur  principally 
at  the  positive  carbon. 

Carbon  being  rather  a  poor  conductor  of  heat,  the  heat  generated 
within  the  small  area  of  the  crater  must  escape  mainly  by  radia- 
tion. 

At  the  negative  electrode  the  heat  production  is  less  rapid  and 
not  so  high  a  temperature  is  reached. 

Between  the  electrodes  the  heat  production  is  fairly  rapid,  but 
the  hot  gases  of  the  arc  stream  with  the  carbon  arc  are  nearly 
transparent  and  radiate  energy  very  slowly. 

Furthermore  the  violet  lines  of  the  spectrum  in  the  arc  stream 
are  brighter  than  from  the  crater  itself  (§  749  a). 

§  750.  Temperature  of  the  crater. — The  temperature  of  the 
positive  crater  rises  until  such  a  temperature  is  reached  that  carbon 

§  749a.  The  great  brilliancy  of  the  violet  lines  in  the  arc  stream  has  received 
two  explanations:  (i)  That  the  arc  stream  is  higher  in  temperature  than  even 
the  crater  itself;  (2)  That  the  electric  current  passing  through  the  gas  causes 
the  gas  to  glow  irrespective  of  its  temperature.  That  is,  it  causes  electro- 
luminescence as  in  the  vacuum  tube  or  the  aurora  borealis. 


CH.  XIII]  LIGHT  FROM  THE  ARC  547 

is  volatilized.  This  is  the  highest  temperature  which  it  is  possible 
to  obtain  artificially.  The  temperature  of  the  positive  crater  of  the 
carbon  arc  has  been  estimated  at  about  3700°  absolute,  that  is, 
3427°  Centigrade  or  6200°  Fahrenheit  (§  7Soa).  Compare  this 
with  the  temperature  of  the  sun,  about  6750°  absolute,  6477°  C; 
the  acetylene  flame,  2330°  absolute,  2057°  C.;  the  gas  flame,  1830° 
absolute,  1557°  C.  (§  75°b). 

§  751.  Parts  of  the  light  source. — Considered  as  a  light  source, 
the  direct  current  arc  may  be  divided  into  four  parts. 

1.  The  positive  crater. 

2 .  The  negative  crater. 

3.  The  hot  ends  of  the  carbons  adjacent  to  the  craters. 

4.  The  arc  stream. 

The  light  emitted  by  the  hot  electrodes  depends  upon  their  vis- 
ible radiation  being  approximately  proportional  to  the  5th  power 
of  their  absolute  temperature.  The  positive  crater  is  the  hottest 
part  of  the  arc  and  furnishes  most  of  the  light.  The  negative  crater 
furnishes  much  less  light  than  the  positive  crater,  being  smaller  and 
not  as  hot. 

The  carbons  are  white  hot  for  some  distance  away  from  the 
craters  and  furnish  some  of  the  light  of  the  arc.  In  calculating  the 
total  light  from  the  arc  it  would  be  necessary  to  consider  the  entire 
area  included  between  the  line  surrounding  the  positive  carbon 
which  is  at  red  heat  and  the  corresponding  line  on  the  negative 
carbon. 

The  arc  stream  with  the  carbon  arc  emits  but  little  useful  light. 
When  flame-arc  carbons  are  used,  however,  the  greater  part  of  the 

§  750a.     Bulletin  of  the  Bureau  of  Standards,  Vol.  I,  p.  909  and  reprint  8. 

§  750b.  Absolute  temperature. — The  absolute  zero  is  defined  as  the  tem- 
perature at  which  a  perfect  gas  would  exert  no  pressure.  This  is  about- 273° 
centigrade,  i.  e.,  273°  centigrade  below  the  melting  point  of  ice.  In  calcula- 
tions of  high  temperature  and  radiation,  all  formulae  are  based  on  absolute 
temperature,  that  is,  the  temperatures  where  the  zero  is  the  absolute  zero  and 
where  the  degree  is  the  degree  centigrade. 

To  find  the  absolute  temperature  of  a  body  add  273°  to  its  temperature  on 
the  centigrade  scale.  Thus  ice  melts  at  o°  centigrade  or  273°  absolute,  and 
water  boils  at  100°  centigrade  or  373°  absolute.  The  temperature  of  the 
human  body,  37.5°  C.  is  310.5°  absolute.  If  the  absolute  temperature  is  given, 
subtract  273°  from  this  value  to  find  the  centigrade  reading. 


548 


LIGHT  FROM  THE  ARC  [Cn.  XIII 


CH.  XIII]  LIGHT  FROM  THE  ARC  549 

FIG.  299.  SIDE  AND  FRONT  VIEWS  OF  THE  INCLINED  CARBON  ARC  WITH  15 
AMPERES  OF  DIRECT  CURRENT  (EWON'S  AUTOMATIC  LAMP). 

The  upper  carbon  (-f-c)  is  soft-cored  and  18  mm.  in  diameter;  the  lower 
carbon  ( — c)  is  solid  and  12  mm.  in  diameter. 

This  is  to  illustrate  an  automatic  lamp  with  a  magnet  (m)  to  control  the 
magnetic  blow;  the  use  of  a  large,  cored  upper  carbon  (-fc)  18  mm.  in  diame- 
ter; and  a  small  solid  lower  or  negative  carbon  ( — c)  12  mm.  in  diameter. 

Incidentally  there  is  shown  the  wandering  of  the  crater  in  the  right  hand 
lower  picture.  When  the  crater  wanders  in  this  way  the  source  of  light  is 
outside  the  principal  optic  axis. 

Photographed  with  an  instantaneous  exposure  for  the  arcs  and  with  an 
additional  exposure  of  90  seconds  for  the  carbons  and  the  blow  magnet  (see  fig. 
292-293). 

light  is  furnished  by  the  incandescent  gases  of  the  arc  stream. 
Flame-arc  carbons  are  not  ordinarily  used  in  projection. 

For  purposes  of  projection,  only  the  light  from  the  positive  crater 
of  the  direct  current  arc,  or  usually  from  only  one  of  the  craters  of 
an  alternating  current  arc  need  be  considered.  The  large  objective 
of  the  magic  lantern  utilizes  the  light  from  both  carbons  with 
alternating  current  and  this  is  important. 

§  752.  The  alternating  current  arc. — Most  conducting  materials 
when  used  as  the  terminals  of  an  arc  lamp  will  not  allow  a  reversal 
or  even  a  very  short  interruption  of  the  current  without  going  out. 
This  property  is  used  in  the  mercury  arc  rectifier. 

When  carbon  electrodes  are  used,  however,  the  current  may  be 
interrupted  for  a  short  interval,  or  the  current  may  be  reversed 
without  putting  out  the  arc. 

When  the  alternating  current  is  used,  first  one  carbon  and  then 
the  other  is  positive.  Craters  of  equal  intensity  are  formed  on  both 
carbons,  but  neither  is  as  bright  nor  as  large  as  is  the  single  positive 
crater  when  direct  current  of  the  same  amperage  is  used. 

The  light  from  a  single  crater  is  not  steady  but  is  intermittent. 

The  process  during  one  cycle  can  be  described  as  follows : 

When  the  current  is  reversed  so  that,  say,  the  upper  carbon 
becomes  positive,  the  crater  is  fairly  cool.  For  the  short  time  it  is 
the  positive  crater,  its  temperature  rises  very  rapidly.  Whether 
or  not  it  momentarily  reaches  the  temperature  which  it  \vould  if 
permanently  the  positive  crater  is  uncertain.  The  current  dies 
out  and  the  crater  cools  rapidly.  When  the  current  has  reversed 


550  LIGHT  FROM  THE  ARC  CH.  XIII 

its  direction  the  crater  is  negative.  The  heating  effect  of  the 
current  is  small  and  the  carbon  tip  continues  to  cool  until  the 
current  has  again  died  out.  This  cooling  still  continues  until  the 
current  has  again  reversed  its  direction,  and  increased  to  a  con- 
siderable positive  value. 

The  temperature  of  an  alternating  current  arc  crater  is  at  no 
instant  higher  than  that  of  a  direct  current  arc  crater  with  the  same 
amperage,  and,  as  part  of  the  time  its  temperature  is  much  lower 
than  this,  the  average  temperature  will  be  lower  than  with  a 
direct  current  crater,  hence  the  light  will  be  less  and  of  a  yellower 
color. 


FIG.  300.     SOME  POSITIONS  OF  THE  CARBON  ELECTRODES  USED  IN  PROJECTION 

LAMPS. 

A  Vertical  carbons.  This  position  gives  the  least  light  along  the  principal 
optic  axis. 

B     Inclined  carbons. 

C  Horizontal  carbons.  This  arrangement  is  common  for  the  search  light, 
and  for  the  reflectors  used  in  projection  (see  fig.  95). 

D  The  usual  arrangement  for  the  carbons  when  at  right  angles.  The 
upper  or  horizontal  carbon  is  positive  with  direct  current.  1  he  crater  on  it  is 
in  the  optic  axis  and  serves  as  the  source  of  light  with  both  direct  and  alter- 
nating current. 

E  Right-angle  carbons  in  which  the  horizontal,  positive  carbon  is  below. 
This  is  an  unusual  arrangement. 

V  V-arrangement  of  the  carbons  for  alternating  current.  With  this 
arrangement  both  craters  supply  light  for  the  projection  of  lantern  slides  or 
opaque  objects. 

THE  ARC  LAMP  AS  AN  ILLUMINANT 

§  753.  The  arc  lamps  suitable  for  projection  purposes  may  have 
the  carbons  in  any  one  of  five  positions. 

1.  With  inclined  carbons  (fig.  297). 

2.  With  carbons  at  right  angles  (fig.  295). 

3.  With  converging  carbons  (fig.  300). 

4.  With  vertical  carbons  (fig.  292). 

5.  With  horizontal  carbons  along  the  axis  (fig.  300). 


CH.  XIII] 


LIGHT  FROM  THE  ARC 


551 


Most  of  the  light  from  the  arc,  and  all  of  the  light  which  is  useful 
for  projection  comes  from  the  craters  of  the  arc;  from  the  positive 
crater,  if  a  direct  current  arc. 

§  753a.  Table  showing  the  proper  size  of  cored-carbons  for 
different  amperages,  and  the  rate  of  wear  in  millimeters  per 
hour;  also  the  relative  rate  of  burning  in  length  and  in  weight. 

(For  the  small  carbons  to  be  used  on  the  house  electric  light- 
ing system  see  §  123,  131,  417-418.) 

Direct  Current,  Right- Angled  Carbons. 

AUTOMATIC    LAMP 


Amperes 

Size  Carbons 

Burns,  mm.                    Relative 
per  Hour                        Volume 

Relative 
Burning  Weight 

10 

ii  upper 

37                        1.8 

.... 

8   lower 

39                         i 

15 

ii       " 

47.                     ±65 

1.69 

8 

54                         i 

I 

15 

jy.      " 

12.                     J-75 

_i.67 

ii 

25                          i 

i 

20 

14      " 

36                       1.87 

1.77 

ii 

3i 

i                             i 

20 

J4       " 

36 

1-53 

1.36 

ii 

38 

i 

i 

25 

j4      " 

41 

_£^5 

_L-92 

ii 

34 

i 

I 

25 

14      " 

4° 

1.61 

1.31 

ii 

40 

i 

I 

HAND-FEED    LAMP 


15 

ii 

53 

1.65 

I-5I 

ii 

32 

I 

i 

20 

14 

37 

1.62 

.... 

14 

23 

I 

40 

14   " 

48_ 

2.00 

2.0O 

14 

24 

I 

I 

4o 

15   " 

44 

2.2 

2.26 

Ts 

20 

I 

I 

552 


LIGHT  FROM  THE  ARC 
Direct  Current,   Inclined  Carbons. 


[CH.  XIII 


HAND-FEED    LAMP. 


Amperes 

Size  Carbons 

Burns  mm. 
per  Hour 

Relative 
Volume 

Relative 
Burning  Weight 

15 

14  upper 

11 

1.62 

1-33 

ii  lower 

41 

I 

I 

20 

Ji       " 

37 

1.84 

1.77 

ii 

34 

I 

I 

25 

J4      " 

^L 

1.9 

1  .81 

ii 

33 

I 

i 

40 

I  8  cored 

26 

1.8 

2.32 

12  solid 

32 

i 

i 

J5 

14 

28 

1.48 

1.72 

"i4 

19 

i 

i 

40 

H       " 

5ft 

2.4 

2_^2 

H 

22 

i 

I 

14                                             19                                      I                                         I 

40             J4       "                      54.                     .M_                      li-S2. 

14                                             22                                      I                                         I 

Alternating  Current,  Hand-Feed  Lamp. 

RIGHT-ANGLED    CARBONS. 

Amperes 

Size  Carbons 

Burns,  mm.  per  Hour 

20 

ii  upper 

34 

ii  lower 

34 

20 

14     " 

20 

14 

20 

25 

14     " 

20 

H     " 

20 

INCLINED    CARBONS 

25 

14  upper 

24 

14  lower 

26 

30 

J[5     " 

_3_o 

~I5 

30 

35 

15     " 

24 

15 

30 

40 

16     " 

24 

16     " 

28 

CH.  XIII]  CANDLE-POWER  OF  ARC  LAMPS 

CANDLE-POWER  OF  ARC  LAMPS 


553 


§  754.  A  number  of  measurements  of  the  candle-power  of  arc 
lamps  have  been  made,  partly  in  the  Physical  Laboratory  at 
Cornell  University,  and  partly  in  the  Illuminating  Engineering 
Laboratory  of  the  General  Electric  Company  at  Schenectady. 
The  experiments  made  at  Cornell  were  for  the  higher  currents  and 
were  made  primarily  to  ascertain  the  efficiency  of  the  mercury  arc 
rectifier  and  the  power  consumption  with  different  forms  of  ballast 
(§  754a). 


FIG.  301. 


CARBONS  IN  THE  CORRECT  RELATIVE  POSITION  FOR  BOTH  DIRECT 
AND  ALTERNATING  CURRENTS. 


A     Inclined  carbons  in  the  correct  position  for  alternating  current. 

B     Inclined  carbons  in  the  correct  position  for  direct  current. 

C  Carbons  at  right  angles  in  the  correct  position  for  either  direct  or 
alternating  current.  Direct  current  is  indicated. 

D  Carbons  arranged  in  a  V-shaped  position.  For  this  position  alternating 
current  only  is  employed;  and  the  crater  on  each  carbon  contributes  to  the 
light.  The  V  may  be  either  in  a  vertical  or  in  a  horizontal  plane.  The  ver- 
tical arrangement  is  the  more  common. 


§  755.  Variation  of  Candle-Power  with  current. — Candle-power 
measurements  were  made  in  the  horizontal  direction,  that  is,  along 
the  axis  of  the  lantern,  using  different  currents  and  with  both  the 
right-angle  and  the  inclined-carbon  arrangements.  Great  care 
was  taken  to  hold  the  position  of  the  electrodes  and  craters  as 
shown  in  fig.  301,  as  these  positions  furnish  the  greatest  amount  of 
light.  With  direct  current  especially,  it  is  necessary  that  the  crater 


§  754a.     The  results  of  the  Schenectady  tests  were  published  in  the  Electrical 
World,  Oct.  13,  1911. 


554 


CANDLE-POWER  OF  ARC  LAMPS 


[Cn.  XIII 


10          15         20        25        30         35        40 


FIG.  302.     VARIATION  IN  INTENSITY  OF  LIGHT  FROM  PROJECTION  ARC  LAMPS 
WITH  DIRECT  AND  WITH  ALTERNATING  CURRENT. 

x     Right-angle  arc. 

o     Inclined  carbon  arc. 

The  small  dotted  curve  is  for  small  currents  with  the  right-angle  arc  burn- 
ing 6mm.  carbons. 

As  shown  by  these  curves,  the  right-angle  arc  lamp  gives  a  greater  candle- 
power  for  the  same  current  than  does  the  inclined  carbon  arc  lamp,  with  both 
direct  and  alternating  current.  Also  that  direct  current  gives  about  four  times 
the  light  that  the  same  number  of  amperes  of  alternating  current  gives. 


CH.  XIII]  CANDLE-POWER  OF  ARC  LAMPS  555 

face  forward  as  is  shown  in  figure  297  for  the  inclined  electrodes  and 
in  figures  294-296  for  the  right-angle  arc.  The  results  of  these 
measurements  are  shown  in  curve  form  in  fig.  302.  These  curves 
show  that  the  greater  the  current  the  greater  is  the  amount  of  light 
given  by  the  arc.  The  increase  in  light  is,  however,  more  rapid 
than  the  increase  in  current  and  no  simple  mathematical  statement 
of  the  relationship  is  possible.  The  crosses  indicate  the  individual 
measurements  with  the  right-angle  arrangement  and  the  circles, 
measurements  with  the  inclined  carbon  arc.  The  upper  curve  is  for 
the  right-angle  arc  with  direct  current.  In  this  case  the  highest 
candle-power  for  the  same  current  (amperage)  is  obtained.  The 
next  curve  is  for  the  inclined  carbon  arc  with  direct  current.  The 
light  is  not  quite  as  much  with  this  arrangement  as  with  the  right- 
angle  arc. 

The  two  lower  curves  are  for  alternating  current..  It  will  be 
noticed  that  there  is  a  greater  difference  in  candle-power  depending 
on  the  electrode  arrangement  with  alternating  than  with  direct 
current.  The  short  dotted  part  of  the  curve  for  the  right-angle 
arrangement  is  for  6  mm.  carbons  and  small  currents,  while  the 
main  part  of  the  curve  is  for  larger  carbons. 

A  table  showing  the  results  of  the  individual  measurements 
might  be  misleading,  as  large  variations  in  the  light  of  the  arc  are 
continually  occurring  and  a  given  measurement  might  be  made 
when  the  arc  was  giving  its  greatest  or  its  least  light.  For  this 
reason  the  values  given  in  the  table  (§  756)  for  the  candle-power  of 
the  arc  with  different  currents  were  taken  from  the  curve  instead  of 
being  from  individual  observations.  These  values  are  good 
averages  and  may  be  accepted  as  close  enough  to  the  actual  candle- 
powers  for  all  practical  purposes  in  projection. 


556  CANDLE-POWER  OF  ARC  LAMPS  [Cn.  XIII 

§  756.     Table  of  Candle-Power  and  Current  with  Arc  Lights. 


Size  Carbons 

Amperes 

Direct  Current 
Carbon? 

Alternating  Curient 
Carbons 

Right-angle 

Inclined 

Right-angle 

Inclined 

6  mm. 

2 

2OO 

3           i           400 

4 

650 

100 

5 

900 

200 

8  mm. 

7-5 

1,500 

1,400 

400 

300 

ii  mm. 

10 

2,200          |         1,900 

500 

4OO 

12.5 

2,900                  2,500 

600 

450 

15 

3,700            3,200 

700 

500 

17-5 

4.500 

3,800 

950 

625 

13  mm. 

20 

5400 

4,550 

I,2OO 

750 

25 

7-500 

6,200                 1,750 

1,100 

15  mm. 

30 

9,500 

8,100            2,300 

1,400 

35 

10,000 

3,ooo 

1,900 

40 

12,000 

2,500 

t 

45 

3,200 

50 

3,700 

60 

1 

4,800 

§  757.    Direct  current;  inclined  electrodes. 


WATTS 

Amps 

Volts 

Watts 

no-Volt  Line 
With  Resistance 

Candle-Power 

15 

50 

750 

1,650 

3-490 

20 

50 

1,000 

2,200 

4,900 

25 

51 

1,270 

2,750 

6,220 

30 

53 

1,590 

3,300 

8,750 

40 

5i 

2,040 

4,400 

12,350 

Mean 

5i 

§  758.    Direct  current;  electrodes  at  right  angles. 


WATTS 

Amps 

Volts 

Watts 

no-Volt  Line 

Candle-Power 

With  Resistance 

IO 

56 

560 

1,100 

2,300 

15 

50 

750 

1,650 

3-680 

20 

52 

1,020 

2,200 

6,230 

25 

62 

1,550 

2,750 

7»5«) 

30 

58 

1,740 

3,300 

10,150 

Mean 

55-6 

CH.  XIII]  CANDLE-POWER  OF  ARC  LAMPS  557 

§  759.     Alternating  current;  inclined  electrodes. 


LINE  WATTS 

Amps 

Volts                Watts 

With  Trans- 

Candle- 
Power 

With  Resistor 

former.  96  per 

cent  Efficiency 

20                          28                         560                    2,200 

585 

620 

25 

27-5 

687              2,750 

715 

894 

30 

26.5 

795              3,300 

830 

1,700 

40 

27 

i,  080              4,400 

U30 

1,830 

50 

35 

1,750              5,500 

1,830 

4,566 

60 

32 

1,920              6,600 

2,000 

4,650 

Mean 

29.2 

Power  factor  (P.  F.)  at  arc  nearly  i.oo. 

§  760.    Alternating  current;  electrodes  at  right  angles. 


10 

15 

20 
25 
30 

Mean 

44 

42 

47 
57 
57 

430 
600 
920 
i,370 
i,  600 

1,100 
1,650 
2,2OO 
2-750 
3,300 

450 
625 
960 
1430 
1,670 

590 
763 
1,050 
1,690 
2,540 

49-6 

Pcrwer  factor  (P.  F.)  at  arc  0.964. 

§  761.    Rectifier; 

inclined  electrodes. 

DIRECT  CURRENT 
SECONDARY 

ALTERNATING  CURRENT  PRIMARY 

Amp? 

Volts 

Watts 

Amps 

Volts 

Watts 

Volt- 
Amps 

P.  F. 

Eff. 

C.  P. 

s 

25 
30 
40 

5i 
54-5 
54 
62 
52 

765 
1,090 
1,350 
1,  860 
2,100 

7 
9-5 

12 
14-5 
19 

J75 
1  88 
194 

220 

215 

1,100 

1,500 
1,900 
2,600 
3,120 

1,225 
1,786 
2,330 
3,190 
4,070 

.898 
.84 
.816 
.816 
.768 

•695 
.727 
.711 
.716 
.672 

3,100 
4,720 
6,470 
8,600 
12,150 

Mean 

54-7 

.828 

.704 

558 


CANDLE-POWER  OF  ARC  LAMPS 


[Cn.  XIII 


§  762.    Rectifier;  Electrodes  at  right  angles. 


10 

58 

58o 

5-5 

195 

850 

1,070 

•794 

•683 

1,900 

15 

45 

675 

7 

1  80 

1,000 

1,260 

•793 

•675 

3,000 

20 

5i 

1,020 

10 

203 

1,500 

2,030 

•739 

.680 

5,600 

25 

66 

1,650 

12 

235 

2,300 

2,820 

.816 

.718 

7.370 

30 

62 

i,  860 

14 

233 

2,600 

3,260 

.798 

.716 

9.450 

Mean 

56.4 

I 

.786 

.694 

§  763.  Power  in  kilowatts  drawn  from  the  line  for  different 
values  of  light.  Inclined  electrodes,  110-volt  supply,  transformer 
96  per  cent  efficiency. 


KILOWATTS 


Candle-Power 

D.  C. 

at  arc 

D.  C. 

Resist. 

A.  C. 

Trans. 

A.  C. 
Resist. 

Rectifier 

I,OOO 

.6 

2-7 



1,500 

.8 

3-2 

•7 

2,000 

•4 

I.I 

i.i 

3-75 

.8 

2,500 

•55 

i-3 

1.2 

4-3 

•9 

3,000 

.6 

i-5 

1.4 

4.9 

i.i 

4,000 

.76 

1.9                1.7 

5-8 

1-3 

5,000 

i.i 

2.25              2.0 

6.9 

i-5 

6,000 

1.2 

2.6 

1.8 

7.500 

i-45 

3-1 



2-15 

10,000 

1.8                 3.8 

2-75 

§  764.     Light    given   for    different   values  of    kilowatt    con- 
sumption. 


Kilowatts 


Candle-Power 


5.5°° 

3.200 

!-5 
2.0 

12,000 

3,OOO 
4.300 

3.40° 
4,800 

500 

6,900 

3-° 

7»3°o 

1,300 

4.0 

.>u 

§  765.  Candle-power  measurements  with  direct  current  sup- 
plied by  a  mercury  arc  rectifier. — By  using  a  mercury  arc  rectifier 
to  convert  alternating  current  to  direct  current,  very  nearly  the 
same  light  intensity  is  obtained  as  if  the  same  amperage  of  direct 


CH.  XIII] 


CANDLE-POWER  OF  ARC  LAMPS 


559 


current  were  supplied  by  a  dynamo.     This  is  shown  in  figures  303- 
304  and  in  the  tables  which  give  the  results  of  the  Schenectady 

tests  (§  757-764). 

§  766.  Relation  between  the  power  consumption  and  candle- 
power.— Besides  the  current  passing  through  the  arc,  it  is  necessary 
to  know  the  power  consumption,  as  it  is  the  power  consumption 
which  determines  the  cost  of  maintaining  the  arc. 

With  direct  current,  the  right-angle  arc,  for  example,  gave  2300 
candle-power  and  required  56  volts  potential  difference  at  the  arc. 
This  means  a  power  consumption  of  560  watts  at  the  arc  with  10 
amperes.  Under  most  circumstances,  however,  the  current  would 
be  supplied  from  a  no  volt  line  and  this  would  represent  power 


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FIG.  303.     RELATION  BETWEEN  CURRENT  AND  CANDLE-POWER. 

^4  The  candle-power  variation  with  right-angle  carbons,  with  alternating, 
direct  and  rectified  current. 

B  The  candle-power  variation  with  inclined  carbons  with  alternating, 
direct  and  rectified  current. 

These  curves  show  that  rectified  and  direct  current  give  approximately  equal 
illumination  and  that  alternating  current  gives  a  much  lower  candle-power  with 
a  given  amperage. 


560  CANDLE-POWER  OF  ARC  LAMPS  [Cn.  XIII 

drawn  from  the  line  to  the  extent  of  15x110  =  1650  watts. 
Hence,  in  fig.  304  there  are  drawn  two  curves  for  direct  current,  one 
for  the  power  consumed  at  the  arc,  and  the  other  for  the  power  drawn 
from  the  line  with  a  no  volt  supply  when  used  with  resistance. 

With  alternating  current  there  are  even  more  possibilities. 
There  is  the  power  consumed  at  the  arc,  the  power  drawn  from  the 
no  volt  line  with  resistance,  and  the  power  drawn  from  the  line  if  a 
suitable  transformer  of  high  efficiency  is  used.  In  calculating  the 
power  consumption  when  using  a  transformer  the  actual  power 
consumed  at  the  arc  was  divided  by  the  efficiency  of  the  trans- 
former. Thus  with  10  amperes  alternating  current  the  right-angle 
arc  consumed  430  watts  at  the  arc.  The  transformer  had  96% 
efficiency,  hence  the  power  drawn  from  the  line  was  430  -=-  .96  = 
450  watts.  .  In  addition,  curves  were  drawn  showing  the  power 
consumption  when  a  rectifier  was  used. 

§  767.  Results. — The  results  as  shown  in  figures  302-304  are, 
that  with  the  same  amount  of  power  drawn  from  the  line,  the  least 
light  is  given  when  alternating  current  is  used  with  a  rheostat  and 
the  most  when  alternating  current  is  used  with  a  rectifier.  With 
the  right-angle  arrangement  there  is  more  light  for  the  same  power 
with  direct  current  and  a  rheostat,  than  with  alternating  current 
and  a  transformer,  but  with  inclined  carbons  there  is  but  very  little 
difference  in  the  light  given  for  the  same  power  supplied,  whether 
alternating  current  is  used  with  a  transformer  or  direct  current  is 
used  with  a  rheostat.  It  is  to  be  noticed,  however,  that  by  using 
sufficient  power  it  is  possible  to  get  more  light  by  the  use  of  direct 
than  with  alternating  current. 

The  power  drawn  from  the  line  depends  on  the  power  consumed 
at  the  arc  and  the  efficiency  of  the  ballast  or  transforming  device. 

§  768.  Efficiencies  with  different  arrangement  of  carbons,  and 
different  forms  of  current. — The  efficiencies  of  these  devices  are : 

With  right-angle        Inclined 
carbons  carbons 

Direct  Current  and  rheostat  =  50%  =46% 

Alternating  Current  and  rheostat  =45%  =  2  7  % 

Alternating  Current  and  transformer  =  96%  =  96% 

Alternating  Current  and  rectifier  =  70%  =  70% 


CH.  XIII] 


CANDLE-POWER  OF  ARC  LAMPS 


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FIG.  304.     RELATION  BETWEEN  POWER  CONSUMPTION  AND  CANDLE-POWER. 

A     Lamp  with  right-angle  carbons. 

Alternating  current  with  a  rheostat  gives  the  least  light. 

Alternating  current  with  a  transformer  gives  more  light  than  when  a  rheostat 

is  used. 
Alternating  current  with  a  rectifier  gives  the  greatest  amount  of  light. 


562  CANDLE-POWER  OF  ARC  LAMPS  [Cn.  XIII 

Direct  current  with  a  rheostat  gives  less  light  than  alternating  current  with  a 

rectifier. 

Direct  current,  if  only  the  power  consumed  at  the  arc  is  counted,  gives  the 
greatest  illumination  of  all  for  a  given  power  input,  (left  upper  curve), 
i.  e.,  10,000  candle-power  for  less  than  two  kilowatts  of  power. 
B     Lamp  -with  inclined  carbons. 

Alternating  current  with  a  rheostat,  the  least  light. 
Direct  current  with  rheostat,  next. 
Alternating  current  with  a  transformer,  next. 

Alternating  current  with  a  rectifier  gives  the  greatest  illumination  for  the 
power  consumed. 

The  upper  left  hand  curve  shows  that  direct  current  gives  the  greatest 
amount  of  light  if  only  the  power  consumed  by  the  arc  is  considered  and  that 
wasted  in  the  rheostat  is  not  counted. 

If  the  sets  of  curves  for  the  right-angle  lamp  and  those  for  the 
inclined-carbon  lamp  are  compared  it  will  be  found  that  the  right- 
angle  lamp  gives  the  most  light  for  the  same  current  in  every  case. 
The  light  given  for  the  same  power  input  is  the  same  with  rectified 
current  for  both  styles  of  lamp.  With  either  alternating  or  direct 
current  and  resistance,  the  right-angle  lamp  gives  the  greater  light, 
but  with  alternating  current  and  a  transformer  the  right-angle  lamp 
gives  less  light.  This  is  due  to  the  higher  voltage  of  the  right-angle 
arc  when  used  with  alternating  current,  the  right-angle  arc  requir- 
ing about  50  volts  while  the  inclined  carbon  arc  requires  but  30 
volts. 

In  the  table  (§  763)  is  shown  the  power  in  kilowatts  drawn  from 
the  line  for  different  intensities  of  the  light.  This  table  was  made 
from  the  curves  in  fig.  3046  and  applies  to  the  inclined  carbon 
lamp,  with  no  volt  supply. 

In  the  table  (§  764)  is  shown  the  candle-power  for  different 
amounts  of  power  consumption. 

§  769.  Distribution  of  intensity  in  the  different  directions  with 
the  different  forms  of  projection  arc. — Fig.  305-306  show  the  dis- 
tribution of  light  around  the  different  forms  of  arc  lamp.  The 
distance  from  the  center  to  the  curved  line  gives  the  candle-power 
of  the  lamp  in  the  given  direction.  Fig.  305  shows  that  the  right- 
angle  arc  has  3, 750  c.  p.  in  a  horizontal  direction,  4,000  c.  p.  15° 
below  the  horizontal,  and  2,900  c.  p.  15°  above  the  horizontal. 
These  curves  show  the  results  of  actual  experiments.  The  light 
coming  mostly  from  the  crater,  a  slight  change  in  the  position  of  the 


CH.  XIII]  CANDLE-POWER  OF  ARC  LAMPS  563 


iao 


FIG.  305.     DISTRIBUTION  OF  LIGHT  INTENSITY  ABOUT  RIGHT-ANGLE  ARCS. 


564  CANDLE-POWER  OF  ARC  LAMPS  [Cn.  XIII 

o     Direct  current  (D.  C.). 

x    Alternating  Current  (A.  C.). 

The  direction  of  a  given  point  on  the  curve  represents  the  direction  in  which 
the  light  intensity  was  measured.  The  distance  of  the  point  from  the  center 
of  the  figure  represents  the  intensity  in  the  given  direction.  For  example,  15° 
above  the  horizontal  the  direct  current  arc  has  2,900  candle-power  while  the 
alternating  current  arc  has  850  candle-power. 

The  numbers  around  the  outside  represent  the  angle  in  degrees  while  those 
on  the  radius  represent  candle-power. 

carbons  or  the  angle  of  the  craters  on  the  carbons  causes  a  great 
change  in  the  distribution  of  light. 

§  770.  Right-angle  electrodes.  —  If  the  right-angle  arc  is  used, 
take  care  to  hold  the  crater  in  the  best  position,  i.  e.,  facing  the 
condenser,  otherwise  a  poor  light  will  result.  Fig.  294-295  show 
about  the  best  position  which  can  be  maintained.  The  distribu- 
tion from  this  arc  with  direct  current  is  shown  in  fig.  305.  The 
distribution  of  light  with  an  alternating  current  right-angle  arc  is 
shown  in  fig.  306. 

§  771.  Converging  electrodes.  —  The  distribution  of  light  with 
converging  carbons  (55°)  with  alternating  current  is  shown  in  fig. 


CANDLE-POWER  OF  ARC  LAMPS 

§  773.  Intrinsic  brilliancy  of  the  crater.  —  Blondel  found  that 
the  intrinsic  brilliancy  of  the  positive  crater  of  the  carbon  arc  was 
nearly  constant,  irrespective  of  the  current,  at  about  158  candle- 
power  per  square  millimeter  for  solid  carbons,  and  130  candle-power 
per  square  millimeter  for  cored  carbons.  This  is  equivalent  to 
97,000  candle-power  per  square  inch  for  solid,  and  84,000  candle- 
power  for  cored  carbons  (§  773a). 

The  increase  in  candle-power  of  the  arc  caused  by  an  increase  in 
current  is  due,  not  to  an  increase  in  the  brightness  of  the  crater, 
but  to  an  increase  in  its  area.  This  is  illustrated  in  fig.  294,  which 
shows  a  photograph  of  a  right-angle  arc  with  10  amperes  and  with 
20  amperes  direct  current.  The  increase  in  the  size  of  the  crater 
is  apparent. 

As  has  been  pointed  out  elsewhere  (Ch.  IX,  XIV),  with  small 
openings  such  as  with  microscopic  objectives,  when  the  crater 


CH.  XIII]  CANDLE-POWER  OF  ARC  LAMPS 

Ob 


565 


FIG.  306.     DISTRIBUTION  OF  LIGHT  INTENSITY  ABOUT  ALTERNATING  CURRENT 
ARCS  WITH  CARBONS  AT  90  AND  AT  55  DEGREES. 


566  CANDLE-POWER  OF  ARC  LAMPS  [Cn.  XIII 

90°     Right-angle  arc  (dotted  lines). 

55°     Arc  with  V-arranged  carbons  (full  lines). 

The  numerals  around  the  semicircle  represent  degrees,  while  those  along  the 
middle  radius  represent  candle-power.  It  is  to  be  noted  that  with  the  V- 
arrangement  where  both  craters  supply  light  that  there  is  considerable  gain 
over  the  right-angle  arrangement. 

image  becomes  too  large  to  enter  the  opening  (objective  front), 
there  is  no  advantage  to  be  gained  by  increasing  the  current,  as 
this  merely  increases  the  size  and  not  the  brightness  of  the  crater 
and  the  crater  image. 

§  774.  Visible  and  invisible  radiation. — It  is  a  well  known  fact 
that,  of  the  total  energy  supplied  to  an  arc  lamp,  but  a  small  part 


FIG.  307.     NORMAL  SPECTRUM  ILLUSTRATING  THE  SEGMENT  OF  RADIATION 

WHICH  is  VISIBLE. 

The  longest  radiation  represented  in  this  diagram  has  a  wave-length  of  2  /*• 
and  is  at  the  base  of  the  triangle.  The  intermediate  wave-lengths  occur  in 
regular  sequence. 

The  segment  of  visible  radiation  occurs  between  wave-lengths  .68  /*  and  .40  ^. 
Other  waves  shorter  than  .40  M  form  the  ultra-violet,  and  those  longer  than 
.68  /*  the  infra-red  part  of  the  spectrum. 

Under  some  conditions  waves  longer  than  .68  /*  and  shorter  than  .40  M  may 
be  seen,  but  the  radiation  for  useful  vision  falls  between  those  wave-lengths. 
The  height  of  the  lines  in  this  diagram  represents  the  wave-lengths  magnified 
20,000  times  at  that  particular  point  in  the  spectrum. 

If  the  visible  radiation  is  passed  through  a  prism  or  a  diffraction  grating,  the 
wave-lengths  are  arranged  in  regular  sequence  from  the  longest  to  the  shortest 
as  shown  in  the  diagram.  The  longest  visible  waves  appear  red  to  the  normal 
eye  and  the  shortest  violet,  with  the  orange,  yellow,  green,  blue,  and  indigo  in 
between. 


§  773a.     Blondel,   Proceedings  of  the   International  Electrical  Congress. 
Chicago,  1893. 

Bulletin  of  the  Bureau  of  Standards,  Vol.  I,  p.  122  and  reprint  8. 


CH.  XIII]          RADIANT  EFFICIENCY  OF  ARC  LAMPS  567 

appears  in  the  form  of  radiation  visible  to  the  eye  as  light.  A  large 
amount  of  energy  is  radiated  in  the  form  of  ether  waves  of  such 
great  length  that  they  do  not  effect  the  eye  and  are  called  infra-red 
radiation.  A  small  amount  of  energy  is  radiated  in  the  form  of 
very  short  invisible  waves  capable  of  exciting  fluorescence  and 
affecting  a  photographic  plate,  this  is  called  ultra-violet  (fig.  307). 

RADIANT  EFFICIENCY  OF  ARC  LAMPS 

§  775,  776.  In  1911  some  experiments  were  made  to  determine 
the  entire  energy  radiated  by  the  arc,  and  the  relation  of  this  energy 
to  the  visible  part  of  the  radiation  (§  776a). 

Briefly,  the  method  consisted  in  getting  side  by  side  two  patches 
of  light,  which  are  photometrically  equal.  One  of  these  patches 


FIG.  308.     ARRANGEMENT  OF  APPARATUS  TO  MEASURE  L'/R. 
(From  tfie  Physical  Review). 

Energy  from  the  source  L  can  reach  the  therrno- junction  of  the  radiomi- 
crometer  Ra  by  either  of  two  paths,  (a)  direct,  no  absorption  except  by  air, 
(b)  through  the  prism  train  P. 

Light  from  the  source  is  focused  by  the  condenser  Cr  on  the  adjustable  slit 
SIf  is  rendered  parallel  by  the  lens  C2,  dispersed  by  the  prism  P  and  focused  as 
a  spectrum  R-  V  by  the  mirror  MT.  The  screen  S2  is  placed  in  the  red  end  of  the 
spectrum  so  that  it  cuts  off  all  of  the  infra-red  to  .68ft.  The  mirror  M 2  reassem- 
bles the  spectrum  to  a  patch  of  white  light  at  the  radiomicrometer. 

The  intensity  of  the  patch  of  direct  light  is  fixed  by  the  brightness  and  dis- 
tance of  the  source  L,  but  that  of  the  other  patch  Wean  be  varied  by  widening 
or  narrowing  the  slit  5,  until  it  is  of  the  same  brightness  as  the  direct  light. 

The  prism  consists  in  a  .60°  hollow  prism  of  carbon  bisulfide  immersed  in  a 
square  glass  cell  filled  with  distilled  water.  It  gives  a  good  dispersion  with  a 
deviation  of  but  20°  from  a  straight  line. 

The  lenses  are  of  glass.  The  mirrors  are  plano-concave  lenses,  silvered  on 
the  concave  side.  The  focal  length  cf  Ml  is  50  cm.  and  of  MI  is  25. 


$68  RADIANT  EFFICIENCY  OF  ARC  LAMPS          [Cn.  XIII 

falls  on  the  comparison  screen  either  directly,  or  after  passing 
through  an  8  cm.  layer  of  water,  as  the  case  may  be.  The  other 
patch  of  light  is  robbed  of  all  of  its  infra-red  by  the  system  of 
prisms  and  lenses  shown  in  fig.  308. 

The  energy  in  these  two  light  patches  was  measured  by  a  radio- 
micrometer.  The  screen  S2  could  be  set  to  remove  all  of  the  infra- 
red radiation  to  any  desired  point  in  the  spectrum.  In  this  work, 
after  careful  experiment,  it  was  decided  to  adopt  the  wave-length 
.68[L  as  best  representing  the  dividing  line  between  the  visible  part 
of  the  spectrum  and  the  infra-red.  The  screen  was  accordingly  set 
to  remove  all  radiation  of  greater  wave-length  than  this. 

By  this  method  it  was  possible  to  measure  the  energy  repre- 
sented by  the  total  radiation  of  the  arc,  and  that  of  the  visible 
portions.  It  was  also  possible  to  insert  a  water-cell  between  the 
source  L  and  the  radiomicrometer  and  compare  the  light  energy, 
with  that  part  of  the  energy  passing  through  an  8  cm.  layer  of 
water.  In  order  to  simplify  the  discussion,  the  total  radiation  of 
the  arc  is  called  R,the  portion  getting  through  the  8  cm.  water-cell 
is  called  W  and  the  luminous  energy  is  called  L.  The  measure- 
ments were  made  in  such  a  way  that  the  ratio  of  L/R  or  radiant 
efficiency  was  determined,  or  else  the  ratio  of  L/W  was  measured. 

In  addition  to  these  values,  the  transmission  of  layers  of  water  of 
different  thickness  was  measured,  that  is,  the  ratio  of  W/R  was 
determined.  This  ratio  is  called  "the  Water-Cell  Efficiency"  and 
was  determined  for  a  layer  of  water  8  cm.  thick.  The  transmission 
of  layers  of  other  thicknesses  is  shown  in  §  849. 

§  777.  The  results  of  these  measurements  for  various  sources 
are  shown  in  the  table  (§  778).  The  most  important  values  are  for 
the  positive  crater  of  the  right-angle  carbon  arc  and  for  the  right- 
angle  arc  with  alternating  current. 

The  positive  crater  shows  a  radiant  efficiency  (L/R)  of  roughly 
10%,  that  is  10%  of  the  energy  radiated  is  visible  as  light,  the  other 
90%  of  the  energy  is  mostly  in  the  infra-red.  "The  water-cell 
efficiency"  (W/R)  varies  from  18%  to  28%,  averaging  roughly 
25%,  that  is,  one  quarter  of  the  energy  radiated  gets  through  the 

§  776a.  See  H.  P.  Gage,  The  Radiant  Efficiency  of  Arc  Lamps,  Physical 
Review,  Vol.  33,  p.  in,  Aug.,  1911. 


CH.  XIII]  RADIANT  EFFICIENCY  OF  ARC  LAMPS 


569 


water-cell.  Of  this  25%,  43%  is  light  and  the  rest  is  infra-red. 
This  shows  the  advantage  of  using  a  water-cell  as  there  is  only 
one  quarter  the  heating  effect  with  the  water-cell  as  without  it. 

With  the  alternating  current  arc,  the  corresponding  figures  are 
approximately;  Radiant  efficiency  (L/R)  6.4%,  Water-cell  effi- 
ciency (W/R)  15.6%,  and  of  the  energy  getting  through  the  water- 
cell  (L/W)  41%  is  light.  In  this  case  it  is  seen  that  the  water-cell 
removes  an  even  greater  proportion  of  energy  and  hence  its  bene- 
ficial effect  is  even  greater  with  alternating  current  than  with 
direct  current.  For  the  practical  application  of  these  values,  see 
§  850,  and  fig.  342. 

§  778.  Table  showing  the  relation  of  light  energy  to  the 
total  radiation  of  various  light  sources. 

(From  the  Physical  Review,  August,  1911) 


Source 

I 

C.  P. 

L/W 

Per 
Cent. 

W/R 

Per 

Cent. 

L/R 

Per 
Cent. 

WATTS 

As  RADIATED 

AT  100%  EFF. 

R 

L 

W.P.C. 

£ 
pj 

u 

0* 

pj 

\t 

£ 

cu 
0 

jj£| 

_j 

Carbon  arc 

7-5 

1550 

18-5 

7-9 

607 

48 

•39 

2.6 

.031 

32 

10 

2300 

42.9 

21.7 

9-3 

785 

73 

•34 

3-0 

.032 

31 

-f  Crater    .  .  . 

15 

3850 

27.0 

n.6 

1148 

133 

•30 

34 

•035 

29 

20 

5600 

28.7 

12.3 

1614 

199 

.29 

3-5 

.036 

28 

—  Crater    .  .  . 
A.C. 

(est.) 

40 

8.25 

3-3 

30 

377 

Shaded    .  .  . 

15 

700 

40.7 

17.8 

7.2 

457 

33             -65 

i-5 

.047 

21 

20 

1200 

20.9 

8-5 

618 

53 

•5i 

2.0 

.044 

23 

Entire  

15 

700 

15-6 

6-3 

590 

37 

.84 

1.2 

.053 

19 

21 

264 

Arc  stream  .  .  . 

21 

6-5 

Flame  arcs 

Entire  arc 

Yellow  ....    13.5 
White    ....  |  13.5 
Arc  stream 

2580 
1440 

57-1 

45-7 

26.9 
31-8 

15-4 
14.6 

430 
476 

66 
69 

•17 

•33 

6.0 

3-o 

.026 
.048 

39 

21 

290 
264 

Yellow  13.5 

79 

49-5 

39 

White             13.5 

54-5 

50-5 

27o 

Nernst  through 

copper  sulphate 

26.4 

100 

.665 

.025 

39-5 

497 

Hefner  (AngstrOm) 

•9 

•363         11.3 

.032 

12.3 

.08 

.047 

21.3 

268 

A.  C.     Alternating  current. 
Amps.     Amperes. 
C.  P.     Candle-Power. 


570  ENERGY  FOR  MOVING  PICTURES  [Ce.  XIII 

L/W  The  ratio  of  the  luminous  energy  (L)  and  the  total  energy  getting 
through  the  water-cell  (W)  (Water-cell  8  cm.  thick). 

W/R  The  ratio  of  the  energy  getting  through  the  water-cell  (W)  and  the 
total  energy  (R)  radiated  by  the  light  source. 

L/R  Ratio  of  light  energy  (L)  and  the  total  energy  (R)  radiated  by  the 
source. 

R     Total  energy  radiated  by  the  source. 

L     The  light  energy  radiated  by  the  source  (fig.  307). 

W.  P.  C.  Number  of  watts  required  for  each  candle-power  with  the  different 
sources. 

C.  P.  W.  Number  of  candle-power  given  by  each  watt  with  the  different 
sources. 

In  the  right-hand  column  are  given  the  meter  candles  or  lumens  for  each  watt 
of  energy  in  the  luminous  part  of  the  spectrum  with  the  different  sources. 


CALCULATION  OF  THE  ENERGY  REQUIRED  FOR  THE  PROJECTION  OP 
MOVING  PICTURES 

§  779.  It  is  interesting  to  calculate,  from  the  data  on  radiant 
efficiency,  how  much  energy  is  required  to  project  a  moving  picture. 
This  has  an  important  bearing  on  the  fire  risk  with  such  projection. 
Stippose,  for  example,  the  picture  is  to  be  3.7x5  meters  in  size 
(i 2  x  16.5  ft.),  a  suitable  size  for  a  30  meter  (90  ft.)  hall.  Its  area 
will  be  18.5  square  meters  (298  sq.  ft.).  A  suitable  average  illumi- 
nation of  the  screen  would  be  100  meter  candles  or  about  10  foot 
candles.  As  the  revolving  shutter  removes  half  the  light,  the 
actual  momentary  illumination  of  the  screen  must  be  200  meter 
candles  or  200  lumens  per  square  meter. 

Basing  the  calculations  on  this,  it  is  seen  that  18.5  x  200  or  3700 
lumens  will  be  required.  When  using  the  right-angle  carbon  arc 
with  direct  current  the  light  represented  by  one  watt  when  radiated 
in  the  visible  part  of  the  spectrum  is  377  lumens  (§  778).  In  order 
to  get  3700  lumens  it  requires  3700/377  =  9.8  watts  of  light  energy. 
This  energy  must  get  through  the  aperture  plate  which  is  2.5  cm.  x 
1.75  cm.  and  which  has  an  area  of  4.2  square  centimeters  (i  in.  x  ^ 
in.,  area  ^  square  inch)  hence  the  light  energy  per  square  centi- 
meter of  film  area  is  9. 8 74. 2  =  2.34  watts  per  square  centimeter 
(§  779a)-  When,  however,  the  entire  radiation  from  the  arc  is 
used,  only  10%  of  which  is  light,  the  energy  is  10  times  as  great,  and 
even  when  a  water-cell  is  used  where  43%  of  the  energy  is  light,  the 
energy  is  2.3  times  as  great.  These  results  are  shown  in  tabular 


CH.  XIII]  ENERGY  FOR  MOVING  PICTURES  571 

form  below,  together  with  the  corresponding  values  when  alternat- 
ing current  is  used. 

§  780.  Radiant  energy  passing  the  aperture  plate  when  using 
right-angle  lamp  with  direct  current. —  power  passing  p&wei  for  each 

through  aperture       square  centi- 
plate  meter  of  film 

Total  radiation  of  arc,  of  which  10%  is  light.  98    watts  23.4  watts 
Radiation  passing  through  water-cell,  of  which 

43%  is  light 22.8  watts  5.44  watts 

Visible  radiation  only,  377  lumens  per  watt .     9.8  watts  2.34  watts 

§  781 .  Radiant  energy  passing  aperture  plate  when  using  right- 
angle  lamp  with  alternating  current.—  Power  pa?sing  Power  for  each 

through  aperture     square  centi- 
plate  meter  of  film 

Total  radiation  of  arc,  of  which  6.4%  is  light  222  watts     53.0  watts 
Radiation  passing  through  water-cell,  of  which 

41%  is  light 34-4  watts  8.2  watts 

Visible  radiation  only,  2 64  lumens  per  watt .      1 4    watts   3 . 2  watts 

§  782.  Effect  of  opacity  of  the  film. — When  a  nearly  trans- 
parent film  is  used,  a  large  proportion  of  this  radiation  passes 
through,  but  when  a  nearly  opaque  film,  such  as  the  title  is  shown, 
almost  all  of  this  energy  is  absorbed  and  converted  into  heat. 
From  these  tables  it  is  not  difficult  to  understand  why,  if  there  is 
no  water-cell  used,  the  film  is  likely  to  spoil  or  ignite  if  it  is  stopped 
for  a  few  seconds  while  the  light  is  falling  on  it.  Take  the  example 
of  the  light  furnished  by  the  alternating  current  arc  such  as  is  used 
in  a  great  many  places.  Here  the  film  is  absorbing  energy  at  the 
rate  of  53  watts  per  square  centimeter,  which  is  faster  than  the 
surrounding  air  can  cool  it.  If  now  a  water-cell  is  used,  the  energy 
rate  is  reduced  to  8.2  watts.  Experiment  has  shown  that  under 
these  conditions  with  the  water-cell,  the  heating  effect  is  not  great 
enough  to  ignite  even  a  black  celluloid  film  if  for  any  reason  it 
should  stop  moving.  But  it  must  be  remembered  that  even  if  a 
water-cell  is  used  the  film  would  catch  fire  if  held  in  an  extremely 
concentrated  beam.  (For  the  time  of  ignition  of  film  see  §  596) . 

§  779a.  If  the  new  standard  size  for  the  opening  in  the  aperture  plate 
(§  5/oa)  were  used,  the  figures  in  the  example  would  be  slightly  different,  but 
the  principle  is  shown  just  as  well  in  the  statement  here  given. 


CHAPTER  XIV 
OPTICS   OF  PROJECTION 
§  790.     Apparatus  and  Material  for  Chapter  XIV: 

See  the  optical  apparatus  in  Chapters  I  to  XL 

§  791.  History  of  the  optics  of  projection  and  references  to 
literature. — See  the  appendix  and  the  works  of  reference  in  Ch.  I, 
§  2 ;  works  on  general  physics,  optics  and  astronomy. 

§  792.  For  the  most  successful  use  of  projection  apparatus  it 
is  necessary  to  understand  some  of  the  simplest  principles  of 
optics,  and  to  keep  in  mind  that  in  the  projection  of  images  two  of 
the  fundamental  phenomena  of  optics  are  constantly  present. 
These  two  phenomena  are:  (i)  Reflection  and  (2)  Refraction. 

§  793.  Reflection.— By  this  is  meant  the  change  in  direction  of 
rays  of  light  when  they  meet  a  surface.  The  change  in  direction 
of  a  beam  of  light  striking  a  surface  depends  upon  the  character  of 
that  surface.  The  principle  kinds  of  reflection  are,  regular  reflec- 
tion, irregular  reflection,  and  semi-regular  reflection. 

§  794. — Regular  reflection. — If  the  surface  is  smooth,  as  in  a 
mirror,  the  incident  and  reflected  ray  will  be  in  the  same  plane  and 
will  make  equal  angles  on  opposite  sides  of  the  normal  erected  at 


FIG.  309.     REGULAR  REFLECTION  AT  A  POLISHED  SURFACE. 
The  angle  of  incidence  *",  is  equal  to  the  angle  of  reflection  r:  and  the  incident 
and  reflected  ray  are  in  a  plane  perpendicular  to  the  reflecting  surface. 

572 


CH.  XIV]  REFLECTION  AND  REFRACTION  573 

the  point  of  reflection  (fig.  309).  Most  cases  of  irregular  and 
semi-regular  reflection  if  considered  from  the  standpoint  of  a  small 
enough  part  of  the  surface  are  really  cases  of  regular  reflection; 
that  is,  any  small  particle  of  which  the  surface  is  made  reflects  the 
light  striking  it  regularly,  but  each  particle  of  the  surface  reflects 
the  light  in  a  different  direction.  Hence,  taken  as  a  whole,  such  a 
surface  will  not  reflect  the  light  regularly.  (See  Mirrors  §  800). 

§  795.  The  use  of  regular  or  mirror  reflection  in  projection  is 
illustrated  by  the  mirrors  used  in  opaque  lanterns  (fig.  95-1 10)  and 
by  the  mirrors  and  prisms  used  with  drawing  apparatus  (fig.  180- 
204). 

With  regular  or  mirror  reflection  the  observer  can  only  see  the 
light  when  he  is  in  the  path  of  the  rays  either  before  or  after  the 
reflection  (§  796a). 

§  796.  Irregular  reflection. — If  the  surface  is  irregular  then 
the  light  striking  it  is  reflected  in  various  directions  depending  upon 
the  position  of  the  irregularities  on  the  surface  receiving  the  light. 
If  these  are  very  small,  as  in  dust  or  smoke  or  as  on  the  surface  of 
snow,  white  cloth  or  paper,  etc.,  then  the  reflected  light  is  scattered 
practically  equally  throughout  the  entire  hemisphere  toward 
which  the  surface  faces  (fig.  310). 

§  797.  The  use  of  irregular  reflection  is  illustrated  by  the 
reflected  rays  from  the  white  screen  upon  which  the  image  is  pro- 
jected by  the  magic  lantern,  pro- 
jection microscope,  etc.  The  im- 
age appears  almost  equally  bright 
from  any  point  in  the  room. 


§  796a.  If  there  is  dust,  smoke  or 
fog  in  the  path  of  the  beam  of  light  either 
before  or  after  reflection,  the  minute 
particles  in  the  smoke  or  fog  irregular- 
ly reflect  some  of  the  light  and  one  can  _ 
see  it  at  any  angle  (fig.  320-323).  Dust  „ 

or  scratches  on  the  surface  of  the  mir-  FIG.  310.  IRREGULAR  OR  DIFFUSE 
ror  enables  one  to  see  where  the  beam  KEFLECTION. 

of  light  strikes  its  surface.  If  there  is  A  ray  of  light  striking  a  rough  sur- 
no  irregular  reflection  then  one  can  only  face  is  scattered  equally  in  all  direc- 
see  a  beam  of  light  when  in  its  path.  tions. 


574 


REFLECTION  AND  REFRACTION 


[Cn.  XIV 


FIG.  311.     SEMI-REGULAR  REFLECTION. 

Light  striking  some  surfaces  is  scattered 
unequally,  being  reflected  to  a  greater  extent 
in  one  direction  than  in  others.  This  repre- 
sents the  kind  of  reflection  produced  by  me- 
tallic-faced screens. 


§  798.  Semi-regular 
reflection. — This  occurs 
when  a  surface  is  imper- 
fectly polished,  that  is,  if 
the  surface  is  an  irregular 
one  but  not  sufficiently 
irregular  to  scatter  the 
light  equally  in  all  direc- 
tions. The  most  famil- 
iar example  is  a  surface 
coated  with  silver  or 
aluminum  powder.  .  Here 
the  individual  metal  par- 
ticles reflect  the  light 


regularly,  but  the  different  particles  lie  at  different  angles  and 
reflect  the  light  in  different  directions.  The  result  is  that 
the  light  is  scattered  in  all  directions,  but  the  greater  part  of 
the  light  is  reflected  in  the  same  general  direction  as  it  would  be  if 
the  surface  were  perfectly  polished.  This  is  shown  in  fig.  247,  311, 
where  the  length  or  number  of  the  rays  after  reflection  indicates 
the  amount  of  light  reflected  in  that  direction. 

§  799.  The  use  of  this  semi-regular  reflection  in  projection  is 
in  the  metallic  screens  or  mirror  screens  sometimes  used  in  long, 
narrow  auditoriums.  Such  screens  do  not  appear  equally  bright 
when  seen  from  all  parts  of  the  room  but  appear  brightest  when 
seen  in  the  direction  of  the  regular  reflection  from  the  lantern,  that 
is,  when  the  observer  is  nearly  in  line  with  the  lantern,  and  they 
appear  very  dim  when  seen  from  the  sides  of  the  room,  1 5°  or  more 
from  the  axis  of  the  lantern  (§  630). 

§  800.  Mirrors. — It  is  possible  to  construct  surfaces  of  metal 
or  silvered  glass  which  are  sufficiently  smooth  to  reflect  nearly  all 
of  the  light  in  accordance  with  the  law  of  regular  reflection  (§  794) . 
Such  a  surface  is  called  a  mirror.  It  may  be  plane  or  curved  (con- 
cave or  convex). 

If  the  mirror  surface  is  plane,  it  follows  from  the  law  that  the 
rays  will  have  the  same  angular  relation  to  one  another  after 


CH.  XIV]  REFLECTION  AND  REFRACTION 


575 


FIG.  312.     REFRACTION  AT  PLANE  AND  AT  CURVED  SURFACES. 
(From    The    Microscope) 

A-C  The  refracted  ray,  changing  its  direction  at  the  point  of  contact  with 
the  denser  medium  (line  shading). 

-Y-JY'     Normal  to  the  refracting  surface. 

B     Point  of  refraction. 

A-C1  In  ( i )  The  course  the  ray  would  have  taken  if  the  medium  from  A-C1 
had  been  homogeneous. 

C-C'1.  Course  the  ray  would  have  taken  from  the  point  C  if  the  medium 
from  Cl  to  C11  had  been  homogeneous. 

(/)     Refraction  from  air  to  water. 

(2-3}  Refraction  from  air  to  crown  glass.  As  shown,  if  the  incident  ray  is 
at  45°,  the  refracted  ray  will  be  at  approximately  28°  with  the  normal. 

reflection  as  before,  that  is,  if  the  rays  were  parallel  before  reflection 
they  will  be  afterward;  and  if  they  were  converging  or  diverging 
before  they  will  converge  or  diverge  after  reflection! 

With  .a  curved  mirror  the  angular  relation  after  reflection  is  not 
the  same  as  before.  For  example,  with  a  concave  mirror,  parallel 
rays  are  bent  towards  one  another  and  finally  meet  at  what  is 
called  the  focal  point.  If  the  mirror  is  convex  then  the  rays  are 
made  to  diverge  on  leaving  the  mirror. 


/     Incident  ray  in  the  air  above  the  glass. 
r     Ray  of  light  below  the  glass,  after  refraction. 
*'     Course  of  the  ray  of  light  if  the  glass  were 
absent. 

The  refracted  bearn  traced  backward  above 


Air 


Glass 


the  glass  to  show  its  apparent  origin. 

n  n'     Normals  where  the  ray  enters  and  leaves 
the  glass. 

This  figure  shows  the  displacement  of  the  light 

by  refraction  through  media  with  plane  surfaces,    FIG.  313.    REFRACTION  BY 
and   that  the  refracted  light  is  parallel  with  the 
incident  light. 


Air 


GLASS  WITH  PARALLEL 
FACES. 


576  LENSES  AND  THEIR  ACTION  [Cn.  XIV 

§  801.  Refraction. — By  refraction  is  meant  the  change  in 
direction  of  a  ray  of  light  in  passing  from  one  transparent  medium 
into  another. 

The  amount  of  bending  depends  upon  two  conditions: 

(1)  The  greater  the  angle  of  incidence  of  the  light,  that  is,  the 
farther  from  the  perpendicular  or  normal  that  the  light  strikes  the 
surface,  the  greater  will  be  the  bending  on  entering  the  second 
medium.     And  this  increase  is  not  simply  with  the  increase  of  the 
angle  of  incidence,  but  proportionally  greater,  that  is,  in  accordance 
with  the  law  of  sines  (fig.  312). 

(2)  The  bending  also  depends  upon  the  difference  of  density 
of  the  two  transparent  media.     If  the  difference  is  great,   the 
refraction  will  be  great,  and  if  the  difference  of  density  is  small,  the 
refraction  will  be  proportionally  small.     See  also  chromatic  aberra- 
tion (§810,  fig.  337). 

The  phenomena  of  refraction  were  worked  out  with  great 
accuracy  by  Ptolemy  in  the  first  and  beginning  of  the  second  cen- 
tury A.D. ;  but  the  precise  mathematical  expression  for  the  law  of 
refraction  was  not  found  until  about  1500  years  later  (Snell's  and 
Descartes'  law  of  sines) .  This  law  of  sines  includes  both  elements 
mentioned  above,  and  is  expressed  thus : 

Sine  of  the  angle  made  by  the  incident  ray       .    ,         -      -        . 

--=  index  of  refraction. 

Sine  of  the  angle  made  by  the  refracted  ray 

§  802.  Lens. — By  making  one  or  two  bounding  surfaces  of  a 
transparent  body  curved,  rays  of  light  traversing  the  body  are 
made  to  converge  or  to  diverge.  Any  transparent  body  having  one 
or  both  of  its  opposite  faces  curved  is  called  a  lens.  The  curved 
surfaces  are  usually  segments  of  spheres,  as  a  spherical  surface  can 
be  ground  and  polished  more  accurately  than  can  any  other. 

§  803.  Principal  axis. — The  straight  line  passing  through  the 
centers  of  the  two  spheres  of  which  the  surfaces  of  a  lens  are  seg- 
ments is  called  the  principal  axis.  This  axis  is  perpendicular  to 
both  surfaces  of  the  lens  (fig.  314). 

§  804.  Optic  center. — This  is  the  point  in  a  lens,  or  near  it, 
through  which  light  rays  pass  without  angular  deviation,  that  is, 


CH.  XIV] 


LENSES  AND  THEIR  ACTION 
2 


577 


FIG.  314.     THE  OPTIC  CENTER  AND  THE  PRINCIPAL  OPTIC  Axis  OF  VARIOUS 

FORMS  OF  LENSES. 

(From  The  Microscope) 

c  c'     Centers  of  curvature  of  the  different  lenses. 

As  shown  in  all  the  figures  each  curved  face  of  the  lens  is  a  part  of  a  sphere 
of  greater  or  less  size. 

c.l     Optic  center  of  the  lens. 

r  Radius  of  the  sphere  from  which  the  lens  is  derived.  (Radius  of  curva- 
ture). 

1.  Double-convex  lens,  the  two  faces  having  different  curvatures. 

2.  Double-concave  lens,  the  two  faces  of  different  curvatures. 

3.  Plano-convex  lens. 

4.  5,  6.     The  same  showing  the  optic  center  (cl). 

7.  Plano-concave  lens  showing  optic  center  (cl). 

8.  Converging,  meniscus  lens.     The  optic  center  (cl)  is  outside  the  lens, 
on  the  convex  side. 

9.  Diverging  meniscus  lens  with  the  optic  center  (cl)  outside  the  lens,  and 
on  the  concave  side. 


578  LENSES  AND  THEIR  ACTION  [Cn.  XIV 

the  ray  before  and  after  it  passes  the  center  of  the  lens  extends  in 
parallel  lines.  As  shown  by  the  following  diagrams  the  optic 
center  is  found  by  drawing  parallel  radii  from  the  two  curved  sur- 
faces, or  from  the  curved  and  plane  surface,  and  joining  the  ends 
of  the  radii.  The  center  of  the  lens  is  the  point  where  the  line 
joining  the  outer  ends  of  the  parallel  radii  cross  the  principal  axis 
(fig.  314). 

The  reason  why  light  rays  traversing  the  optic  center  have  no 
angular  deviation  is  as  follows :  The  radii  are  perpendicular  to  the 
surfaces  of  the  lens;  and  the  tangent  plane  perpendicular  to  the 
radius,  is  tangent  to  the  sphere  at  the  end  of  the  radius.  As  the 
two  tangents  to  parallel  radii  must  themselves  be  parallel,  it  follows 
that  a  ray  of  light  passing  from  one  tangential  point  to  the  other  is 


FIG.  315.     CONJUGATE  Foci  C,C2  ON  THE  PRINCIPAL  Axis. 

traversing  a  body  with  parallel  surfaces  at  the  point  of  entrance 
and  departure,  and  hence  it  will  suffer  no  angular  deviation 
although  the  ray  may  be  displaced,  as  in  traversing  any  thick 
transparent  body  with  plane  faces  (fig.  313).  With  meniscus 
lenses  the  crossing  point  (optic  center)  is  on  an  extension  of  the  line 
joining  the  centers  of  curvature  (fig.  314), 

§  805.  Secondary  axis. — Every  line  traversing  the  optic  center 
of  a  lens,  except  the  principal  axis,  is  a  secondary  axis.  It  follows 
therefore  that  every  secondary  axis  must  be  more  or  less  oblique 
to  the  principal  axis  (fig.  317). 

§  806.  Principal  focal  point. — The  principal  focus  or  focal 
point  of  a  lens  or  of  a  lens  system  like  a  condenser  or  a  projection 
objective,  is  the  point  on  the  principal  axis  where  rays  of  light 
parallel  with  the  principal  axis  before  entering  the  lens  or  combina- 
tion, cross  the  principal  axis  after  leaving  the  lens  or  objective. 
It  is  also  sometimes  called  the  burning  point  (fig.  319). 


CH.  XIV] 


LENSES  AND  THEIR  ACTION 


579 


With  a  concave  mirror  it  is  likewise  the  point  on  the  principal 
axis  where  rays  parallel  with  the  principal  axis  before  striking  the 
mirror  are  made  to  cross  the  principal  axis  after  being  reflected  by 
the  curved  face  of  the  mirror.  This  point  is  situated  half  way 
between  the  mirror  face  and  the  center  of  curvature. 

§  807.     Conjugate  foci,  and  the  mutual  relation  of  images. — 

In  figures  3 1 5-318,  are  shown  conjugate  foci  on  a  principal  and  on  a 
secondary  axis.     In  each  case  the  object  and  the  image  might 


o, 


FIG.  316.     METHOD  OF  IMAGE  FORMATION  ON  THE  PRINCIPAL  Axis. 

In  this  case  the  object  (CJ  and  the  image  (C2)  are  equally  distant  from  the 
center  of  the  lens,  hence  they  are  of  the  same  length,  and  the  distance  between 
them  is  four  times  the  principal  focus  of  the  lens. 


Axis 


FIG.  317.     CONJUGATE  Foci  ON  A  SECONDARY  Axis. 
The  secondary  axis  passes  through  the  optic  center  of  the  lens,  and  the  con- 
jugate focus  C2  is  below  the  principal  axis  if  the  point  CI  is  above  it. 

change  places  without  any  change  in  the  mutual  relation  of  the 
object  and  image.  For  example,  if  the  screen  picture  with  a  magic 
lantern  were  an  actual  scene  and  the  magic  lantern  pointed  toward 
it  as  in  projection,  a  small  image  exactly  like  the  lantern  slide 
would  be  formed  at  the  level  of  the  lantern  slide.  It  is  from  this 
mutual  relation  of  object  and  image  that  they  are  said  to  be 
conjugates. 

§  808.     How  to  obtain  the  principal  focus  experimentally.— 

This  is  accomplished  by  holding  the  lens  or  combination,  or  the 
mirror,  with  the  principal  axis  pointing  directly  toward  the  sun. 


580  LENSES  AND  THEIR  ACTION  [Cn.  XIV 

The  point  where  the  image  of  the  sun  appears  indicates  the  prin- 
cipal focal  point,  or  the  burning  point. 

Another  way  to  get  the  equivalent  focal  length  or  focus  of  an 
objective  is  to  put  it  in  position  on  an  optical  bench  like  that  shown 
in  fig.  159  and  then  to  use  a  metric  rule  (fig.  178),  or  a  lantern  slide 
of  such  a  rule  as  object,  and  a  white  screen  or  a  ground-glass  on  the 
other  side  of  the  objective.  The  object  and  the  screen  are  then 
moved  toward  and  from  the  objective  until  the  image  is  of  exactly 
the  same  size  as  the  object.  The  distance  apart  of  the  image  and 
the  object  is  four  times  the  focal  length  of  the  objective  (fig.  316). 


r^^.  _PCJ  "sip*  L  .A*!* 


FIG.  318.     IMAGE  FORMATION  ON  A  SECONDARY  Axis. 

With  a  good  lens'the  arrows  CT  and  C  2  are  both  perpendicular  to 
the  principal  axis. 

C,     Object. 

C2  Image.  When  object  and  image  are  of  the  same  size,  as  here,  the  image 
is  as  far  below  the  principal  axis  as  the  object  is  above  it. 

SPHERICAL  AND  CHROMATIC  ABERRATION  AND  MEANS  OF  CORRECT- 
ING THESE  DEFECTS 

§  809.  Spherical  aberration. — By  this  is  meant  the  unequal 
bending  of  the  light  rays  in  different  zones  of  a  lens.  As  shown  in 
fig.  320,  the  rays  passing  through  the  outer  zones  of  a  spherical 
lens  are  proportionally  more  bent  than  those  which  pass  nearer 

§  808a.  Equivalent  focus. — The  term  equivalent  focus  is  often  employed 
for  compound  optical  systems  like  objectives.  This  means  simply  that  the 
objective  gives  the  same  magnification  or  reduction  in  a  given  case  as  a  simple 
lens  of  that  focus  would  give. 

For  example,  the  simple  lens  in  fig.  209,  with  the  object  2  cm.  from  the 
center  of  the  lens,  gives  an  image  at  8  cm.,  four  times  as  large  as  the  object. 
Now  any  compound  system  of  lenses  which  gives  a  magnification  of  four  under 
similar  conditions  is  said  to  be  equivalent  to  this  simple  lens.  The  expression* 
equivalent  focus  is  frequently  designated  by  the  initial  letters  of  the  words, 
e.  f. 


CH.  XIV] 


LENSES  AND  THEIR  ACTION 


the  axis.  It  results  from  this  that  the  border  rays  cross  the  axis 
considerably  nearer  the  lens  than  the  central  rays,  hence,  with 
parallel  rays,  instead  of  one  focus,  there  are  many  foci  drawn  out  in 
a  line.  This  is  shown  by  the  bright  core  in  the  photograph  of  the 
cone  of  rays  in  fig.  322. 

Except  with  a  symmetri- 
cal, double  convex  lens  the 
amount  of  spherical  aberra- 
tion depends  upon  which  face 
of  the  lens  receives  the  inci- 
dent light,  and  whether  the 
incident  light  is  parallel, 
diverging  or  converging. 

With  plano-convex  lenses, 


[Fic.~3i9.     THE  PRINCIPAL  Focus  OF  A 
CONVEX  AND  OF  A  CONCAVE  LENS. 
(From  The  Microscope) 

Axis,  Axis.   The  principal  optic  axis  of 
the  lenses. 

F    The  focus.  In  the  convex  lens  it  is 


as  shown  in  fig.  320—323,  the 
spherical  aberration  with 
parallel  incident  light  is  less 
when  the  parallel  light  is 

incident   on  the  convex  face   where  the  light  rays  actually  cross  the  axis, 
than  whpn  the  lens  k  turned    In  the  concave  lens  it  is  where  they  would 

1   cross  if  produced  backward  as  indicated 
so  that  the  light  is  incident  by  the  broken  lines. 

upon  the  plane  face. 

For  diverging  rays  the  plane  face  should  receive  the  incident 
light,  and  for  converging  rays  the  convex  surface  should  receive  the 
light  to  insure  minimum  spherical  aberration.  With  all  lenses, 
the  general  rule  to  follow  is  that  for  minimum  spherical  aberration, 
the  light  rays  should  be  equally  bent  on  entering  and  on  leaving  the 
lens  i.  e.,  at  both  refracting  surfaces.  Furthermore,  with  the  same 
light  beam,  the  aberration  is  greater  for  lenses  of  large  curva- 
ture than  for  lenses  of  small  curvature. 

To  overcome  this  aberration,  a  concave  lens  is  combined  with  a 
convex  lens,  and  so  proportioned  that  the  too  great  converging 
effect  of  the  outer  zone  of  the  convex  lens  is  just  counterbalanced 
by  the  diverging  effect  of  the  concave  lens  in  its  various  zones  (fig. 
324).  A  perfectly  corrected,  or  aplanatic  combination  brings  all 
the  parallel  rays  to  one  focus. 


582 


LENSES  AND  THEIR  ACTION 


[Cn.  XIV 


FIGS.  320-321.     BEAMS  OF  PARALLEL  LIGHT  TRAVERSING  PLANO-CONVEX 

LENSES  TO  SHOW  MAXIMIUM  AND  MINIMUM  SPHERICAL 

ABERRATION. 

Figure  320  shows  the  lens  in  the  position  to  give  the  maximum  aberration. 
That  is,  the  border  rays  cross  the  axis  much  nearer  the  lens  than  the  inter- 
mediate rays. 

Figure  321  shows  the  lens  in  the  position  to  give  the  minimum  aberration. 
The  border  and  the  intermediate  rays  cross  the  axis  mere  nearly  in  the  same 
place. 

This  picture  was  made  in  a  dark-room  (fig.  179).  The  room  was  filled  with 
smoke,  and  the  light  was  partly  scattered  by  the  smoke,  thus  making  the  rays 
visible  from  the  side  (§  7Q6a).  The  first  element  of  the  triple  condenser  was 
covered  in  the  parallel  team, with  a  perforated  metal  disc.  This  permitted  only 
minute  cylinders  of  light  to  escape  along  the  diameter  of  the  condenser. 

The  lens  in  fig.  320  appears  dark  as  it  was  clean  .  The  one  in  fig.  321  appears 
white  because  there  was  some  very  fine  talcum  dust  on  the  face. 


CH.  XIV]  LENSES  AND  THEIR  ACTION  583 

§  810.  Chromatic  aberration. — By  this  is  meant  the  separation 
of  the  images  produced  by  the  different  wave  lengths  of  which 
white  light  is  composed.  Newton  thought  this  was  a  purely 
refractive  action  and  therefore  could  not  be  corrected  without  at  j 
the  same  time  overcoming  all  the  refraction,  hence  he  thought 
there  could  be  no  images  formed  by  lenses  or  combinations  of 
lenses  without  the  presence  of  the  color  defect.  But  later  it  was 
found  that  some  glass  separated  the  light  into  colors  more  markedly 
than  others  of  the  same  refraction.  Now  by  combining  two  kinds 
of  glass  which  act  differently  in  this  respect  it  was  found  possible 


FIG.  324.    ACHROMATIC  LENSES. 
(From  Lewis  Wright,  Optical  Projection). 

By  combining  a  convergent  or  convex  crown  glass  lens  with  a  divergent  or 
concave  flint  glass  lens  it  is  possible  to  get  a  combination  which  is  largely  free 
from  chromatic  as  well  as  spherical  aberration.  In  all  but  D  and  the  right- 
hand  combination,  but  two  lenses  are  used;  in  those,  one  flint  and  two  crown 
glass  lenses  are  used. 

to  bring  two  or  three  of  the  colors  to  one  focus,  and  thus  to  produce 
practically  colorless  images  by  means  of  lenses  (fig.  324). 

Usually  an  objective  for  forming  images — photographic  objec- 
tive, microscopic  objective,  projection  objective — is  corrected  both 
for  spherical  and  for  chromatic  aberration,  so  that  the  image  is 
correct  in  every  way.  This  is  accomplished  by  combining  concave 
and  convex  lenses  of  the  right  form  and  composition.  Sometimes 
also,  as  with  the  apochromatic,  microscope  objectives,  a  natural 
mineral — fluorite — is  introduced  to  make  a  more  perfect  correc- 
tion than  could  be  accomplished  by  artificial  glass. 


584          IMAGE  FORMATION,  INVERSION  OF  IMAGES        [Cn.  XIV 

IMAGE  FORMATION  WITH  THE  MAGIC  LANTERN 

§  811.  Ideal  case. — When  using  transparent  lantern  slides 
with  a  magic  lantern  and  a  small  source  of  light  the  ideal  arrange- 
ment is  that  shown  in  fig.  325. 

L,  is  a  point  source  of  light  (crater  of  the  arc  light).  The  con- 
denser C,  focuses  this  light  at  the  point  O,  in  the  optical  center  of 
the  objective.  The  slide-carrier  S,  is  placed  just  in  front  of  the 
condenser.  The  objective  O,is  at  the  proper  distance  from  S,to 
form  a  real  image  of  the  slide  on  the  screen.  All  of  the  rays  of 
light  from  S  pass  directly  through  the  center  of  the  objective  O,  and 


FIG.  325.     LANTERN-SLIDE  PROJECTION;    No  SPHERICAL  ABERRATION. 

This  shows  an  ideal  case  where  there  is  a  point  source  of  light,  and  a  con- 
denser without  spherical  aberration.  The  light  from  the  condenser  crosses  at 
the  center  of  the  objective  (O)  and  goes  on  without  deviation  to  the  image 
screen. 

L     Light  source. 

C    Condenser. 

S     Lantern  slide. 

O     Projection  objective. 

hence  undergo  practically  no  deviation.  If  the  source  of  light  L, 
were  really  a  point  source,  and  the  condenser  C,  had  no  spherical 
aberration,  the  shadow  of  the  lantern  slide  S,in  the  screen  without 
an  objective  would  be  just  like  the  image  which  is  projected  by  the 
objective. 

§  812.  Inversion  of  the  image. — In  their  passage  from  the 
lantern  slide  to  the  screen  the  rays  pass  from  the  top  of  the  slide 
to  the  bottom  of  the  screen,  and  from  the  bottom  of  the  slide  to  the 
top  of  the  screen.  In  liketmanner  the  rays  from  the  two  sides  of 
the  slide  cross  before  reaching  the  screen  (fig.  i.) 

This  crossing  of  the  rays  gives  what  is  known  as  an  inverted 
image. 


CH.  XIV]          IMAGE  FORMATION,  INVERSION  OF  IMAGES 


585 


§  813.  Actual  case. — The  actual  case  differs  from  the  ideal  case 
in  that  the  condenser  has  a  considerable  amount  of  spherical  aber- 
ration and  that  the  source  is  not  a  point  but  is  somewhat  extended. 

§  814.  Spherical  aberration  of  the  condenser. — The  effect  of 
the  spherical  aberration  of  the  condenser  has  not  been  sufficiently 
studied  up  to  the  present,  but  it  exerts  a  good  deal  of  influence  in 
projection  especially  with  micro-projection  and  with  moving 
pictures. 


B 


FIG.  326A.     THE  PATH  OF  THE  LIGHT  RAYS  FROM  THE  DIFFERENT 
ZONES  OF  THE  CONDENSER. 

FIG.  3266.     THE  APPEARANCES  ON  A  CARD  HELD  IN  THE  DIFFERENT  PARTS 
OF  THE  CONE  OF  LIGHT. 

a,  b,  c,  d,  e,  f     Lines  showing  where  the  card  was  held  in  the  light  cone  to 
give  the  appearances  in  B. 
g     Diaphragm. 

As  was  shown  in  §  809,  the  light  from  a  point  source  will  not  come 
to  a  focus  in  a  point,  but  the  rays  passing  through  the  margin  of  the 
lens  will  be  relatively  more  bent  and  will  cross  the  axis  sooner  than 
those  which  pass  through  the  lens  near  the  axis  (fig.  320-323,  337). 

A  curious  phenomenon  is  the  effect  on  the  illumination  of  the 
screen  when  a  diaphragm  is  used  to  cut  off  the  margin  of  the  cone. 


586         IMAGE  FORMATION,  INVERSION  OF  IMAGES         [Cn.  XIV 

If  placed  at  b  or  e  (fig.  326),  it  cuts  off  the  margin  of  the  cone  and 
lets  the  center  through  but  if  placed  at  g,  light  from  the  center  and 
the  margin  of  the  condenser  gets  through,  but  light  from  a  zone  part 
way  out  is  removed  (fig.  327). 

The  result  on  the  illumination  of 
an  object  placed  in  the  converging 
cone  of  light  will  be  as  shown  in  fig. 
3266.  An  object  placed  near  the 
condenser  will  be  evenly  illumina- 
ted. As  it  is  moved  away  from  the 
condenser  face  towards  the  crossing 
of  the  rays  the  outer  edge  first  be- 
comes more  brightly  illuminated 
than  the  center  and  then  the  spotted 
effects  shown  in  the  figure  will  be 

FIG.  327.    APPEARANCE  ON  THE  seen.     At  no  position  will  there  be 
SCREEN  WHEN  ILLUMINATED  BY  ...       ...          r   ,,         ,  . 

THE  CONDENSER   SHOWN   IN       an  even  illumination  of  the  object 
FIG.  326A  IF  DIAPHRAGM  is         when   using  a  point  source  except 

when  the  object  is  placed  next  to  the 

condenser  face,  a.  If  it  is  necessary  to  eliminate  the  spotted  effect 
due  to  spherical  aberration  as  when  exhibiting  moving  pictures  one 
must  use  an  extended  source  of  light,  so  that  the  aberration  figures 
from  the  different  points  of  the  source  overlap.  The  arc  lamp  with 
15  to  20  amperes  direct  current  is  sufficiently  extended  to  give  an 
even  illumination  provided  a  short  focus  condenser  is  used. 

§  815.  Spherical  aberration  of  the  condenser  with  the  magic 
lantern. — When  using  the  magic  lantern  the  spherical  aberration 
of  the  condenser,  unless  exceedingly  great,  is  of  no  special  disad- 
vantage. The  rays  from  the  different  parts  of  the  slide  will  not  all 
cross  at  the  center  of  the  objective  but  will  cross  at  different  points 
on  the  axis  (fig.  320,  337).  If  the  objective  is  of  good  quality  and 
of  large  enough  diameter  to  include  all  of  the  beam  of  light  there 
will  result  a  good  screen  picture. 

§  816.  Effect  of  an  extended  source. — Let  a,  fig.  330,  represent 
a  point  in  the  slide  S.  Light  which  has  come  from  all  parts  of  the 


CH.  XIV[         IMAGE  FORMATION,  INVERSION  OF  IMAGES         587 

source  L,  after  passing  through  (a)  will  spread  out  over  an  angle 
and  strike  the  objective  lens  O.  The  purpose  of  the  objective  lens 
O,  being  to  collect  all  of  the  light  from  the  point  (a)  on  the  slide  and 
to  bring  it  together  at  the  point  (a')  on  the  screen.  This  is  exactly 
similar  to  the  case  of  image  formation  of  a  self-luminous  or  diffusely 
reflecting  surface  described  in  (Ch.  VII,  §  273,  fig.  90)  except  that 
the  light  from  the  point  (a)  does  not  spread  out  in  all  directions, 
but  only  over  the  angle  x'  a  y'. 
C 


FIG.  328.     INTERCHANGEABLE  MAGIC  LANTERN  AND  MOVING  PICTURE 
PROJECTION  WITH  POINT  SOURCE  AND  CONDENSER  FREE  FROM 

SPHERICAL  ABERRATION. 
a     Magic  lantern  arrangement. 
b     Moving  picture  arrangement. 
L     Crater  of  the  arc  lamp  as  a  source  of  light. 
c     Condenser. 

s,s     The  lantern  slide  in  (a),  and  the  film  in  (6). 
o,  o     Projection  objectives. 

§  817.  Simplicity  of  the  Magic  Lantern. — From  the  above  it  is 
seen  that  with  the  arc-light  magic  lantern  the  actual  case  is  nearly 
the  same  as  the  ideal  case  and  the  manipulation  of  the  apparatus 
is  relatively  simple. 


RELATION  OF  THE  FOCAL  LENGTH  OF  THE  CONDENSER  TO  THE 
FOCAL  LENGTH  OF  THE  PROJECTION  OBJECTIVE 

§  818.  Types  of  condensers. — There  are  in  general  use  two 
main  types  of  condenser;  the  two-lens  type,  as  shown  in  fig.  331, 
and  the  three-lens  type,  as  shown  in  fig.  332.  The  two-lens  type 


588 


CONDENSERS  FOR  PROJECTION 


[Cn.  XIV 


has  the  advantage  of  simplicity  and  cheapness  while  the  three- 
lens  type  has  the  advantage  that  it  has  very  little  spherical  aberra- 
tion and  at  the  same  time  it  is  possible  to  bring  the  lamp  closer  to 
the  first  surface  of  the  condenser,  thus  utilizing  a  greater  proportion 
of  the  light  of  the  illuminant  (§  8i8a). 


FIG.    329.    ARRANGEMENT    FOR    INTERCHANGEABLE    LANTERN-SLIDE    AND 
MOVING  PICTURE  PROJECTION  WHEN  THE  CONDENSER  HAS  SPHERICAL 

ABERRATION. 
Projection  of  lantern  slide. 
Projection  of  moving  picture  film. 
Arc-lamp  crater,  the  source  of  light. 
Condenser  of  two  plano-convex  lenses. 
In  (a)  the  lantern  slide  near  the  condenser. 
In  (b)  the  moving  picture  film. 
Objective  for  projection. 
The  image  of  the  condenser  face. 


§  818a.  Types  of  condensers. — In  the  development  of  projection  apparatus 
almost  every  form  of  condenser  has  been  used  from  a  single  piano-  or  double- 
convex  lens  to  one  composed  of  three  or  more  lenses.  In  form,  the  lenses  have 
been  plano-convex,  double-convex,  meniscus  and  parabolic. 

For  artificial  light  the  condenser  is  now  almost  always  composed  of  two  or 
more  lenses. 

Fkst  element  (fig.  332). — The  first  element  may  be  composed  of  a  single 
plano-convex  lens  or  a  meniscus,  or  it  may  be  composed  of  two  lenses.  A 
meniscus  and  a  plano-convex,  or  a  meniscus  and  a  double  convex  or  finally  of 
two  meniscus  lenses.  The  first  element  in  all  cases  collects  the  light  fronTthe 
source  and  renders  it  more  or  less  parallel. 

Second  element,  (fig.  332). — The  second  element  of  the  condenser  may  be 
a  plano-convex  or  a  double  convex  lens  or  an  achromatic  combination. 

We  have  tried  the  different  forms  of  condensers  and  have  found  those  com- 
posed of  two  plano-convex  lenses,  or  those  with  two  plano-convex  lenses  and  a 
meniscus  next  the  light,  most  satisfactory  (fig.  1,2). 

Finally  there  has  been  recently  produced  a  parabolic  condenser  for  projec- 
tion with  the  microscope.  This  form  eliminates  almost  all  the  spherical 
aberration  and  is  promising. 


CH.  XIV] 


CONDENSERS  FOR  PROJECTION 


589 


§  819.  The  two-lens  type  of  condenser. —  In  choosing  the 
objective  and  other  optical  parts  of  the  lantern  one  must  first 
consider  the  room  in  which  the  projection  is  to  be  done  and  then 
choose  an  objective  of  such  a  focal  length  that  the  picture  will  be 
of  the  desired  size  (Ch.  XII,  §  635).  After  the  objective  is  deter- 
mined upon,  it  is  necessary  to  select  the  condenser  lenses  of  such 
focal  length  that  the  best  results  may  be  obtained.  There  are  two 
factors  which  must  be  balanced  in  this  choice.  First;  the  closer 
the  light  is  to  the  condenser,  that  is  the  shorter  is  its  focus,  the 


FIG.  330.     EFFECT  OF  AN  EXTENDED  SOURCE  OF  LIGHT.  CL 

Z/  (w,  x,  y,  z)     Extended  light  source. 

Single  Lens  Condenser  near  the  source. 

S    Lantern  slide. 

a  and  b     Points  on  the  lantern  slide. 

O     Single  lens  objective. 

y',  xr     Image  of  the  extended  light  source  on  the  objective. 

/,  a     Screen  image  of  the  lantern  slide. 

If  the  light  source  xy,  is  not  too  large,  all  of  the  light  collected  by  the  con- 
denser gets  through  the  objective. 

If  the  light  source  wz,  is  too  large,  the  image  w'  z',  will"  be  larger  than  the 
objective  and  much  light  will  be  lost  by  falling  outside  the  objective. 

greater  will  be  the  amount  of  light  which  it  will  collect.  Second; 
the  shorter  the  focus  of  the  condenser,  the  greater  will  be  its 
spherical  aberration.  In  order  to  get  the  minimum  of  spherical 
aberration  with  two  plano-convex  lenses  such  as  are  generally  used 
for  condensers  it  is  necessary  to  turn  them  so  that  parallel  or  nearly 
parallel  light  strikes  the  curved  surfaces  and  the  diverging  light 
from  the  source  strikes  the  plane  surface  (fig.  321,323).  When  the 
parallel  beam  strikes  the  curved  surface  of  the  lens,  all  of  the  rays 
come  to  a  focus  more  nearly  at  the  same  point  than  when  the 
parallel  beam  strikes  the  plane  surface  of  the  lens  (fig.  320,  321). 
This  requires  that  the  curved  surfaces  of  the  lenses  shall  be  turned 
towards  each  other  as  in  fig.  329. 


590 


CONDENSERS  FOR  PROJECTION 


[CH.  XIV 


In  order  that  the  combination  of  the  two  lenses  of  the  condenser 
shall  have  as  little  spherical  aberration  as  possible  they  should  be 
of  about  equal  focal  length.  If  there  is  a  difference  in  focal  length, 
the  thicker  lens,  i.  e.,  the  one  of  shorter  focal  length,  should  be 
placed  next  the  radiant  and  the  thiftner  lens,  i.  e.,  the  one  of  greater 
focal  length,  should  be  away  from  the  radiant.  The  best  lenses  to 
use  in  a  given  case  can  only  be  determined  by  experiment  but  as  a 
first  trial  we  would  suggest  the  following  foci  for  condensers  of  11.4 
cm.  (4^2  inches)  diameter  for  magic  lantern  work  and  moving 
picture  projection. 

§  820.     Table  of  condenser  lenses. — 


Focus  of  objective  or  Distance 
of  Aperture  Plate 

FOCUS  OF  THE  LENSES  OF  THE  CONDENSER 

Lens  next  the  radiant 

Lens  away  fiom  the  radiant 

15     cm.  (  6  in.) 
1  8     cm.  (  7  in.) 
20     cm.  (  8  in.) 
25     cm.  (10  in.) 
30     cm.  (12  in.) 
38     cm.  (15  in.) 
45.7  cm.  (18  in.) 

15     cm.  (  6      in.) 
15     cm.  (  6      in.) 
16.5  cm.  (  6K  in.) 
16.5  cm.  (  6^2  in.) 
1  7.  8  cm.  (  7      in.) 
17.  8  cm.  (  7      in.) 
19     cm.  (  7K  in.) 

15     cm.  (  6      in.) 
16.5  cm.  (  6>£  in.) 
16.5  cm.  (  61/,  in.) 
1  7.  8  cm.  (  7      in.) 
1  7.  8  cm.  (  7      in.) 
19     cm.  (  7>4  in.) 
19     cm.  (  7>£  in.) 

With  these  lenses  the  light  from  the  first  lens  will  be  somewhat 
diverging  before  it  strikes  the  second  lens.  The  best  results  are 
obtained  when  the  two  lenses  are  as  close  together  as  they  can  be 
put  without  touching. 

§  821.  The  three-lens  type  of  condenser. — The  three-lens  type 
of  condenser  illustrated  in  figure  332  is  designed  on  a  different 
principle.  Here  the  first  combination,  consisting  of  a  meniscus 
lens  and  a  plano-convex  or  a  double-convex  lens  (fig.  in),  is 
designed  to  render  the  light  from  a  point  source  parallel  with  but 
a  very  small  amount  of  spherical  aberration.  In  order  that  the 
light  shall  be  focused  at  the  center  of  the  objective  it  is  then  neces- 
sary that  the  last  lens  of  the  condenser  shall  bring  the  parallel  beam 
to  a  focus  where  it  is  wanted,  that  is,  it  must  have  a  focus  approxi- 
mately equal  to  that  of  the  objective.  For  long  focus  objectives 
38-46  cm.  (15-18  in.)  this  of  course  necessitates  a  rather  thin  lens 
next  to  the  lantern  slide  (fig.  332). 


CH.  XIV]       IMAGE  FORMATION  WITH  MOVING  PICTURES        591 

In  microscopic  projection  (Ch.  IX)  and  in  drawing  with  the 
microscope  (Ch.  X)  the  three-lens  condenser  with  its  small  spheri- 
cal aberration  is  of  great  advantage.  For  micro-projection  without 
a  substage  condenser,  the  final  plano-convex  lens  of  the  triple  con- 
denser should  have  a  focus  of  about  1 5-20  cm.  (6-8  in.).  This  will 
answer  well  for  objectives  as  high  as  4  mm.  equivalent  focus  (>6 
in.).  Where  a  substage  condenser  is  used,  the  focus  of  the  last 


FIG.  331.     TWO-LENS  CONDENSER  FOR  PROJECTION. 

The  condenser  (Cond)  is  shown  in  connection  with  the  lamp-house  and  right- 
angle  arc  lamp. 

The  first  lens  of  the  condenser  (i)  is  of  shorter  focus  (i.  e.,  thicker)  than  the 
second  lens  (2).  The  condenser  is  nearer  the  source  of  light  (L)  than  the 
principal  focus  of  the  first  condenser  lens,  hence  the  light  beam  between  the 
condenser  lenses  is  diverging.  With  this  arrangement  and  the  lenses  close 
together  a  wider  beam  of  light  can  be  utilized  for  projection  than  as  if  the 
condenser  were  farther  from  the  lamp  (see  fig.  343).  (For  a  more  complete 
explanation  of  this  figure  see  fig.  379.) 

plano-convex  lens  of  the  large  condenser  should  be  longer  than 
15  cm.  One  of  25-40  cm.  (10-16  in.)  is  more  satisfactory  (See 
Ch.  IX,  §  402). 

§  822.  Image  formation  with  moving  pictures. — When  moving 
pictures  are  to  be  projected,  the  conditions  to  be  met  are  not  so 
simple  as  with  the  magic  lantern,  and  one  must  bear  in  mind  the 
actual  requirements. 

§  823.  Practical  requirements. — These  requirements  are :  (The 
figures  for  equivalent  focus  refer  to  an  actual  case).  The  moving 


592        IMAGE  FORMATION  WITH  MOVING  PICTURES       [Cn.  XIV 


picture  objective  was  13.3  cm.  e.  f.  ($%  m-)  and  the  focal  length 
of  the  magic  lantern  objective  to  go  with  it  is  indicated. 

1.  The  dimensions  of  the  aperture  plate  were  23  mm.  x  17.3 
mm.,  diagonal    28.0  mm.  (If  in.  x  H  in.,  diagonal  ij/g  in.)     (For 
new  standard  aperture  see  §  57oa). 

2.  The  moving  picture  is  to  be  thrown  on  the  screen  with  an 
image  either  as  wide  or  as  high  as  the  magic  lantern  picture  or  of 
the  same  diagonal. 

Lantern  slides  have  a  maximum  opening  of  7.5  cm.  wide,  7  cm. 
high,  diagonal  10.2  cm.  (3  in.  wide,  2^4  in.  high,  4  in.  diagonal). 


Condenser 


FIG.  332.     THREE-LENS  CONDENSER. 

With  a  three-lens  condenser  the  source  of  light  is  placed  at  the  principal 
focus  of  the  first  element  of  the  condenser,  the  meniscus  and  the  plano-convex 
lens,  the  position  of  the  lenses  being  as  here  shown.  This  gives  a  parallel 
beam  of  light.  The  second  element  of  the  condenser  should  then  be  of  a  focal 
length  to  cause  the  rays  to  cross  at  the  center  of  the  projection  objective  (fig.  2). 

That  one  dimension  shall  be  the  same  with  both  the  moving  pic- 
ture and  the  magic  lantern,  requires  that  the  magic  lantern  objec- 
tive have  a  focus  which  is  from  3  to  4  times  as  long  as  the  moving 
picture  objective,  as  is  shown  in  the  following  table: 


The  two  pictures  are  to  be  of 


Lantern  slide  objective  has  an  equivalent 
focus  of 


The  same  width 
The  same  height 
The  same  diagonal 


3.17  times  that  of  M.  P.  objective 
4.00  times  that  of  M.  P.  objective 
3.65  times  that  of  M.  P.  objective 


CH.  XIV]       IMAGE  FORMATION  WITH  MOVING  PICTURES       593 

In  the  above  case,  with  a  moving  picture  objective  of  13.3  cm. 
focus,  the  focus  of  the  magic  lantern  objective  to  use  with  the 
moving  picture  objective  is,  for  the  same: 

Width  of  picture  42.1  cm.  i6Kin. 

Height  of  picture  53- 1  cm.  21      in. 

Diagonal  of  picture  48.5  cm.  19      in. 

3.  The  same  arc  lamp,  condenser,  etc.,  are  to  be  used  inter- 
changeably for  either  films  or  slides  by  simply  pushing  the  appara- 
tus sidewise.     Usually  the  slide-carrier  is  mounted  permanently 
with  the  condenser  so  that  the  opening  is  not  a  circle  of  the  diame- 
ter of  the  condenser  but  a  rectangle  7.5  cm.  x  10  cm.  (3  in.  x  4  in.). 

4.  Even  illumination  of  the  screen. — If  the  light  is  not  quite 
uniform  it  is  better  to  have  the  center  the  brighter  rather  than  the 
edge. 

§  824.  Ideal  case,  moving  pictures. — The  ideal  case  of  projec- 
tion (shown  in  fig.  328  a  and  b)  is  where  the  light  is  a  point  source 
and  the  condenser  has  no  spherical  aberration.  This  is  the  case 
which  is  usually  figured,  but  it  is  not  the  best  in  practice  if  an 
extended  source  is  used. 

When  changing  over  to  moving  picture  films  the  lamp  and 
condenser  are  moved  to  the  position  b.  The  objective  O,  is  still 
45  cm.  (18  in.)  from  the  condenser  face  where  the  rays  will  cross 
in  the  diaphragm  plane,  and  the  film  is  placed  13.3  cm.  (5^  in.) 
from  the  objective  so  that  it  will  be  in  focus  on  the  screen. 

§  825.    Illumination  of  moving  pictures,  practical  method.— 

The  method  which  has  been  found  most  successful  in  lighting 
moving  pictures  is  to  focus  the  image  of  the  crater  not  on  the  objec- 
tive but  on  the  aperture  plate.  This  is  because  a  moving  pictur^ 
objective  usually  has  a  diameter  greater  than  the  diagonal  of  tlJe 
film  (40  mm. to  65  mm. against  28.5  mm. diagonal;  i^in.  to  2^  in. 
against  i  J^  in.  diagonal),  hence  the  important  point  is  to  get  the 
light  through  the  film;  the  large  objective  will  take  in  all  the  light 
which  can  get  through  the  film. 

Figures  333-335  show  the  effects  of  different  methods  of  lighting. 
In  practice  all  three  are  used  together,  that  is,  the  film  is  illum- 
inated by  the  area  of  the  condenser  which  is  not  covered  by  the 


594       IMAGE  FORMATION  WITH  MOVING  PICTURES       [Cn.  XIV 

slide-carrier  and  is  evenly  illuminated  by  the  combined  effect  of 
spherical  aberration  and  an  extended  image  of  the  crater. 

§  826.     Image  formation  with  moving  pictures. — Let  us  trace 
the  course  of  the  rays  from  the  condenser  to  the  screen  assuming 
a 

o 


.  333.     IMAGE  FORMATION  OF  A  MOVING  PICTURE  FILM  WITH  AN  EXTENDED 

SOURCE  OF  LIGHT. 

a, b     Second  element  of  the  condenser. 
L'     Image  of  the  source  of  light. 
5,  /     Film. 
O     Objective. 

w,  y,  x,  z     Points  on  the  face  of  the  objective, 
ft',  b'     Image  of  the  condenser. 
s',  t'     Image  of  the  film  on  the  screen. 


334- 
A  POINT  SOURCE  AND  A  CONDENSER  HAVING  SPHERICAL  ABERRATION. 

ft,  b,  c,  d,  e,  /,  g     Points  on  the  condenser  face. 

r,  s,  t    Film. 

D     Diaphragm  in  front  of  the  objective. 

the  crater  image  to  cover  the  entire  opening  of  the  aperture  plate. 
From  every  point  of  the  condenser  as  a,  fig.  333,  light  spreads  out 
over  the  area  5  /.  Light  will  reach  the  point  s,  on  the  film  from 
every  part  of  the  condenser  between  a  and  b.  From  s,  light 
spreads  out  in  every  direction  between  the  limiting  rays  b  s  w,  and 
a  s  y,  and  the  objective  0,  collects  all  of  this  light  to  the  point  s  'on 
the  screen.  Light  from  s,  reaches  5',  between  the  limiting  rays 


CH.  XIV]     IMAGE  FORMATION  WITH  MOVING  PICTURES         595 

5  w  sr  and  s  y  s.'    In  the  same  way  light  from  /,  reaches  /',  between 
the  limiting  rays  toot'  and  t  z  tf. 

The  objective  O,  will  bring  an  image  of  the  condenser  face  to  a 
focus  somewhere  between  it  and  the  screen.  In  fig.  333  the  image 
of  the  condenser  face  is  at  the  point  a'  bf.  With  the  magic  lantern 
the  condenser  face  and  the  lantern  slide  being  so  close  together  the 
image  of  the  condenser  face  is  nearly  in  focus  on  the  screen. 


FIG.  335.     THE  DOTTED  LINES  SHOW  THE  MARGINAL  RAYS  REMOVED  BY  THE 

SLIDE-CARRIER. 

s,  t     Film. 
0     Objective. 

0 


FIG.  336.     SMALL  CONDENSER  FOR  MOVING  PICTURES. 
This  is  exactly  comparable  to  lantern  projection  except  that  the  condenser 
and  the  object  are  smaller. 

§  827.  Image  formation  when  using  a  point  source  and  a 
condenser  with  no  spherical  aberration. — The  crater  image  in  this 
case  would  be  focused  at  o,  and  only  the  rays  a  s  y  s',  and  b  t  x  t', 
would  be  used  (fig.  333). 

§  828.  Image  formation  with  a  point  source  and  a  condenser 
having  spherical  aberration. — The  condenser  must  have  either  no 
spherical  aberration  at  all  or  just  the  right  amount.  Fig.  334 
represents  a  condenser  having  the  right  amount  of  spherical  aberra- 
tion. Consider  the  effect  of  each  zone  of  the  condenser  in  illum- 
inating the  film.  The  center  zone  from  d  to  e,  lights  most  of  the 
center  of  the  film.  With  this  zone  onlv,  the  illumination  would 


SQ6        IMAGE  FORMATION  WITH  MOVING  PICTURES       [Cn.  XIV 

be  dim  but  fairly  uniform.  The  zones  from  c  to  d,  and  e  to/,  light 
a  narrow  ring  of  the  film  near  5  and  t,  i.  e.,  a  dim  center  and  a  bright 
outside  ring  would  be  produced  by  the  zone  from  c  to  f  (fig.  326, 
position  c).  The  zone  b  to  c,  lights  the  part  of  the  film  between  s 
and  r,  and  /  to  g,  lights  the  part  between  t  and  r,  the  addition  of 
these  zones  is  to  increase  the  illumination  of  the  center,  making  the 
illumination  more  uniform.  The  narrow  zones  a-b,  and  g-h,  out- 
side this,  further  illuminate  the  region  in  the  center  of  the  film.  It 
is  necessary  to  remark  that  with  an  actual  point  source  the  illum- 
ination with  this  arrangement  can  never  be  really  uniform  but  the 
"aberration  figure"  will  consist  of  a  bright  ring  5  t,  a  bright  point  r, 


FIG.  337.     CONVEX  LENS  SHOWING  CHROMATIC  ABERRATION. 

(From  The  Microscope] 

The  ray  of  white  light  (w)  is  represented  as  dividing  into  the  short  waved, 
blue  (6)  and  the  long  waved,  red  (r)  light.  The  blue  (b)  ray  comes  to  a  focus 
nearer  the  lens  and  the  red  ray  (r)  farther  from  the  lens  than  the  principal 
focus  (/).  Principal  focus  (/)  for  rays  very  near  the  axis;  /'  and/",  foci  of  blue 
and  red  light  coming  from  near  the  edge  of  the  lens.  The  intermediate  wave 
lengths  would  have  foci  all  the  way  between  /'  and  f". 

at  the  center  (white  ghost)  and  between  will  be  a  more  or  less 
evenly  lighted  disc.  When  a  slightly  extended  source  is  used  how- 
ever, the  aberration  figures  for  the  different  points  of  the  source  will 
overlap  and  if  the  dimensions  of  the  crater  image  are  about  one- 
third  as  great  as  the  aberration  figure  an  even  illumination  may  be 
secured. 

§  829.     Effect  of  the  diameter  of  the  objective. — If  for  any 

reason,  as  the  insufficient  diameter  of  the  objective  lenses,  some 
of  the  light  rays  are  lost  after  passing  the  film,  the  effect  on  the 
screen  image  is  the  same  as  if  these  rays  never  reached  the  film. 
Thus,  if  the  objective  0,  fig.  333,  has  such  a  small  diameter  that  it 
would  not  admit  the  ray  b  s  w,  the  effect  would  be  the  same  as  if  no 
light  reached  s,  from  the  point,  b,  of  the  condenser. 


CH.  XIV]       IMAGE  FORMATION  WITH  MOVING  PICTURES        597 

By  reference  to  fig.  334,  it  can  be  seen  that  if  the  objective  has  a 
small  diameter,  or  if  an  iris  diaphragm  D,  with  a  small  opening  is 
present,  only  light  from  the  central  zones  from  b  to  e,  is  permitted 
to  pass.  Increasing  the  diameter  of  the  objective  or  diaphragm 
opening  has  practically  the  same  effect  as  increasing  the  diameter 
of  the  condenser.  As  the  diaphragm  is  opened  the  effect  is  striking, 
it  is  as  if  there  were  three  layers  of  light  upon  the  screen:  First, 
the  bright  spot  in  the  center  of  the  screen  increases  in  size  until  it 
covers  the  entire  opening  of  the  aperture  plate,  then  the  light  has 
the  appearance  of  folding  over  on  itself  and  the  second  layer  spreads 
over  the  picture  starting  from  the  edges.  During  this  stage  the 
illumination  is  uneven,  there  being  a  dark  spot  in  the  center  of  the 
field  (dark  ghost).  The  second  layer  of  light  reaches  the  center 
and  goes  beyond  so  there  is  a  layer  of  light  which  starts  at  the 
center  and  spreads  out  towards  the  periphery  of  the  field.  In  this 
stage  the  illumination  is  brighter  at  the  center  of  the  field  than  at 
the  edges,  there  being  a  bright  spot  in  the  center  (light  ghost). 
With  a  larger  aperture  yet  the  third  layer  spreads  over  the  entire 
field.  For  this  reason  one  is  more  likely  to  secure  an  evenly 
illuminated  field  having  no  shadows  in  the  center  if  an  objective 
with  large  lenses  is  used  than  if  one  with  small  lenses  is  used. 

§  830.     Advantage  in  using  a  large  diameter  objective. — The 

difficult}*  of  lighting  a  picture  evenly  when  using  an  objective  of 
small  diameter  is  often  very  great  and  requires  a  good  deal  of 
rather  careful  adjusting  to  eliminate  a  shadow  in  the  center  of  the 
field  and  to  get  rid  of  the  reddish  brown  corners  at  the  same  time. 
It  is  necessary  to  try  various  distances  from  the  lamp-house  to  the 
aperture  plate,  different  positions  of  the  arc  with  respect  to  the 
condensers  and  it  will  perhaps  be  necessary  to  try  condensers  of 
different  focal  lengths.  It  will  generally  be  found  more  satisfactory 
and  convenient  to  have  an  objective  of  large  diameter  which  will 
allow  quite  a  range  of  adjustment  either  side  of  the  very  best 
without  materially  damaging  the  result. 

§  831.  Special  condenser  for  moving  pictures. — If  the  con- 
denser of  a  moving  picture  outfit  were  designed  especially  for  that 


5Q8      IMAGE  FORMATION,  PROJECTION  MICROSCOPE    [Cn.  XIV 

purpose  and  was  not  intended  to  serve  for  lantern  slides  also,  the 
design  would  be  exactly  similar  to  that  for  lantern-slide  projection 
except  that  everything  would  be  on  a  smaller  scale,  the  condenser 
lenses  being  of  smaller  diameter  and  of  shorter  focal  length.  This 
would,  of  course,  necessitate  placing  the  lamp  very  close  to  the 
lenses,  but  they  will  be  small  and  correspondingly  thin  and  will  not 
crack  as  easily  as  larger  ones.  Whether  or  not  this  would  be  a  good 
design  for  a  large  size  outfit  using  35  to  50  amperes  is  not  certain, 
but  there  is  no  doubt  that  good  results  can  be  obtained  for  projec- 
tion on  a  small  scale  using  three  to  four  amperes  which  would  not 
be  possible  on  account  of  the  difficulty  of  getting  even  illumination 
if  the  big  standard  size  condenser  were  used. 

§  832.  Experiment  with  small  size  condenser. — The  method 
of  image  formation  using  a  small  size  condenser  is  shown  in  fig.  336. 
L,is  the  source,  an  arc  using  5  mm.  carbons  and  3  amperes  of  cur- 
rent. It  is  practically  a  point  source.  Condenser  lenses  58  mm. 
(2% in.)  in  diameter  and  63  mm.  2^in.)  focal  length  placed  25  mm. 
(i  in.)  from  the  source  were  used. 

Even  when  using  this  very  small  source  (3  mm.  circle)  a  perfectly 
uniformly  illuminated  field  was  obtained,  a  thing  which  could  not 
be  done  when  a  large  condenser  having  the  usual  amount  of 
spherical  aberration  was  tried. 

The  diameter  of  the  light  cone  through  the  objective  was  2  cm. 
(24  m-)  •  When  large  carbons  were  used  and  the  current  increased 
to  1 2  amperes  the  effects  were  to  increase  the  brightness  of  the  pic- 
ture and  to  increase  the  diameter  of  the  cone  of  light  through  the 
objective  to  3  cm.  (1^4  in.).  It  is  seen  that  in  either  case  the  lenses 
of  the  objective  did  not  need  to  be  of  as  large  diameter  as  when 
using  the  ordinary  large  condenser. 

IMAGE  FORMATION  WITH  THE  PROJECTION  MICROSCOPE 

§  833.  Illumination  for  low  powers. — For  low  powers  (20  to  100 
mm.  objectives)  the  principle  is  that  the  focus  of  the  condenser 
should  fall  at  the  center  of  the  projection  objective  and  that  the 
object  should  be  placed  in  the  converging  cone  of  light  in  the  posi- 


CH.  XIV]     IMAGE  FORMATION,  PROJECTION  MICROSCOPE      599 

tion  to  give  a  sharp  image  on  the  screen.  To  accomplish  this  best, 
the  objective  is  so  placed  that  the  focus  of  the  condenser  is  at  the 
center  of  the  projection  objective,  and  then  the  stage  of  the  micro- 
scope is  moved  back  and  forth  until  the  image  is  sharp  upon  the 
screen.  If  a  three-lens  condenser,  without  a  substage  condenser  is 
used,  it  will  be  found  best  for  low  powers  (20-125  mm.  focus)  to 
have  a  condensing  lens  next  the  objective  (2d  element  of  the  con- 
denser, fig.  332)  of  20  to  25  cm.  (8  to  10  in.)  principal  focus.  For 
the  higher  powers  where  greater  numerical  aperture  is  needed,  a 
condenser  lens  of  15  cm.  focus  is  better. 


FIG.  338.     IMAGE  FORMATION  WITH  AN  AMPLIFIER. 

O     The  back  lens  of  the  objective. 

A     Amplifier  (divergent  lens). 

/'  The  image  which  would  be  projected  by  the  objective  if  no  amplifier  were 
in  place. 

/     Image  projected  with  the  amplifier  in  place. 

Note  that  the  rays  from  A  diverge  more  rapidly  than  from  O  making  the 
image  larger  than  without  the  amplifier.  (See  also  fig.  126). 

§  834.  Illumination  for  high  powers. — In  all  high  power  micro- 
scopic projection  (2  to  16  mm.  objectives)  any  source  of  light  should 
be  considered  as  an  extended  source  whether  lime  light,  arc  light, 
or  the  sun  is  used. 

The  best  method  of  illuminating  microscopic  specimens  has  been 
found  to  be  to  place  the  microscope  so  that  the  front  lens  of  the 
objective  is  in  the  image  of  the  crater  (or  the  sun),  (fig.  140)  and 
then  the  specimen  is  moved  up  toward  the  objective  until  its  image 
is  in  focus  on  the  screen.  Light  will  extend  from  every  point  of  the 
object  as  shown  in  fig.  347  and  strike  the  front  lens  of  the  objective. 
The  action  of  the  objective  is  to  bring  all  of  the  light  leaving  a  point 
of  the  object  to  a  single  point  on  the  screen.  The  details  of  image 


6oo     IMAGE  FORMATION,  PROJECTION  MICROSCOPE     [Cn.  XIV 

formation    are    taken    up   later  in    §   858,   in   connection  with 
aperture. 

§  835.  Amplifiers  and  oculars. — When  using  the  projection 
microscope  it  is  often  desirable  to  magnify  the  screen  image  without 
changing  to  another  objective.  This  may  be  done  with  an  ampli- 
fier or  an  ocular. 

§  836.  Image  formation  with  an  amplifier. — The  amplifier  is 
a  negative  lens  or  combination  placed  some  distance  beyond  the 
objective.  Without  the  amplifier  the  objective  would  form  an 
image  at  I'  (fig.  338).  The  effect  of  the  amplifier  (A)  is  to  cause 
rays  to  cross  at  /  which  would  otherwise  cross  at  /'  and  at  the  same 
time  the  light  from  the  objective  0  is  rendered  more  divergent  and 
it  covers  a  larger  area  on  the  screen  than  it  would  without  the 
amplifier  (Fig.  126). 

When  using  the  amplifier  one  must  focus  the  objective  slightly 
farther  from  the  specimen. 

§  837.  Magnification  due  to  the  amplifier. — The  magnification 
due  to  the  amplifier  is  greater  the  shorter  its  focal  length  and  the 
farther  it  is  from  the  objective.  The  same  principle  is  employed 
as  with  the  telephoto-attachments  to  photographic  objectives.  It 
has  been  found  that  an  amplifier  of  -5  diopters  (20  cm.  focus)  11.3 
cm.  from  the  objective  will  give  a  magnification  of  1.68  times  and 
an  amplifier  of  -10  diopters  (10  cm.  focus)  at  the  same  distance  will 
magnify  2.5  times.  (See  also  §  356a  for  diopter,  and  for  the 
amplifier  §  3Q2a). 

§  838.  Projection  ocular. — A  projection  ocular  is  required  for 
certain  apochromatic  objectives  which  are  designed  to  be  used  only 
with  a  compensation  ocular,  and  when  the  microscope  is  used  with 
polarized  light,  otherwise  it  is  not  necessary  to  use  an  ocular, 
although  one  may  be  used  with  any  microscopic  objective,  see  Ch. 
IX  and  Ch.  X  under  demonstrations  and  drawing  with  high  powers 
(§  401,  405,  477). 

The  field  lens  O\  (fig.  339)  in  connection  with  the  objective 
forms  an  inverted  image  of  the  object  at  D.  This  image  is  in  turn 
projected  by  the  eye  lens  or  combination  Oz,  to  the  screen  at  /. 


CH.  XIV]        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        601 

This  image  is  inverted  with  respect  to  J9,but  erect  with  respect  to 
the  original  object  (fig.  207).  A  diaphragm  at  D,  limits  the  size 
of  the  field  and  makes  its  boundaries  sharp.  Often  owing  to  the 
small  size  of  the  diaphragm  D,  the  field  is  not  as  large  as  desirable 
on  the  screen. 

Besides  limiting  the  size  of  the  field  there  is  a  greater  loss  of  light 
with  the  ocular  than  with  the  amplifier  as  the  ocular  is  made  of  at 
least  two  separated  lenses  while  the  amplifier  consists  of  but  one 
lens  or  a  cemented  combination. 


FIG.  339.     IMAGE  FORMATION  WITH  A  PROJECTION  OCULAR. 

0  Objective  forming  a  real  inverted  image  D,  with  the  help  of  the  field  lens 
of  the  ocular  Ox. 

O,     Field  lens  of  the  ocular. 

O2  Eye  or  projection  lens  of  the  ocular.  It  projects  a  screen  image  /,  of 
the  real  image  D. 

The  image  D,  wa?  inverted  by  the  objective.  O2,  also  inverts  the  image  D,  in 
projecting  it,  hence  the  final  image  /  is  erect  like  the  object.  (See  also  fig.  207). 

APERTURE  AND  LIGHT  LOSSES 

§  839.  So  far  the  path  of  the  light  from  the  source  to  the  screen 
has  been  considered  mainly  from  the  standpoint  of  image  formation, 
no  account  having  been  taken  of  the  amount  of  light  needed  or  of 
the  losses  of  light  and  energy  in  the  apparatus. 

Light  losses  may  occur  from  three  causes: 

1.  Removal  of  the  margin  of  a  beam  of  light  due  to  lenses  of 
insufficient  diameter. 

2 .  Reflection  of  light  both  regular  and  diffused,  at  the  surfaces 
of  the  lenses. 

3.  Absorption  of  light  by  the  glass  of  the  lenses,  by  the  partial 
opacity  of  the  object  and  by  dirt. 

4.  Special  light  losses  due  to  the  nature  of  the  experiment. 


6o2        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        [Cn.  XIV 

§  840.    Losses  by  the  removal  of  the  margin  of  the  beam.— 

From  the  source,  light  spreads  out  in  all  directions.  Only  the  light 
that  strikes  the  front  surface  of  the  first  lens  of  the  condenser  is 
available,  hence  the  first  lens  should  be  of  such  a  diameter  and  so 
placed  that  it  takes  in  as  large  an  angle  of  light  from  the  source  as 
possible.  The  use  of  a  meniscus  lens  next  the  radiant  allows  a  much 
larger  angle  of  light  to  be  used  than  does  a  condenser  without  such 
a  lens  (fig.  332,  §  821). 

The  lenses  after  the  first,  should  not  remove  any  of  the  border 
rays  of  the  light  transmitted  by  the  first  lens,  or  the  first  lens  need 
be  of  only  sufficient  diameter  to  furnish  a  beam  of  light  which  will 
just  fill  the  opening  of  the  other  lenses. 

After  passing  through  the  condenser  the  light  is  available  for 
illuminating  the  object.  With  the  magic  lantern  the  entire 
diameter  of  the  cone  of  light  passes  through  the  objective  and 
reaches  the  screen.  With  moving  picture  projection  the  entire 
cone  of  light  may  or  may  not  get  through  the  objective  (§  825,  829). 
With  the  microscope,  except  for  the  lowest  powers,  the  objective 
lenses  are  smaller  than  the  image  of  the  crater  which  is  thrown  on 
the  front  of  the  objective,  and  much  loss  of  light  occurs  from  this 
cause. 

§  841.  Losses  by  reflection. — The  polished  surfaces  of  a  lens 
reflects  some  light,  about  4  to  5  per  cent,  at  each  surface  between 
glass  and  air;  8  to  10  per  cent,  for  each  lens  or  plate  of  glass.  If 
the  surfaces  of  the  glass  are  not  perfectly  clean  or  perfectly  polished 
the  light  losses  may  amount  to  much  more,  sometimes  15%  at  each 
surface.  All  reflected  light  being  lost,  this  effect  is  generally  much 
more  important  than  the  slight  absorption  in  the  body  of  the  glass 
itself.  A  good  illustration  of  this  reflection  by  glass  surfaces  is  the 
brilliant  reflection  from  windows  often  seen  at  sunset. 

§  842.  Light  losses  by  absorption. — The  object  (slide,  film,  or 
specimen)  absorbs  some  of  the  light  incident  upon  it.  This  is  a 
necessary  accompaniment  of  showing  the  object  at  all,  but  an 
object  which  does  not  absorb  too  much  light  is  to  be  preferred 
whenever  obtainable. 


CH.  XIV]        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        603 

The  glass  of  which  lenses  are  made  is  not  perfectly  transparent 
but  absorbs  some  light.  This  is  especially  true  of  the  thick  con- 
denser lenses  which  usually  look  green  when  laid  on  a  piece  of  white 
paper.  Such  green  lenses  will  be  found  to  absorb  an  appreciable 
amount  of  light.  Some  condenser  lenses  made  of  cheap  glass  will 
turn  purple  after  being  in  use  for  some  time. 

§  843.  Special  light  losses. — The  use  of  polarized  light  neces- 
sarily entails  the  loss  of  one-half  of  the  light  in  the  polarizing  nicol. 
The  analyzing  nicol  may  transmit  most  of  the  remaining  light  but 
generally  it  is  turned  to  transmit  but  a  small  portion  of  it  (§  884). 

In  moving  picture  projection  the  shutter  covering  the  lens  while 
the  film  is  in  motion  removes  part  of  the  light.  In  this  case  it  has 
been  found  by  careful  experiment  that  removing  all  of  the  light 
part  of  the  time  has  exactly  the  same  effect  as  removing  part  of  the 
light  all  of  the  time.  Some  shutters  remove  but  /4  of  the  light 
while  others  remove  y£  of  the  light  (§  591).  The  latter  are,  how- 
ever, sometimes  to  be  preferred,  the  avoidance  of  nicker  being  of 
more  importance  than  the  slight  dimming  of  the  image. 

ENERGY  LOSSES 

§  844.  Of  the  energy  which  is  radiated  by  the  source  only  a 
comparatively  small  part,  from  2  to  10  per  cent,  is  of  those  wave 
lengths  which  affect  the  eye,  the  major  part  of  the  energy  being  in 
the  infra-red  part  of  the  spectrum  (fig.  307).  This  infra-red 
radiation  accompanies  the  light  radiation  and  is  bent  by  a  lens  in 
very  nearly  the  same  manner.  It  has  been  found  that  the  differ- 
ence in  focus  between  the  infra-red  and  the  red,  for  glass,  is  no 
greater  than  the  difference  in  focus  between  the  red  and  the  blue. 
This  is  due  to  the  special  dispersive  qualities  of  glass. 

§  845.  Disadvantage  of  the  infra-red. — As  the  infra-red 
radiation  has  such  great  energy  and  consequently  so  great  a  heating 
effect  wherever  it  is  absorbed  (fig.  307),  and  as  at  the  same  time 
it  has  no  effect  upon  the  eye,  it  is  advantageous  to  remove  it  as  far 
as  possible.  Energy  losses,  in  so  far  as  they  are  not  accompanied 
by  light  losses,  are  of  advantage. 


604        LIGHT  AND  ENERGY  LOSSES  IN. PROJECTION        [Cn.  XIV 

§  846.  The  energy  losses. — The  energy  losses  occur  principally 
in  three  places  (fig.  342). 

1.  In  the  condenser  lenses. 

2.  In  the  water-cell,  if  there  is  one. 

3 .  In  the  specimen. 

Energy  losses  beyond  the  specimen  are  not  considered  separate 
from  light  losses. 

§  847.  Losses  in  the  condenser. — The  glass  of  which  the  con- 
densers are  made,  even  if  perfectly  transparent  to  visible  light, 
absorbs  a  large  amount  of  infra-red.  A  piece  of  condenser  glass 
2  cm.  thick  was  found  to  absorb  41%  of  the  radiant  energy  from 
the  positive  crater  of  the  right-angle  arc  incident  upon  it,  while 
absorbing  but  i o%  of  the  incident  light.  This  has  two  effects. 

1 .  The  absorbed  energy  heats  the  condenser  very  greatly. 

2 .  The  light  which  gets  through  the  condenser  has  a  much  less 
heating  effect  on  the  specimen  than  it  would  have  otherwise. 

The  first  effect  (heating  the  condenser)  is  a  distinct  disadvantage 
to  the  condenser  as  it  is  one  of  the  causes  of  condenser  breakage. 
Most  of  the  energy  absorbed  will  be  by  the  first  lens,  and  in  that 
one,  more  will  be  absorbed  at  the  surface  near  the  lamp  than  away 
from  it ;  a  circumstance  which  leads  to  unequal  heating  and  puts  a 
strain  on  the  lens. 

Different  kinds  of  glass,  equally  transparent  to  visible  light 
absorb  different  amounts  of  infra-red.  For  example,  crown  glass 
will  be  found  to  be  opaque  to  some  of  the  longer  waves  to  which 
flint  glass  is  perfectly  transparent. 

The  second  effect  of  heating  the  condenser  is  an  advantage,  as 
the  specimen  is  relieved  of  a  good  deal  of  the  heating  effect.  Lan- 
tern slides  are  less  likely  to  be  cracked,  moving  picture  films  are 
less  likely  to  curl  or  catch  fire,  and  microscopic  specimens  can  be 
shown  for  a  longer  time  before  they  are  injured. 

§  848.  Energy  losses  in  the  water-cell. — Water  is  very  opaque 
to  radiation  of  great  wave  length,  even  the  thinnest  films  being 
absolutely  opaque  to  certain  wave-lengths. 

The  table  (§  849)  and  the  curves  (fig.  340-341)  show  the  energy 
of  the  positive  crater  of  the  right-angle  arc  transmitted  by  layers 


CH.  XIV.        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        605 

of  water  of  different  thickness.  The  lower  line  represents  the 
energy  transmission.  About  10%  of  the  light  (and  energy)  is  lost 
by  non-selective  reflection.  If  one  wished  to  know  the  amount  of 
energy  transmitted  to  get  the  same  light  transmission,  it  is  neces- 
sary to  add  10%  to  the  above  values.  The  upper  line  represents 
the  transmitted  energy  after  the  correction  for  reflection  has  been 
made.  These  curves  show  that  after  the  first  four  or  five  centi- 
meters of  water,  increased  thickness  does  not  reduce  the  energy 
very  much.  A  6  cm.  layer  of  water  transmits  22.5%,  a  10  cm. 
layer  transmits  20%,  the  difference  absorbed  in  the  last  4  cm.  being 
but  2.5%  of  the  incident  energy. 

The  water-cell,  by  absorbing  a  great  deal  of  the  energy,  reduces 
the  heating  effect  of  the  light  on  the  specimen.     This  energy 


so 

40 

c 
«30 

3 
* 

020 

u5 

J* 

10 


r      li  1 


0  I         4         6 

Thichness 


8        i.O       12       1.4 
Centimeters 


0         t       4        6       6       10      l£ 

Thichness      Centimeters 


FIG.  340-341.     PERCENTAGE  OF  ENERGY  FROM*"THE"CRATER  OF  THE  RIGHT- 
ANGLE  ARC  TRANSMITTED  BY  LAYERS  OF.WATER  OF  DIFFERENT  THICKNESS. 

The  lower  curve  shows  the  actual  energy[  transmission  in  each  case,  and 
the  upper  curve  shows  the  actual  energy  transmission  corrected  for  reflection. 


6o6        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        [Cn.  XIV 


absorbed  in  the  water-cell  heats  the  water,  but  water  is  peculiarly 
adapted  for  this  purpose  for  it  is  the  best  known  absorbent  of  the 
infra-red,  "heat  rays." 

Water  is  easily  obtained  and  put  into  the  cell.  It  has  the  highest 
specific  heat  of  any  known  substance;  i.  e.,  a  given  quantity  of 
water  will  absorb  more  energy  when  being  warmed  a  given  amount 
than  will  anything  else.  If  the  water  in  a  water-cell  becomes  so 
hot  that  it  gives  off  bubbles,  a  cool  cell  can  be  substituted  for  it. 
Cooling  a  cell  by  the  circulation  of  cold  water  through  it  has  not 
proved  successful. 

The  temperature  of  the  water  has  no  appreciable  effect  upon  the 
energy  absorption,  boiling  water  serving  as  well  as  ice  water.  The 
energy  transmission  for  a  water-cell  was  found  when  hot  (80°  C.) 
to  be  18.4%;  when  cold  (22°  C.)  19.2%.  The  water  was  slightly 
turbid,  being  more  so  when  hot  than  cold. 

§  849.  The  energy  transmission  of  layers  of  water  of  different 
thickness. — The  source  of  light  is  the  crater  of  the  right-angle 
carbon  arc. 

WITH  12  AMPERES  DIRECT     CURRENT 


Thickness  of  the  layer 
of  water 

ENERGY  TRANSMITTED 

Observed 

Corrected  for  reflection 

.15  cm. 

43-5% 

48.4% 

•30 

38.9% 

43-2% 

•45 

35-8% 

39-8% 

.60 

34-i% 

37-9% 

75 
.90 

32.9% 
31-9% 

36.6% 
354% 

1.  00 

29.5% 

32.8% 

1.05 

30.8% 

34-2% 

1.20 

27-8% 

30.8% 

i-35 

27.0% 

30.0% 

6.00 

22.5% 

25-2% 

8.00 

22.0% 

24.0% 

10.00 

20.0% 

22.5% 

WITH  15  AMPERES  ALTERNATING  CURRENT 


8.00  cm. 


14-4% 


15-6% 


CH.  XIV]        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION        607 

§  850.    Table   of    the    energy    and    light    transmission    and 
absorption. 


ABSORBING  ELEMENT 

PHOTOMETRIC 

ENERGY 

Absorp. 

Trans. 

Absorp. 

Trans. 

Plane  Glass  
Six  Centimeter  Cell  

10-55% 
10.75% 

28.8  % 

H.3  % 

22.6    % 

7.9    % 
14-7    % 
II.  I    % 

15-3    % 

1  1.6  % 

25-3  % 
36.0  % 
24.6  % 

8945% 
89.25% 

79-2  % 
88.7  % 
774  % 

92.1   % 

85.3  % 
88.9  % 

847  % 
88.4  % 

74-7  % 
64.0  % 

754  % 

41-5    % 

77-i  % 
82.0  % 

77-9  % 
78.8  % 

81.6  % 
80.8  % 

79-6  % 
77-2  % 
75-0  % 
27-5  % 
35-0  % 
68.6  % 
32.3  % 
23.5  % 
37-0  % 
274  % 

58.5    % 
22.9    % 

18.0  % 

22.1    % 
21.2    % 

18.4    % 
19.2    % 

20.4    % 
22.8    % 
25-0    % 
72.5    % 
65.0    % 
314    % 
67.7    % 
76-5    % 
63.0    % 
72.6    % 

PI.  glass  and  6  cm.  Cell  
Condenser  Cell  Clear 

Condenser  Cell,  Muddy  
Condenser  Water-Cell,   Hot. 
and  turbid  

Same,  Cold  and  turbid  
Stage  Water-cell  Clr 

Stage  Water-cell,  Muddy  .  .  . 
Ten  Centimeter  Cell 

Alum,  6  cm.  Cell  

Glycerin  6  cm.  Cell  
Mica,  thin  
Mica,  thick  
Balsam,  Stage  Cell  

Glass  Slide  green 

Glass  Slide,  white 

Green  Slide  with  Balsam  
White  Slide  with  Balsam  

§  85 1 .  Other  substances  dissolved  in  water. — Other  substances 
dissolved  in  water  have  not  been  found  to  improve  its  energy 
absorbing  qualities.  For  a  long  time  it  was  supposed  that  a 
saturated  solution  of  alum  was  more  effective  than  pure  water  but 
this  is  not  so ;  moreover  it  is  very  difficult  to  prepare  a  saturated 
solution  of  alum  which  is  not  turbid.  Tests  show  energy  trans- 
mission of  22.8%  for  an  alum  solution  as  against  22.9%  for  clear 
water,  while  alum  absorbs  15%  of  light  as  against  only  10%  for 
clear  water.  The  alum  only  serves  to  dilute  the  water.  Crystals 
of  alum,  K2  SO4  A12  (SO4)3  24  H2O,  absorb  much  energy,  but  it  has 
been  proved  that  this  is  entirely  due  to  the  water  of  crystallization. 

§  852.  Energy  losses  hi  the  specimen. — All  the  light  energy 
absorbed  by  the  specimen  is  converted  into  heat ;  hence,  an  opaque 
specimen  or  one  which  is  black  would  become  heated  in  the  con- 
centrated beam  of  light  necessary  for  the  microscope  even  if  only 
the  radiation  in  the  visible  spectrum  were  used.  As  the  visible 


6o8        LIGHT  AND  ENERGY  LOSSES  IN  PROJECTION         [Cn.  XIV 

radiation  constitutes  only  about  10%  of  the  energy  radiated  by  the 
arc,  this  effect  is  insignificant  in  comparison  to  the  heating  due  to 
the  infra-red.  Even  when  the  water-cell  is  used  only  43%  of  the 
energy  which  gets  through  is  visible  as  light.  A  greater  thickness 
of  water  would  reduce  this  effect  but  little,  hence  it  is  necessary  to 
carry  the  heat  away  from  the  specimen  as  rapidly  as  possible. 
This  is  done  by  the  stage  cooling  cell  which  is  in  contact  with  the 
glass  slide.  The  effect  is  purely  one  of  conduction,  and  a  thick 
piece  of  any  transparent  substance  would  answer.  But  water  has 
been  chosen  because  of  its  great  specific  heat,  and  the  comparative 
cheapness  of  hollow  glass  cells. 


3,000  C.P. 


Condenser         L  2 
Water  Cell 


FIG.  342.     ILLUSTRATION  OF  THE  LIGHT  AND  ENERGY  LOSSES  IN  THE 

PROJECTION  MICROSCOPE. 

Starting  from  the  arc  lamp  the  light  and  energy  reaching  the  first  face^of 
the  condenser  are  each  designated  by  100%.  Opposite  each  element  of  the 
optical  system  is  given  the  percentage  of  light  and  of  energy  transmitted  by 
each.  With  the  16  mm.  objective  only  about  6%  of  the  original  light  is  avail- 
able for  the  screen  picture. 

§  853.  Keeping  the  condenser  cool. — One  of  the  causes  of  the 
condenser  breakage  is  the  stream  of  hot  gases  from  the  arc  which 
strikes  the  upper  part  of  the  condenser  and  heats  it  unequally. 
This  is  specially  troublesome  when  the  lantern  is  tipped  up  at  an 
angle.  To  prevent  this  a  thin  sheet  of  glass  (watch  glass)  or  mica 
may  be  used  between  the  arc  and  the  condenser.  Glass  is  to  be 
preferred  as  it  is  more  transparent  than  mica  and  has  less  defects 
to  cause  shadows  on  the  screen.  The  following  data  refer  to  two 
sheets  of  mica  new  and  in  good  condition. 

Light  absorbed       Energy  absorbed 

Thin  piece... 25.3%  27.5% 

Thick  piece 36.0%  35-°% 


CH.  XIV]  EFFECT  OF  APERTURE  IN  PROJECTION  609 

This  shows  a  heavy  loss  in  light,  the  absorption  being  non- 
selective,  that  is  the  total  energy  transmitted  is  in  proportion  to 
the  light. 

§  854.  Example  of  light  and  energy  losses. — In  fig.  342  is  a 
diagrammatic  representation  of  the  light  and  energy  losses  actually 
found  in  a  certain  projection  system.  An  arc  light  was  used. 
The  light  and  energy  from  the  arc  striking  the  first  surface  of  the 
condenser  were  each  called  100%. 

After  passing  the  first  part  of  the  condenser  LI,  there  remains  82% 
of  the  light  and  54%  of  the  energy. 

After  passing  the  condenser  water-cell  the  light  was  reduced  to 
73%  while  there  was  left  only  16%  of  the  energy. 

After  leaving  the  second  part  of  the  condenser  L/2,  there  was  68% 
of  the  light  and  14%  of  the  energy.  Of  this  remaining  14%  of  the 
energy,  57%  is  invisible  and  43%  is  visible  as  light. 

When  used  with  a  magic  lantern  the  projection  objective  trans- 
mits only  70%  of  the  light  reaching  it.  As  68%  of  the  original 
light  reaches  the  objective,  the  screen  image  must  be  formed  by 
68  x  70  =  47.6%  of  the  original  light. 

With  the  microscope  further  losses  occur  due  to  the  presence  of 
the  stage  water-cell.  The  microscope  objective  lets  through  but  a 
small  amount  of  the  light  incident  upon  it,  the  loss  being  greater 
the  higher  the  power  of  the  objective.  A  16  mm.  objective,  for 
example,  transmits  10%  of  the  incident  light.  In  the  case  investi- 
gated only  10  x  60  =  6%  of  the  light  originally  striking  the  first 
lens  of  the  condenser  reached  the  screen.  If  a  substage  condenser 
and  an  ocular  are  used  the  light  for  the  screen  image  is  still  further 
reduced. 

EFFECT  OF  APERTURE 

§  855.  Increasing  the  aperture  of  a  perfect  lens  or  a  combina- 
tion of  lenses  with  undirected  light  has  two  effects. 

i.  It  increases  the  definition,  that  is,  the  image  shows  finer 
structures  than  does  a  lens  of  smaller  aperture,  i.  e.,  it  will  show 
more  lines  to  the  millimeter  or  inch.  See  under  Abbe  diffraction 
theory  §  910. 


6 io  EFFECT  OF  APERTURE  IN  PROJECTION  [Cn.  XIV 

2.     It  lets  through  more  light. 

The  effect  of  increasing  the  aperture  of  a  lens  when  using 
directed  light  as  with  the  magic  lantern  depends  somewhat  upon 
circumstances.  If  the  directed  light  spreading  out  in  the  form  of  a 
cone  has  a  greater  diameter  than  that  of  the  lens,  the  larger  the 
lens  up  to  the  full  diameter  of  the  cone,  the  greater  the  amount  of 
light  which  gets  through,  and  the  brighter  will  be  the  screen  image 
just  as  with  undirected  light. 

If  however,  directed  light  from  a  point  in  the  object  spreads  out 
over  a  cone  which  has  a  smaller  diameter  than  the  lens,  then  the 
size  of  the  lens  is  immaterial,  for  all  the  light  which  gets  through 
the  lens,  gets  through  a  small  part  of  its  area,  and  the  rest  of  the 
lens  is  not  used  at  all.  Increasing  the  diameter  of  the  lens  will  not 
increase  the  brightness  of  the  screen  image. 

Another  method  of  looking  at  the  problem  is  to  suppose  the 
diameter  of  the  lens  fixed  and  that  of  the  cone  of  directed  light  to 
be  increased  in  diameter,  assuming  the  light  source  to  have  the 
same  intrinsic  brilliancy,  i.  e.,  same  brightness  per  square  centi- 
meter. Suppose  we  start  with  a  very  small  source  of  light  behind 
the  pinhole  in  the  screen  5,  fig.  343,  This  light  will  get  through  a 
small  part  of  the  lens.  Now  increase  the  area  of  the  light  source  L, 
keeping  everything  else  the  same,  the  cone  of  light  from  5,  will  for 
a  time  all  get  through  the  lens  and  be  collected  at  a  point,  hence  the 
image  I,  of  the  pin  hole  will  keep  getting  brighter  until  the  cone  of 
light  just  fills  the  aperture  of  the  lens.  When  this  occurs  a  further 


FIG.  343.     SIZE  OF  THE  LIGHT  SOURCE  AND  BRILLIANCY  OF  THE  SCREEN  IMAGE. 

Up  to  a  certain  point  the  larger  the  light  source  L,  the  greater  will  be  the 
amount  of  light  from  the  specimen  S,  which  gets  through  the  objective  0,  but 
beyond  this  point  increase  in  the  size  of  the  light  source  produces  no  further 
increase  in  the  intensity  of  the  screen  image  as  the  light  (u,  v)  passes  outside 
the  objective. 


CH.  XIV] 


EFFECT  OF  APERTURE  IN  PROJECTION 


611 


increase  of  the  diameter  of  the  cone  of 
light  will  do  no  good  because  the  light 
falls  outside  of  the  opening  of  the  lens. 
The  conclusions  from  this  are: 

1.  If  there  is  a  cone  of    directed 

light  from  a  given  point  it  is  of  no  ad-  FIG.  344.    THE  "CLOSING  AN- 
,  .     ..  .,,       GLE"  OF  LIGHT  FORMING  A 

vantage    to   use    an    objective    with  SCREEN  IMAGE. 

lenses    of    larger   diameter  than   this  The  shaded  portion  shows 

cone,  the  closing  angle  when  an  or- 

TTT.  .         .           .        .  .                  .  dinarv  magic  lantern  is  used 

2.  With  a  given  size  objective,  when  with  an  arc  lamp  as  a  SOUrce 

the  object  is  illuminated  by  directed  of  light;  and  the  outside  lines 
. .  ,  A  .  .  .  , ,  show  the  closing  angle  when  an 

light  an  increase  in  area  of  the  source  extended  source  of  light,  like  a 
beyond  that  which  will  fill  the  aper-  gas  or  acetylene  flame,  is  used. 
ture  of  the  objective  is  of  no  advantage. 

§  856.  Method  of  determining  the  aperture  of  the  objective 
which  is  used. — The  simplest  way  is  to  look  directly  into  the 
objective  when  in  use;  of  course,  using  a  colored  glass  or  smoky 
mica  to  protect  the  eyes.  Do  not  hold  the  head  too  close  to  the 
objective.  If  the  whole  of  the  back  lens  of  the  objective  is  filled 
with  light  the  aperture  is  filled.  If  the  bright  light  is  only  in  the 
center  of  the  back  lens  the  bright  spot  is  the  part  of  the  aperture 
which  is  used.  It  often  occurs  that  different  parts  of  the  objective 
are  used  by  the  light  from  different  parts  of  the  object.  This  can 
be  determined  by  looking  at  the  objective  and  moving  the  head 
from  side  to  side. 


FIG.  345. 


THE  CLOSING  ANGLE  OF  LIGHT 
TO  FORM  AN  IMAGE. 


In  order  that  the  images  i  and  j,  shall  be  equally  bright  with  opaque  pro- 
jection, the  closing  angle  of  light  from  the  objectives  must  be  the  same. 
As  the  distance  bj,  is  twice  as  great  as  ai,  the  diameter  of  the  lens  (b)  must  be 
twice  as  great  as  that  of  (a)  and  its  area  must  be  four  times  as  great. 


612  EFFECT  OF  APERTURE  IN  PROJECTION         [Cn.  XIV 

BRIGHTNESS  OF  THE  SCREEN  IMAGE 

§  857.  The  brightness  of  the  image  can  be  calculated  in  either 
of  two  ways. — 

1 .  The  relative  area  of  the  object  and  image  and  the  illumina- 
tion of  the  object. 

2.  The  intrinsic  brilliancy  of  the  source  and  the  closing  angle 
of  the  rays  forming  the  image. 

The  first  case  is  more  applicable  to  directed  light  where  all  of  the 
light  illuminating  the  object  gets  through  the  objective.  For 
example,  with  a  magic  lantern,  let  the  area  of  the  slide  be  50  sq.  cm., 
7.1  x  7.1  cm.  and  the  area  of  the  screen  2x2  meters  =  40,000  sq. 
cm.  or  800  times  as  much  surface.  Let  the  brightness  of  the  slide 
illumination  be  48,000  meter  candles.  The  illumination  of  the 
screen  will  be  ~o  of  this  or  60  meter  candles. 

Actually  only  70%  of  the  light  from  the  slide  will  get  through  the 
objective,  and  the  illumination  will  be  42  meter  candles.  For 
ordinary  reading  with  artificial  light  one  needs  an  illumination  of 
from  30  to  50  meter  candles. 

The  second  case  is  most  useful  where  the  entire  aperture  of  the 
lens  is  filled  with  light,  as  with  a  large  light  source  with  the  micro- 
scope, and  with  opaque  projection.  Consider  the  same  example  as 
above  except  that  the  object  is  a  white  opaque  body  illuminated 
from  the  front.  More  data  concerning  the  lens  will  be  needed. 
Let  the  lens  be  one  of  14  cm.  (5^2  in.)  focus,  6.25  cm.  (2% in.)  diam- 
eter, the  size  of  the  picture  being  two  meters  square  as  before ;  and 
as  before  the  object  illuminated  with  an  intensity  of  48,000  meter 
candles.  To  secure  the  same  magnification  as  before  requires  a 
distance  of  396  cm.  (4  meters  approximately)  from  the  screen  with 
this  focus  objective.  Suppose  the  objective  is  looked  at  from  the 
screen.  Its  entire  opening  will  appear  of  the  same  brightness 
(except  for  absorption)  as  if  there  were  no  glass  present  and  the 
illumination  on  the  screen  will  be  just  the  same  as  if  the  light 
reaching  it  were  from  a  piece 'of  white  paper  having  an  area  of  50 
sq.  cm.  illuminated  by  48,000  meter  candles.  Considered  as  a 

source  of  light  this  paper  disc  would  have  a  candle-power  of '- 

io,ooox 


CH.  XIV]          EFFECT  OF  APERTURE  IN  PROJECTION  613 

per  sq.  cm.  (§  85 ;a)  50  sq.  cm.  gives  76  candle-power.  At  a  dis- 
tance of  4  meters  from  the  screen  this  gives  ^  =  4.8  meter  candles. 
Counting  the  losses  due  to  the  lens  as  30%  the  illumination  of  the 
screen  would  be  3.16  meter  candles.  This  is  about  a  third  of  the 
minimum  illumination  for  projection  in  a  perfectly  dark  room,  and 
about  one-tenth  of  what  would  be  required  for  good  projection. 

If  the  intrinsic  brilliancy  of  the  source  is  the  same  and  the  closing 
angle  is  the  same  the  illumination  will  be  the  same,  thus,  if  the 
screen  is  twice  as  distant  and  the  objective  has  twice  the  diameter 
the  illumination  would  be  the  same  (fig.  344-345). 

In  the  above  example  no  use  has  been  made  of  the  focal  length  of 
the  objective  nor  the  magnification  of  the  object,  these  having  no 
direct  influence  on  the  screen  illumination.  If  a  higher  magnifica- 
tion were  desired  a  shorter  focus  objective  would  be  substituted 
and  the  object  brought  nearer  to  it.  The  apparent  brightness  of 
the  paper  seen  through  the  objective  will  not  change  if  the  paper 
is  moved  closer  to  the  objective.  Therefore,  if  the  objective  has 
the  same  diameter  the  illumination  on  the  screen  will  be  just  as 
before. 

Another  way  of  looking  at  the  matter  is  this :  with  the  shorter 
focus  objective  a  certain  small  area  of  the  object  will  be  spread 
over  a  larger  area  on  the  screen,  but  bringing  the  object  nearer  the 
face  of  the  objective,  more  light  from  the  small  area  of  the  object 
will  enter  it.  These  two  effects  exactly  counterbalance  each  other, 
the  increased  light  taken  in  by  the  objective  being  sufficient  to 
illuminate  the  larger  area. 


§  857a.  Formula  for  finding  the  candle-power  of  a  surface  illuminated  at  a 
given  intensity. — Suppose  a  perfectly  diffusing,  perfectly  white  surface  to  be 
illuminated  at  a  given  intensity,  say  the  intensity  of  the  incident  illumination 
is  I  meter  candles,  i.  e.,  the  incident  light  flux  is  I  lumens  per  square  meter. 
The  light  falling  on  one  square  centimeter  will  be  1/10,000  lumens.  This  light 
will  be  scattered  in  all  directions  so  that  the  surface  appears  equally  bright 
when  seen  from  any  direction,  but  as  the  surface  appears  fore-shortened  when 
seen  from  any  other  direction  than  the  perpendicular,  more  light  will  be 
reflected  perpendicular  to  the^surface  than  in  any  other  direction.  The  candle- 
power  of  one  square  centimeter  of  this  surface  in  any  given  direction  can  be 
expressed  as  B  cos  6,  where  the  constant  B,  is  the  intrinsic  brilliancy  in  candle- 
power  per  square  centimeter  of  the  surface  considered  as  a  source  of  light,  and 
6,  is  the  angle  between  the  normal  to  the  surface  and  the  given  direction. 


614  EFFECT  OF  APERTURE  IN  PROJECTION  [Cn.  XIV 

Consider  a  sphere  of  one  meter  radius  having  this  lighted  surface  at  its 
center.  The  light  received  by  one  square  meter  of  the  surface  of  this  sphere 
will  then  be  B  cos  6  lumens.  Only  half  of  the  sphere  can  receive  light  from 
this  opaque  surface  and  the  entire  light  received  by  this  hemisphere  will  be : 


27rB  sine  cos6  d6  = 


Now  if  the  reflecting  surface  is  perfectly  white  there  will  be  no  light  lost  and 
the  entire  light  received  by  the  hemisphere  will  equal  the  light  incident  upon 
the  reflecting  surface,  that  is  irB  =  1/10,000  and  B  =  I/IO,OOOTT  candle- 
power  per  square  centimeter.  In  the  above  example  where  the  incident 
illumination  is  48,000  meter  candles,  the  surface  considered  as  a  source  of 
light  will  have  48,000  candle  power  per  square  centimeter. 

IO.OOOTT 

This  same  formula  will  apply  to  the  case  of  opaque  projection  (§  2743)  where 
it  is  desired  to  determine  the  ratio  of  the  light  getting  through  the  objective  to 
form  the  screen  image  and  the  light  falling  on  the  opaque  surface,  assuming  that 
this  opaque  surface  is  perfectly  diffusing  and  perfectly  white.  In  the  case  of 
the  objective,  light  over  a  certain  zone  of  the  hemisphere  is  used.  If  the  angle 
which  the  objective  subtends  with  a  point  on  the  object  taken  as  the  center  is 
called  28,  then  the  angle  between  the  axis  and  the  edge  of  the  objective  is  6, 
and  the  above  formula  will  apply,  i.  e.,  the  light  flux  striking  the  objective 
from  one  square  centimeter  is  7rB/2  (i  —  cos  26).  Also  the  total  light  flux 
reflected  from  the  surface  over  the  entire  hemisphere  is  TrB,  hence  the  ratio  of 
the  light  flux  striking  the  objective  to  form  the  screen  image  to  the  light  flux 
received  by  the  reflecting  surface  is  i — cos  26.  This  takes  no  account  of  losses 

2 
due  to  reflection  and  absorption  by  the  objective. 


CH.  XIV]  EFFECT  OF  APERTURE  IN  PROJECTION  615 


^     — :::-v::t:::::::-:;;;    ^o 

£ N«  -    *»«       ::.::-:---.,,Xa 


FIG.  346.     CANDLE-POWER  OF  A  SURFACE  ILLUMINATED  AT 
A  GIVEN  INTENSITY. 

O  A  surface  having  an  area  of  one  square  centimeter  located  at  the  center 
of  a  hemisphere  of  one  meter  radius.  This  surface  is  illuminated  at  an  inten- 
sity of  I  meter  candles.  It  receives  and  reflects  1/10,000  lumens.  As  a  light 
source  it  has  in  the  direction  OP,  B  candle-power. 

OP     Perpendicular  to  the  surface. 

b'a'  ab    A  zone  on  the  surface  of  the  hemisphere.     This  zone  is  located  at 
an  angular  distance  of  B,  from  the  perpendicular  to  the  surface.     The  angle' 
subtended  by  this  zone  from  the  center  is  dO,  and  its  width  ab,  is  d6  meters. 
The  radius  aN,  of  the  circle  aa^is  sin  0  meters,  and  its  circumference  is  2ir  sin  0. 
The  area  of  the  zone  is  then  2/^in  0  d0. 

The  intensity  of  illumination  of  this  zone  is  B  cos  0,  meter  candles.     The 
light  flux  received  by  this  zone  is  then  (illumination  x  area)  equal  to 
B  cos  6  x  2  TT  sin  6  d  6  =  2  v  B  cos  6  sin  6  d  0. 

IMAGE    FORMATION    WITH    THE    MICROSCOPIC    OBJECTIVE    WITH 
REFERENCE  TO  APERTURE 

§  858.  Let  a  b,  fig.  347,  represent  the  face  of  the  condenser 
which  is  in  such  a  position  with  respect  to  the  objective  that  its 
image  sr  t',  is  in  focus  on  the  screen.  With  high  powers  the  speci- 
men will  be  very  close  to  the  front  of  the  objective. 

The  front  lens  or  combination  of  the  objective  Oi,  will  form  an 
image  a'  bf,  of  the  condenser  face  which  may  or  may  not  coincide 
with  the  back  lens  O2,  of  the  objective  as  here  shown. 

Tracing  the  light  from  the  condenser  we  see  that  all  the  light 
from  a,  which  gets  through  the  front  lens  passes  through  a',  and  all 
light  from  b,  passes  through  6',  and  so  on. 


6i6 


EFFECT  OF  APERTURE  IN  PROJECTION          [Cn.  XIV 


The  light  from  the  point  s,  spreads  out  over  the  angle  xsy,  which 
equals  angle  asb.  Light  from  s,  which  has  come  from  a,  can  reach 
the  screen  along  the  path  s  x  a'  s'.  From  b,  the  light  follows  the 
path  syb's',  and  from  the  central  parts  of  the  condenser  light  will 
go  from  5  to  s',  along  the  paths  which  lie  between  a'  and  b'. 

The  result  is  that  the  light  which  goes  to  make  up  the  point  5', 
of  the  screen  image  has  come  from  the  entire  area  of  the  circle  a  'b'. 
That  is,  the  circle  a'b',  is  the  diaphragm  which  limits  the  illum- 
inated aperture  of  the  objective. 


FIG.  347.     IMAGE  FORMATION  WITH  A  MICROSCOPIC  OBJECTIVE. 

The  shaded  portion  shows  the  cone  of  light  which  illuminates  the  point  t,  of 
the  specimen  and  which  goes  to  make  its  image  /',  on  the  screen;  all  other 
points  are  similarly  lighted  and  similarly  pass  on  to  form  the  screen  image. 

a,b     The  last  element  of  the  condenser  (see  fig.  332). 

s,  t     Two  points  of  the  specimen  to  be  projected. 

OI?  02     The  front  and  back  combinations  of  the  objective. 

•w  x  y  z     Points  on  the  front  lens  of  the  objective. 

a',  b'     Image  of  the  condenser  face. 

s',  t'     The  inverted  screen  image  of  the  object  s,  t. 

When  used  with  a  magic  lantern,  the  projection  objective  transmits  only 
70%  of  the  light  reaching  it.  As  only  68%  of  the  original  light  reaches  the 
objective,  the  screen  image  must  be  formed  by  (68  x  70  =  47.6%)  of  the 
original  light. 

With  the  microscope,  however,  only  about  6%  of  the  original  light  gets 
through  the  objective  and  goes  on  to  form  the  screen  image  (fig.  342).  If  a 
substage  condenser  and  an  ocular  are  used,  the  light  for  the  screen  image  is  still 
further  reduced. 


The  illuminated  aperture  of  the  objective  may  be  increased  by: 

1.  Using  a  larger  diameter  condenser  of  the  same  focal  length. 

2.  Using  a  shorter  focus  condenser  of  the  same  diameter. 
Either  method  will  increase  the  angle  asb,  and  the  diameter  of 

the  disc  a'b'. 


CH.  XIV]  EFFECT  OF  APERTURE  IN  PROJECTION  617 

The  illuminated  aperture  might  be  decreased  by  using  a  large 
iris  diaphragm  to  cover  part  of  the  condenser  face  a  b. 

In  the  figure  the  aperture  illuminated  a'b',  is  less  than  the 
diameter  of  the  rear  lens.  If  the  size  of  the  condenser  were  greatly 
increased  until  its  image  was  as  large  as  the  rear  lens  of  the  objec- 
tive, the  marginal  ray  from  s,  would  move  from  sxa's'  to  szs'. 
The  entire  aperture  of  the  objective  would  be  illuminated  and  no 
more  light  would  be  used  by  a  further  increase  in  the  size  of  the 
condenser  (Fig.  347). 

§  859.  Image  formation  of  a  point  not  on  the  axis. — Light  from 
t,  will  spread  out  over  the  angle  w  t  y,  which  equals  angle  a  t  6,  will 
pass  through  a'  b',  and  be  collected  to  a  point  t',  on  the  screen. 
This  light  will  of  course  fill  a  cone  of  which  the  limiting  rays  are 
t  w  b'  t'  and  t  y  a'  t'  (Fig.  347). 

§  860.  Illumination  of  the  screen  image. — Any  single  point  on 
the  screen  as  s'  or  f,  will  be  illuminated  by  light  which  has  come 
from  the  bright  disc  a'  b'.  The  illumination  will  therefore  depend 
on  the  three  factors,  the  brightness  and  area  of  the  disc  a'b',  and 
its  distance  from  the  screen  (Fig.  347). 

The  area  of  the  disc  can,  of  course,  be  no  greater  than  the  area  of 
the  back  lens  of  the  objective,  and  is  usually  smaller.  For  this 
reason  the  brightest  projection  in  a  given  case  is  obtained  when  the 
back  lens  of  the  objective  appears  to  be  entirely  filled  with  light. 

The  brightness  of  this  disc  of  light  would,  if  it  were  not  for  light 
losses,  be  exactly  the  same  as  that  of  the  original  source.  This 
follows  from  the  fact  that  the  brightness  of  an  object  remains  the 
same,  except  for  light  losses,  when  seen  through  a  lens  or  a  system 
of  lenses  as  when  viewed  directly.  A  lens  can  only  change  the 
direction,  not  the  intensity  of  light,  or  in  other  words  it  can  only 
change  the  apparent  size  of  an  object. 

This  being  the  case  the  screen  brightness  is  limited  not  by  the 
candle-power  of  a  source  but  by  its  intrinsic  brilliancy  (candle- 
power  per  square  centimeter).  This  assumes  the  image  of  the 
light  to  have  an  area  great  enough  to  cover  the  front  lens  of  the 
objective,  which  is  the  case  with  most  microscopic  projection. 


6i8  EFFECT  OF  APERTURE  IN  PROJECTION          [Cn.  XIV 

The  effect  of  light  losses  by  reflection  and  by  absorption  is  to 
reduce  the  brilliancy  of  the  bright  disc  a'  b'.  These  losses  are  very 
great,  and  as  only  a  small  amount  of  light  is  available  anyway,  that 
is  the  reason  we  do  not  recommend  the  use  of  the  substage  con- 
denser except  in  the  special  cases  of  high  power  demonstration,  for 
photography  and  for  high  power  drawing,  where  fine  details  are  of 
more  importance  than  brilliancy  (see  Ch.  IX,  X,  §  401,  477). 

A  substage  condenser  will  reduce  the  brilliancy  of  the  disc  to 
70%  of  its  former  value,  and  our  experience  has  been  that  the  full 
aperture  of  all  but  the  highest  power  objectives  (8,  6,  4,  2  mm. 
equivalent  focus  §  8o8a)  can  be  entirely  filled  without  its  use. 

§  861.  Appearance  when  one  looks  into  the  objective. — If  the 
eye  is  held  at  5'  (a  dark  glass  being  of  course  held  in  front  of  the 
eye  or  better  yet  held  just  before  the  front  of  the  objective  at  5) 
light  will  strike  the  pupil  from  all  parts  of  the  condenser  image 
a'  b',  the  appearance  being  that  of  a  bright  disc  of  light. 

The  larger  this  disc,  the  greater  the  aperture  of  the  objective 
illuminated.  With  low  powers  the  entire  aperture  will  be  illum- 
inated by  the  use  of  the  large  condenser  alone.  With  high  powers 
only  the  central  part  of  the  back  lens  will  appear  bright.  When 
the  bright  disc  spreads  over  the  entire  back  lens  the  aperture  of 
the  objective  is  fully  illuminated  and  no  further  increase  of  light 
is  possible  with  a  given  source. 

As  often  happens,  the  back  lens  appears  illuminated  not  with  a 
uniform  bright  disc  but  by  a  bright  ring  with  a  bright  center 
separated  by  a  dark  ring  or  crescent.  This  is  due  to  the  spherical 
aberration  of  the  condenser. 

§  862.     Appearance  when  an  amplifier  or  an  ocular  is  used.— 

An  amplifier  or  an  ocular  will  spread  the  light  from  the  objective 
over  a  larger  area  than  before,  of  course  decreasing  the  brightness 
of  the  screen  image.  This  effect  could  be  foretold  by  looking 
directly  at  the  instrument  from  the  screen  for  the  bright  disc  of 
light  a'  b'  (fig.  347)  will  appear  smaller  when  the  ocular  or  amplifier 
is  in  place. 

§  863.    Limit  of  brightness  with  the  projection  microscope.— 

The  screen  image  with  microscopic  projection  apparatus  is  not  as 


CH.  XIV]          EFFECT  OF  APERTURE  IN  PROJECTION  619 

bright  as  could  be  obtained  with  the  magic  lantern.  The  reason 
is  the  physical  impossibility  of  crowding  more  than  a  limited 
amount  of  light  through  the  very  small  opening  of  a  microscopic 
objective.  The  objective  can  be  illuminated  so  that  light  comes 
from  the  entire  area  of  the  rear  objective  lens  to  form  the  screen 
image.  When  this  occurs  this  objective  will  act  like  a  luminous 
source  having  the  same  intrinsic  brilliancy  (except  for  losses)  as  the 
original  source,  and  having  the  area  of  the  rear  lens.  The  area  of 
this  rear  lens  is  fixed  and  cannot  be  increased.  The  intrinsic 
brilliancy  of  a  given  source  is  fixed.  Hence,  only  by  using  a 
brighter  source  as  changing  from  a  lime  light  to  the  alternating 
current  arc,  from  the  alternating  current  arc  to  the  direct  current 
arc,  or  from  the  direct  current  arc  to  sunlight,  or  by  reducing  the 
losses  due  to  unnecessary  complication  of  lenses  and  condensers 
between  the  source  and  the  screen,  can  the  image  brightness  be 
increased.  When  using  sunlight  one  has  reached  the  limit  of 
possibility  for  light  brilliancy. 

KOEHLER'S  METHOD  OF  ILLUMINATING  A  SPECIMEN  FOR 

MICROSCOPIC  PROJECTION 

§  864.  The  simple  method  of  lighting  a  specimen  for  micro- 
scopic projection  by  a  condenser  is,  as  stated  above,  to  focus  the 
image  of  the  crater  upon  the  front  of  the  objective  (fig.  140).  For 
high  powers  this  is  practically  the  same  as  if  the  crater  image  were 
focused  upon  the  specimen. 

With  the  Kcehler  method  a  substage  condenser  is  used.  The 
large  condenser  forms  an  image  of  the  crater  on  the  diaphragm  of 


FIG.  348.     ORDINARY  METHOD  OF  ILLUMINATING  MICROSCOPIC  SPECIMENS. 

(See  §  376,  833). 


620 


EFFECT  OF  APERTURE  IN  PROJECTION 


[Cn.  XIV 


the  substage  condenser  at  di  (fig.  170,  349).  The  effect  of  the 
diaphragm  at  d\,  in  front  of  the  large  condenser,  and  at  J2,  where  the 
crater  image  is  formed,  is  just  opposite  in  the  two  cases.  With  the 
usual  arrangement  (fig.  348)  the  diaphragm  A,  will  limit  the  aper- 
ture of  the  objective  while  the  diaphragm  D2,  will  limit  the  size  of 
the  field  illuminated.  With  the  Kcehler  arrangement,  however, 
(fig.  349)  the  diaphragm  PI,  limits  the  size  of  field  illuminated,  while 
the  diaphragm  D2,  limits  the  aperture  of  the  objective  used. 


FIG.  349.     KOEHLER'S  METHOD  OF  ILLUMINATING  MICROSCOPIC  SPECIMENS. 

L  Light  source. 

Ct  Condenser. 

DT  First  diaphragm. 

D2  Second  diaphragm. 

L'  Image  of  light  source  on  second  diaphragm. 

C2  Substage  condenser. 

61  Specimen. 

D'  Image  of  diaphragm  DJt  on  the  specimen. 

0  Objective. 

D\  L"     Image  of  diaphragm  D2  and  light  source. 

§  865.    Advantages  and  disadvantages  of  Koehler  method. — 

The  Kcehler  method  has  the  advantage  that  it  enables  an  easy 
control  of  the  size  of  the  field  illuminated  and  of  the  aperture  of  the 
objective  used.  If  a  larger  field  than  necessary  is  illuminated, 
there  may  be  undue  heating  of  the  specimen  and  the  best  results 
are  obtained  only  when  just  the  right  objective  aperture  is  used. 
On  the  other  hand,  the  use  of  a  substage  condenser  precludes  the 
use  of  a  cooling  stage,  (fig.  134),  except  a  very  thin  form.  It 
limits  the  size  of  field  which  may  be  used,  and  transmits  only  70% 
of  the  incident  light  and  reduces  the  general  flexibility  and  ease  of 
handling  the  apparatus. 


CHAPTER  XV 

SOME  USES  OF  PROJECTION  IN  PHYSICS;  EXPERIMENTS 

ILLUSTRATING  NORMAL  VISION  AND  SIMPLE, 

REFRACTIVE  EYE  DEFECTS. 

§  875.    Apparatus  and  Material  for  Chapter  XV: 

For  polarized  light,  see  §  879;  For  projection  of  spectra,  see 
§  885;  For  photography,  see  §  908;  For  Abbe  Diffraction  Theory, 
see  §  909;  For  eye  defects,  see  §  916. 

§  876.    Historical  development  of  experimental  projection.— 

Works  of  reference  giving  methods  of  projecting  experiments. 

In  every  book,  and  in  every  article  on  projection,  directions  and 
hints  are  given.  The  following  are  especially  full  in  directions,  and 
rich  in  suggestions:  Dolbear. — The  Art  of  Projecting;  Fourtier  et 
Moltini . — Les  Projections  Scientifiques ;  Hassack  und  Rosenberg.— 
Projektions-apparate ;  Lehmann. — Flussigekrystalle ;  Trutat.  - 
Traite  des  Projections;  Tyndall. — Six  lectures  on  light;  Wright. — 
Light,  a  course  of  experimental  optics  chiefly  with  the  lantern; 
Wright. — Optical  Projection. 

INTRODUCTION 

§  877.  Many  physical  and  chemical  experiments  can  be 
exhibited  to  an  entire  class  in  a  striking  manner  by  the  aid  of 
projection  apparatus.  Sometimes  transparent  objects  are  used, 
and  then  again,  as  suggested  by  Dolbear,  many  experiments  with 
opaque  objects  show  very  clearly  as  shadows  on  the  screen  if  they 
are  performed  in  the  beam  of  the  magic  lantern. 

Indeed,  for  exhibiting  to  a  class  or  any  large  audience  the  varied 
experiments  necessary  in  physical  and  chemical  work,  all  the  pro- 
jection apparatus  described  in  this  book  and  combinations  of  them 
may  be  needed.  By  thoroughly  mastering  the  principles  of  projec- 
tion one  can  so  adjust  and  combine  the  different  pieces  of  appara- 
tus that  almost  any  phenomenon  can  be  shown  on  the  screen. 

One  can  find  many  suggestions,  and  often  detailed  directions  for 
showing  various  experiments  in  the  works  referred  to  at  the  begin- 
ning of  this  chapter. 

621 


622  PROJECTION  EXPERIMENTS  IN  PHYSICS          [Cn.  XV 

No  directions  for  the  ordinary  experiments  shown  by  projection 
apparatus  in  every  university  course  in  physics  and  chemistry  are 
given  here,  but  we  thought  it  wise  to  include  a  few  special  projec- 
tion experiments  that  have  been  found  by  us  to  be  especially 
instructive  or  difficult  to  perform  by  the  means  ordinarily  used. 

The  experiments  illustrating  normal  vision  and  the  simpler  re- 
fractive eye  defects  are  included  because  of  the  importance  of 
these  defects  and  their  prevalence  with  school  children  and  stu- 
dents and  others  doing  close  work;  and  because,  with  projection 
apparatus,  it  is  so  easy  to  show  in  a  striking  manner  what  is 
meant  by  the  defects,  and  how  certainly  the  defects  can  be  cor- 
rected by  using  the  proper  spectacles. 

SOME  SPECIAL  EXPERIMENTS  IN  PHYSICS 
§  878.  Kind  of  apparatus. — Most  projection  experiments  in 
physics  are  only  shown  occasionally,  hence  permanent  apparatus 
to  demonstrate  many  physical  phenomena  is  so  costly  as  to  be  out 
of  the  question.  The  apparatus  here  described  consists  in  lenses, 
objectives,  prisms,  clamps,  arc  lamps,  etc.,  generally  to  be  found  in 
any  laboratory  where  such  experiments  would  be  shown.  The 
apparatus  can  either  be  clamped  to  rods  or  laid  upon  a  horizontal 
table.  The  former  method  has  the  advantage  that  one  can  project 
in  an  inclined  as  well  as  in  a  horizontal  direction ;  the  latter  method 
is  easier  to  set  up.  When  permanent  apparatus  is  used  the 
principle  is  exactly  the  same,  permanent  instead  of  temporary 
supports  being  used  to  hold  the  lenses. 

An  optical  bench  like  that  shown  in  fig.  158-159  is  satisfactory 
where  the  apparatus  is  set  up  on  a  table.  When  it  must  be  held 
at  various  angles  some  clamping  arrangement  is  desirable. 

EXPERIMENTS  WITH  POLARIZED  LIGHT 
§  879.    Apparatus.— 

1.  Right-angle  arc  light;    Condenser;   Water-cell; 

2.  Small  Nicol  prisms  with  openings  of  i  to  2  cm.  which  are 
mounted  so  that  they  can  be  rotated. 


CH.  XV] 


EXPERIMENTS  WITH  POLARIZED  LIGHT 


623 


3.  Pile  of  glass  plates  to  polarize  light.     Prefer- 
ably thin  sheets  of  plate  glass,  but  a  pile  of  lantern- 
slide  cover-glasses  can  be  made  to  answer. 

4.  Plano-convex  condensing  lens,  5  to  10  cm.  in 
diameter  and  20  to  30  cm.  focus.     This  lens  must  be 
free  from  strain. 

5.  Projection  objective,  preferably  of  large  diam- 
eter and  short  focus. 

6.  Two  sets  of  lenses  to  give  converging  polarized 
light.     Two  substage  condensers  of  microscopes  will 
answer  if  free  from  strain. 

7.  Objective  of  short  focus  and  large   diameter. 
A  plano-convex  lens  will  answer  (see  §  88 1). 

8.  Specimens.     Pieces  of  mica,  crystal  sections, 
plate  of  glass  on  which  crystals  have  formed,  an- 
nealed and  unannealed   pieces   of  glass,  clamp  for 
putting  the  glass  under  strain  when  in  the  field  of 
the  lantern. 

Many  of  the  most  beautiful  experiments  in  optics 
require  the  use  of  polarized  light.  The  demonstra- 
tion of  this  phenomenon  is  growing  more  difficult 
owing  to  the  increasing  scarcity  of  the  natural 
mineral  calcite,  which  is  used  to  make  the  Nicol 
prisms  needed  for  polarizing  and  analyzing  the 
light.  Clear  pieces  of  calcite  are  getting  so  rare 
that  except  for  a  few  large  Nicol  prisms  in  private 


FIG.  350. 


PROJECTION  WITH  POLARIZED  LIGHT,  USING  SMALL 
NICOL  PRISMS. 


Source  of  light,  right-angle  arc. 

Ordinary  magic  lantern  condenser. 

Water-cell. 

_ .  j    Nicol  prism  of  i  to  2  cm.  opening  (polarizer). 
C2    Condenser  free  from  strain  which  renders  the  polarized 
light  parallel  or  slightly  converging. 
S      Specimen. 
0     Magic  lantern  objective. 
N2   Nicol  prism  (analyzer)  of  i  to  2  cm.  opening. 


624 


EXPERIMENTS  WITH  POLARIZED  LIGHT 


[Cn.  XV 


collections  and  a  few  large  crystals  in   museums,   no   prisms  of 
large  openings  (5  to  8  cm.)  are  now  to  be  obtained. 

§  880.  Use  of  small  Nicol  prisms. — A  method  will  be  described 
for  using  Nicol  prisms  of  small  openings  (i  to  2  cm.).  This 
method  consists  in  concentrating  the  light  to  small  diameter  in 
those  places  where  it  must  pass  through  the  Nicol  prisms. 

Let  L  fig.  350  be  the  source  of  light,  preferably  a  small  right- 
angle  direct  current  arc.     W,  is  a  water-cell.     It  is  imperative  to 
use  a  water-cell,  otherwise  the  polarizing  Nicol  will  be  ruined, 
vv          The  polarizing  Nicol  NI,  is  placed  at  the  image  of  the 
crater.     Light  spreading  out  from  the  farther  side  of  this 
prism  is  polarized.     This  Nicol  is  treated  exactly 
o      as  if  it  were  an  original  source  of  polarized  light. 
The    second  condenser   C2,   which   must   be    free 


FIG.  351.     LIGHT  POLARIZED  BY  A  PILE  OF  GLASS  PLATES. 
L     The  light  source. 
C     Plano-convex  condenser  lens. 
C-2    Condenser  of  long  focus  and  free  from  strain. 
S    The  specimen. 
0     Magic  lantern  objective. 
N2  Nicol  prism  with  i  to  2  cm.  opening  (analyzer). 

from  strain,  renders  the  light  parallel  or  slightly  convergent.  The 
specimen  is  at  S.  The  objective  O,  is  placed  at  such  a  distance 
from  the  specimen  that  the  image  of  the  latter  will  be  in  focus  on 
the  screen.  The  analyzing  Nicol  No,  is  placed  at  the  right  in  front 
of  the  objective  at  the  point  where  the  rays  cross,  i.  e.,  in  the  image 
of  NI,  cast  by  the  lenses  C2  and  O. 

When  either  the  polarizing  or  analyzing  Nicol  NI  or  N2,  is  rotated, 
two  positions,  180°  apart,  will  be  found  in  which  the  screen  is  dark. 
If  when  these  positions  are  found,  a  piece  of  mica,  for  example,  is 
put  in  the  field,  it  will  change  the  plane  of  polarization  and  will  give 
on  the  screen  the  most  beautiful  colors. 


CH.  XV] 


EXPERIMENTS  WITH  POLARIZED  LIGHT 


625 


Another  method  of  producing  polarized  light  is  to  reflect  the 
light  from  a  pile  of  glass  plates  (lantern-slide  cover-glasses  will 
answer).  At  an  angle  of  incidence  of  about  57°  from  the  normal 
as  shown  in  figure  351,  the  light  reflected  from  the  glass  surfaces 
will  be  plane  polarized.  The  specimen  is  placed  at  S,  the  objective 
at  O,  and  the  analyzer  N2,  at  the  crossing  of  the  rays  as  before 
(fig.  351).  The  heating  effect  of  the  light  reflected  from  the  glass 
surface  is  so  small  that  a  water-cell  is  unnecessary. 

§  881.  Converging  polarized  light  to  show  rings  and  brushes. — 
If  the  polarized  light  passing  through  a  crystal  is  a  converging 
cone,  the  most  beautiful  phenomena  are  shown.  Fig.  352  shows 
the  apparatus  used  to  project  rings  and  brushes.  A  parallel  beam 
of  polarized  light  obtained  as  before,  strikes  the  lens  system  LI, 
designed  to  bring  parallel  light  to  a  focus  in  a  strongly  convergent 
beam.  I>2,  renders  the  light  again  nearly  parallel.  LI,  is  the  usual 
form  for  this  work  and  L®,  is  a  substage  condenser  from  a  micro- 
scope, either  form  will  give  good  results  if  free  from  strain.  The 
objective  O,  is  a  lens  of  short  focus  and  large  diameter.  It  need  not 
be  a  special  projection  objective.  Three  objectives  were  tried 
which  gave  good  results,  (i)  A  photographic  objective,  focus 
12  cm.,  diameter  2  cm.  (2)  A  magnifying  glass,  focus  6  cm., 
diameter  4  cm.  (3)  A  plano-convex  lens,  focus  5  cm.,  diameter 
6  cm.  The  single  plano-convex  lens  gave  the  best  results  except 
that  the  figures  were  slightly  distorted.  The  analyzing  prism  N2, 
must  have  a  medium  sized  opening,  15  to  20  mm.  free  aperture, 
othenvise  it  will  cut  off  part  of  the  field. 


FIG.  352.     CONVERGING  POLARIZED  LIGHT  TO  SHOW  RINGS  AND  BRUSHES. 

Lz  is  the  usual  form  of  a  lens  system  designed  to  bring  parallel  light  to  a 
strongly  convergent  beam. 

L2  is  a  microscope  condenser,  to  parallelize  the  light  from  LT. 

O  objective  to  converge  the  light  beam  upon  the  Nicol  prism  (N2),  and  to 
focus  the  rings  and  brushes  on  the  screen. 

N2     Nicol  prism  with  at  least  1.5  to  2  cm.  opening. 


626  EXPERIMENTS  WITH  POLARIZED  LIGHT          [Cn.  XV 

§  882.  Detecting  strain  in  lenses. — Substage  condensers  from 
the  microscope  if  free  from  strain  are  excellent  for  showing  rings 
and  brushes.  The  glass  of  which  they  are  made  should  be  per- 
fectly homogeneous  and  should  have  no  effect  at  all  on  polarized 
light.  If  the  glass  of  which  they  are  made  is  imperfectly  annealed, 
the  lenses  will  affect  polarized  light.  An  achromatic  condenser  is 
more  likely  to  be  free  from  strain  than  an  ordinary  Abbe  condenser, 
because  it  is  likely  to  be  made  of  better  glass.  Strain  may  be 
tested  for  in  each  lens  by  holding  it  between  crossed  Nicols.  A 
strained  condenser  if  put  in  position  L2,  will  show  a  black  cross  on 
a  white  ground  when  no  crystal  is  used. 

§  883.  Setting  up  the  experiment. — The  condensing  lenses  LI, 
and  L2  can  be  clamped  on  a  ring  stand.  The  distance  between 
them  is  adjusted  until  the  emerging  light  is  practically,  parallel,  the 
objective  O,is  put  in  place  so  that  the  image  of  the  lens  L2,  is  in  focus 
on  the  screen,  a  Nicol  prism  N2,  is  put  in  the  narrowest  part  of  the 
beam  of  light  from  the  objective.  When  the  analyzer  N2,  is  turned 
to  give  a  dark  field  on  the  screen  and  a  crystal  section  (mica,  for 
example)  is  placed  in  the  converging  light  between  the  two  con- 
densers the  field  becomes  illuminated  with  bright  colors  and 
beautiful  patterns. 

The  final  adjustment  of  the  apparatus  is  now  made,  the  distance 
between  the  condensers  is  adjusted  until  the  screen  has  the  most 
uniform  light.  The  objective  O,  is  moved  until  the  figures  on  the 
screen  are  as  sharp  as  possible;  the  analyzer  may  also  require  a 
slight  adjustment. 

§  884.  Brightness. — The  screen  image  with  polarized  light  is 
never  very  bright,  hence  a  very  dark  room  is  needed.  A  screen 
picture  over  one  or  two  meters  in  diameter,  (3  to  6  feet)  should 
never  be  attempted. 

PROJECTION  OF  SPECTRA 

§  885.     Apparatus. — Magic  lantern. 

Stand  to  hold  the  objective  in  proper  position  if  the  lantern 
bed  is  not  long  enough. 

Slit  with  adjustable  blades. 


Ce.  XV]  PROJECTION  OF  SPECTRA  627 

Glass  prism,  hollow  prism  to  hold  carbon  bisulphide,  direct- 
vision  prism. 

Diffraction  grating,  ruled  on  speculum  metal,  or  glass,  or  one  of 
the  replica  gratings,  6,000  to  8,000  lines  to  the  centimeter, 
(15,000  to  20,000  lines  to  the  inch). 

Glass  cell  to  hold  colored  liquids. 

Colored  liquids,  solutions  of  didymium  nitrate,  copper  sulphate, 
analine  dyes,  etc. 

Colored  glass,  especially  red  and  blue. 

Arc  lamp  with  vertical  carbons. 

Hollow  carbons  stuffed  with  salts  as:  lithium  chloride,  sodium 
chloride,  potassium  chloride,  calcium  chloride,  aluminum. 

Flame  arc  electrodes,  "Yellow"  and  "Brilliant  white." 

Metallic  electrodes,  iron,  copper,  aluminum,  uranium  oxide  in 
tin  tube,  rutile  in  tin  tube  or  else  luminous  arc  electrodes  §  88 5a. 

Screen  coated  with  anthracene,  50  x  150  cm.  to  use  instead  of 
a  white  screen  to  show  ultra-violet. 


APPARATUS  FOR  THE  DEMONSTRATION  OF   ULTRA- 
VIOLET LIGHT. 

Quartz  condensing  lens  and  two  plano-convex  quartz  lenses. 
Source  of  ultra-violet  light  as : 

Quartz  mercury  arc. 

Arc  lamp  with  carbons  filled  with  various  salts. 

Arc  lamp  with  carbons  filled  with  metallic  aluminum. 

Arc  lamp  with  brilliant  white  flame  arc  carbons. 

Arc  lamp  with  iron  tube  filled  with  uranium  oxide. 
Small  card  coated  with  anthracene  to  render  the  ultra-violet 
visible. 

Especially  since  the  days  of  Newton,  the  exhibition  of  the 
spectrum  has  been  one  of  the  most  fascinating  experiments  in 
physics.  It  is  also  one  of  the  simplest  of  the  "special  projection" 
experiments  to  perform. 

§  885a.     These  tubes  can  be  made  by  rolling  strips  of  tinned  iron  into 

tubes. 


628  PROJECTION  OF  SPECTRA  [Cn.  XV 

§  886.  Source  of  light. — In  order  to  demonstrate  an  absorption 
spectrum  of  a  substance  it  is  necessary  to  use  a  source  of  light 
which  has  a  continuous  spectrum  in  order  not  to  confuse  the  dis- 
continuities of  the  light  source  with  those  caused  by  the  absorbing 
medium. 

§  887.  Sunlight. — Sunlight  while  not  having  a  perfectly  con- 
tinuous spectrum  is  near  enough  to  it  for  most  purposes.  The  im- 
age of  the  sun  should  be  focused  on  the  slit  as  described  for  the 
crater  of  the  arc  lamp  (§  376,  833-834). 

§  888.  Carbon  arc  lamp. — In  the  visible  parts  of  the  spectrum 
the  carbon  arc  lamp  will  give  a  continuous  spectrum,  using  pref- 
erably the  positive  crater  of  the  direct  current  arc,  although  alter- 
nating current  will  answer. 

§  889.  The  uranium  arc. — Using  the  vertical  arc,  the  lower 
electrode  made  of  uranium  oxide  is  connected  to  the  positive 
terminal  as  described  in  §  905.  About  4  to  6  amperes  should 
be  used  and  a  fairly  continuous  spectrum  will  be  obtained.  This 
continuous  spectrum  will  extend  well  into  the  ultra-violet  and  can 
be  observed  by  using  an  anthracene  screen  (§  899). 

§  890.  The  Nernst  lamp. — This  can  be  used  if  necessary. 
Focus  one  filament  on  the  slit  or  use  a  very  long  focus  lens  (one  of 
100  cm.  focus  or  longer)  and  use  the  filament  as  a  line  source  of 
light.  The  spectrum  is  continuous  but  not  very  bright. 

The  tungsten  incandescent  lamp. — One  filament  is  focused  on 
the  screen  by  the  objective  and  a  slit  is  placed  over  the  side  of  the 
bulb  so  that  none  of  the  other  filaments  show  on  the  screen.  This 
single  filament  acts  as  a  line  source  of  light. 

OPTICAL  SYSTEM 

§  891.  The  optical  system  for  the  projection  of  spectra  is 
shown  irj  fig.  353.  First  a  condenser  C,  is  put  in  front  of  the  arc. 
This  condenser  brings  an  image  of  the  arc  to  a  focus  at  the  slit. 
An  objective  is  put  at  O,  so  as  to  focus  an  image  of  the  slit 
on  a  distant  screen  at  i.  When  a  prism  is  used  to  disperse 
the  light  into  a  spectrum,  it  is  placed  at  P  and  turned  as  shown. 


CH.  XV]  PROJECTION  OF  SPECTRA  629 

If  a  source  is  used  which  shows  a  line  spectrum,  this  spec- 
trum will  be  found  approximately  in  focus  at  R  V.  By  focus- 
ing the  objective  and  by  rotating  the  prism  the  spectrum  can  be 
sharply  focused  on  the  screen.  The  sharpness  of  the  spectrum  can 
be  increased  by  narrowing  the  slit  or  the  brightness  of  the  spectrum 
can  be  increased  by  opening  the  slit.  If  the  slit  is  adjustable  in 
width,  judgment  must  be  exercised  to  secure  a  spectrum  which  is 
as  sharp  as  the  occasion  requires  and  at  the  same  time 
sufficiently  bright. 


FIG.  353.     OPTICAL  SYSTEM  FOR  THE  PROJECTION 

OF  SPECTRA. 

L     Light  source,  arc  lamp. 
C    Condenser. 
Slit. 

0  Objective. 

1  Image  of  the  slit  focused  on  the  wall  before  the 
prism  is  in  place. 

R  V  Spectrum  focused  on  the  wall  after  placing  the  prism  in  the  path  of 
the  rays. 

The  distance  from  P  to  R  V  must  be  relatively  much  greater  than  here 
shown,  2  meters  (6  ft.)  or  more,  in  order  to  get  a  sharply  denned  spectrum. 

Note  that  the  red  (R)  is  deviated  less  than  the  violet  (V).  Compare  with 
a  grating  spectrum,  fig.  360. 

§  892.  Slit. — The  slit  for  projection  should  be  a  fairly  large  one ; 
12  cm.  square,  with  jaws  5  cm.  long. 

If  one  is  limited  by  time  or  expense  a  very  serviceable  slit  may  be 
made  by  soldering  a  couple  of  pieces  of  tinned  iron  on  a  larger  piece 
with  a  hole  in  it,  or  one  of  the  pieces  may  be  fastened  so  it  can  be 
slid  closer  or  farther  from  the  other.  The  main  point  to  be 
observed  is  that  the  edges  forming  the  slit  opening  must  be  per- 
fectly smooth  and  straight  so  that  when  they  are  brought  close 
together  there  will  be  no  unevenness  in  the  light  which  gets 
through. 

Large  slits  with  adjustable  jaws  may  be  obtained  of  dealers  in 
projection  apparatus. 


630 


PROJECTION  OF  SPECTRA 


[Cn.  XV 


§  893.  Prism. — A  glass  prism  may  be  used,  but  it  is  much 
better  to  use  a  hollow  prism  filled  with  carbon  bisulphide  as  this 
liquid  gives  a  much  higher  dispersion  than  glass,  thus  enabling 
one  to  obtain  a  more  extended  spectrum  than  would  be  possible 
with  a  glass  prism. 

Caution. — In  using  carbon  bisulphide  remember  that  it  is  very 
volatile  and  its  vapor  is  easily  ignited.  Hence  this  liquid  should 
not  be  poured  or  left  in  unstoppered  vessels  in  the  neighborhood  of 
the  lighted  arc.  Also  be  sure  that  the  hollow  prism  has  no  leaks 
around  the  stopper  or  elsewhere. 


Slit    Opening 


V*riaWe 
Slit  Open, 03 


FIG.  354.     HOME-MADE  SLITS  FOR  PROJECTION. 

Slit  with  stationary  blades. 
Slit  with  one  movable  blade. 
Side  view  of  b. 


§  894.  Other  prisms:  gratings.  —  The  60°  prism,  either  solid 
or  filled  with  liquid  is  usually  the  most  available,  but  other  forms 
are  often  at  hand. 

The  compound  prism  due  to  Rutherford  (fig.  356),  composed  of  a 
dense  flint  glass  prism  of  a  large  angle  and  two  crown  glass  prisms 
cemented  to  it,  will  give  a  much  higher  dispersion  than  will  a  single 
prism  of  even  very  dense  glass.  Such  a  prism  is  used  in  practically 
the  same  way  as  a  simple  prism. 

With  a  direct-vision  prism  (fig.  357)  the  axis  of  the  spectrum  is  not 
turned  to  one  side.  Such  a  prism  may  be  constructed  of  pieces  of 
different  kinds  of  glass  or  it  may  be  made  by  filling  the  hollow  cells 
of  a  prism  with  different  liquids. 


CH.  XV] 


PROJECTION  OF  SPECTRA 


631 


-      /I 


A  convenient  way  of  get- 
ting a  prism  which  has  a  de- 
viation of  but   about     150° 
while  having  a  very  good  dis- 
persion is  to  immerse  a  hol- 
low 60°  prism,  filled  with  car- 
bon bisulphide,  in  a  cubical  FIG  ^    HOLLOW  GLASS 
glass  dish  filled  with  water     PRISM  WITH  COVER  AND 
(Go-   orjn  STOPPER  FOR  CARBON 

BISULPHIDE  (CS8). 

§    895.      A     transmission 

diffraction  grating,  either  one  ruled  on  glass  or  a 
replica  grating,  held  in  front  of  the  projection  objec- 
tive will  give  wide  but  rather  faint  spectra  on  the 
screen. 

If  a  grating  with  rather  coarse  lines,   100-200  to 
the  centimeter  (250-500  lines   to  the  inch)  is  used 


FIG.  356.     RUTHERFORD  PRISM. 

This  prism  consists  of  a  flint  glass  prism  F,  with  an  angle 
of  about  90°  and  two  crown  glass  prisms  C  C,  cemented  to  it. 
The  combination  as  here  shown  has  a  prism  angle  of  about 
30°  and  has  the  same  deviation  as  a  60°  flint  prism,  but  has  a 
much  higher  dispersion  than  could  be  obtained  with  a  simple 
prism  of  even  dense  flint  glass. 

FIG.  357.     USE  OF  A  DIRECT-VISION  PRISM  FOR  THE 
PROJECTION  OF  SPECTRA. 

5    Slit. 
O     Objective. 

Flint     Flint  prism  to  cause  dispersion. 
Crown,  Crown     Prisms  of  crown  glass  to  obviate  the  devi- 
ation. 

V  R     Spectrum. 


632 


PROJECTION  OF  SPECTRA 


[Cn.  XV 


there  will  appear  on  the  screen  not  only  the 
central  image  O,  of  the  slit  but  also  fainter 
diffraction  images  i,  2,  3,  on  both  sides  of  the 
central  one  (fig.  357).  These  diffraction  im- 
ages are  really  short  spectra.  By  using  colored 
glasses  it  can  be  shown  that  with  red  light  the 
images  are  farther  apart  than  with  green  or 
blue  light. 


»-      <M      CO 


FIG.  358.     LIQUID  PRISM  OF  GOOD  DISPERSION 

BUT  SMALL  DEVIATION. 

It  consists  of  a  hollow  prism  filled  with  carbon  bi- 
sulphide (CS2)  immersed  in  a  glass  box  filled  with 
water. 


§  896.     Grating  spectra. — If  a  grating  with  CD 

fine  lines,  5,000  to  10,000  lines  per  centimeter 
(12,500  to  25,000  lines  to  the  inch),  is  used  the 
diffraction  images  are  spread  out  farther  and  ^ 
appear  as  extended  spectra.  In  case  the  de- 
tails of  a  spectrum  are  to  be  studied  it  is 
necessary  to  turn  the  axis  of  the  lantern  to  one 
side  as  in  the  case  where  a  prism  is  used.  The 


FIG.  359.     USE  OF  A  GRATING  WITH  COARSE  LINES. 
S    Slit. 
O     Objective. 
G     Grating. 

0  Primary  image  of  slit  formed  without  grating. 

1  2  3,  i  2  j     Diffraction  images  of  the  slit  formed 
by  the  grating.     These  images  are  short  spectra. 


CO 


CH   XV] 


PROJECTION  OF  SPECTRA 


633 


diffraction  spectra  are  not  as 
bright  as  are  the  spectra  obtained 
with  prisms  and  cannot  well  be 
used  except  to  demonstrate  that 
such  spectra  can  be  produced. 

Sometimes  if  a  very  high  order 
spectrum  is  to  be  shown  the  grating 
can  be  held  obliquely  as  in  fig.  361. 

§  897.    Use  of  reflection  gratings. — In 

case  a  reflection  grating  is  to  be  used  the 
lantern  is  pointed  directly  away  from  the 
screen  and  the  reflecting  grating  held  so  as 
to  reflect  the  light  back  to  the  screen.  The 
objective  is  focused  until  the  central  image 
is  sharp  on  the  screen  and  the  spectra  are 
observed  at  both  sides,  or  the  grating  may 
be  tipped  so  as  to  reflect  the  central  image  to 
one  side,  when  the  spectra  will  appear  in 
the  center  of  the  screen. 

In  this  work  it  is  essential  in  order  that  the 
comparatively  faint  spectra  can  be  seen,  to  have 
the  room  perfectly  darkened ;  the  arc  house  per- 
fectly light-tight  and  the  lantern  well  enclosed  to 
avoid  stray  light. 

Concave  reflection  gratings  can  be  used  by  a 
method  similar  to  that  for  plane  gratings,  but  iri 
order  to  have  the  central  image  sharply  in  focus 
on  the  screen  the  objective  must  be  closer  to  the 


FIG.  360.     USE  OF  A  GRATING  WITH  FINE  LINES  FOR  THE 
PROJECTION  OF  SPECTRA. 

S    Slit. 

0  Objective. 
G     Grating. 

1  Primary  image  of  the  slit. 

i  V  R     Primary  image  of  the  slit  (*"),  and  the  first  dif- 
fraction image  of  the  slit  spread  out  into  a  spectrum  (R  V). 
Note  that  with  a  grating,  the  red  is  deviated  more  than  the  violet.     Compare 
with  the  prism,  fig.  353. 


O 


PROJECTION  OF  SPECTRA 


[Cn.  XV 


FIG.  361.  USE  OF  A  GRAT- 
ING FOR  PROJECTING  A 
HIGH  ORDER  SPEC- 


TRUM. 


S    Slit. 

0  Objective. 
G     Grating. 

1  Image  of  the  slit  if 
there  is  no  grating  in  position. 

R   V    Spectrum. 

The  grating  is  tilted  as  shown. 


These  high  order  spectra  are  very  faint. 


slit  than  for  the  plane 
grating,  that  is,  the  light 
from  the  objective  must 
be  diverging  instead  of 
slightly  converging  when 
it  strikes  the  grating  (fig.  363). 

§  898.     Direction  of  the  light.— As 

the  direction  of  the  light  from  a  60° 
prism  is  oblique  to  the  axis  of  the  lan- 
tern (fig.  353),  it  is  necessary  to  turn 
the  entire  projection  apparatus  to  one 
side  so  that 'the  spectrum  will  strike 
the  screen. 

§  899.  Screen,  white  and  anthra- 
cene.— A  white  screen  such  as  is  suit- 
able for  ordinary  lantern  projection 


FIG.  362.     USE  OF   A    PLANE    REFLECTING 
DIFFRACTION  GRATING  FOR  THE 

PROJECTION  OF  SPECTRA. 
i     Primary  image  of  the  slit  as  it  would  be 
reflected  by  a  plane  mirror. 

R   V    Diffraction   spectrum   produced  by 
the  reflecting  grating. 


CH.  XV] 


PROJECTION  OF  SPECTRA 


635 


will  show  the  visible  parts  of  the  spectrum  very  well,  that  is,  it 
will  show  the  red,  green,  and  blue  parts. 

In  order  to  illustrate  the  ultra-violet  parts  of  the  spectrum,  use 
a  screen  coated  with  anthracene.     A  suitable  screen  can  be  made 


FIG.  363.     USE  OF  A  CONCAVE  REFLECTING  GRATING  FOR  THE  PROJECTION 

OF  SPECTRA. 
5    Slit. 

0  Objective. 

G     Concave  reflecting  grating. 

1  V  R     Image  of  the  slit  (i)  and  of  the  spectrum  (R  V). 

Note  that  in  order  to  get  a  sharp  image  of  the  slit  at  i,  it  is  necessary  to  have 
diverging  light  strike  the  grating.  The  spectrum  R  V  will  be  sharply  denned 
under  these  conditions. 

by  taking  a  piece  of  white  cardboard  50  cm.  x  75  cm.  and  painting  it 
with  a  suspension  of  anthracene  in  xylene.  The  anthracene  used 
is  the  ordinary  commercial  variety  of  resublimed  anthracene. 
Only  enough  xylene  is  used  to  make  the  mixture  so  it  can  be  put  on 


636 


PROJECTION  OF  SPECTRA 


[Cn.  XV 


the  paper  with  a  brush.  This  screen  will  show  a  brilliant  green 
fluorescence  wherever  ultra-violet  light  strikes  it.  When  the 
spectrum  of  an  arc  is  projected  upon  such  a  screen,  not  only  is  the 
visible  red,  green,  and  blue  to  be  seen,  but  also  beyond  the  blue  end 
is  a  vivid  green  fluorescence  which  indicates  the  presence  of  ultra- 
violet light. 


FIG.  364.     ILLUMINATION  OF  THE  SLIT  FOR  THE  PROJECTION  OF  SPECTRA. 

A  The  image  of  the  arc  focused  to  a  small  spot  on  the  slit.  The  objective  0 
is  filled  with  light.  The  spectrum  will  be  bright  but  a  mere  line. 

B  The  image  of  the  arc  slightly  out  of  focus  giving  a  higher  spectrum  but 
not  so  bright. 

C  The  slit  next  to  the  condenser  in  the  lantern-slide  position.  This  gives 
a  relatively  dim  spectrum  and  illuminates  a  greater  height  of  slit  than  is  used 
for  the  spectrum. 

§  900.  Illumination  of  the  slit. — Excellent  results  may  be 
obtained  by  focusing  the  arc  on  the  slit  by  the  condenser.  This 
gives  an  intense  illumination  but  the  spectrum  is  not  very  high,  in 
fact,  it  may  be  a  mere  line  of  color.  To  remedy  this,  the  slit  may 
be  brought  closer  to  the  condenser  than  the  crater  image.  Thit 
increases  the  height  of  the  spectrum  but  reduces  its  intensity.  Is 


CH.  XVI  ABSORPTION  SPECTRA  637 

is  a  common  practice  in  experiments  with  spectra  to  put  the  slit 
close  to  the  condenser  as  for  a  lantern  slide,  but  this  lessens  the 
brilliancy  of  the  spectrum. 

ABSORPTION  SPECTRA 

§  901.  The  apparatus  being  arranged  as  above  indicated  to 
project  a  continuous  spectrum,  all  that  remains  is  to  insert  the 
absorbing  medium  between  the  light  source  and  the  screen,  it 
makes  little  difference  where.  The  appearance  of  the  spectrum 
would  be  the  same  even  if  the  absorbing  medium  were  held  between 
the  eye  of  the  observer  and  the  screen.  As  a  practical  matter  it  is 
best  to  place  the  absorbing  substance  just  in  front  of  the  slit.  In 
this  position  any  slight  lack  of  planeness  of  its  surfaces  will  not 
cause  any  interference  with  the  optical  system  nor  reduce  the  sharp- 
ness of  the  spectrum  on  the  screen.  The  specimen  may  cover  the 
entire  slit,  in  which  case  the  entire  spectrum  will  show  the  absorp- 
tion bands  of  the  substance,  or  the  specimen  may  be  made  to  cover 
part  of  the  slit,  in  which  case,  part  of  the  spectrum  will  be  that  of 
the  light  source  and  part  will  show  the  absorption  bands  of  the 
specimen.  The  advantage  of  having  this  comparison  spectrum 
of  white  light  is  to  bring  out  much  more  clearly  any  faint  absorp- 
tion of  one  end  of  the  spectrum  as  with  dilate  copper  sulphate  or 
with  amber  glass.  Liquids  may  best  be  shown  by  placing  them 
in  hollow  glass  boxes  (fig.  365).  Many  variations  of  this  method 
and  many  fascinating  experiments  will  soon  suggest  themselves  to 
the  experimenter  once  the  apparatus  is  set  up. 

§  902.  Suitable  substances. — The  following  substances  will 
show  interesting  absorption  bands: 

Colored   glasses.      Red,   blue,    purple,  canary-yellow. 

Gelatines  colored  with  solutions  of  ana- 
line  dyes,   for  example,    methyl    violet, 
eosine  (red  ink),  fuchsine. 
Blood  diluted  with  water. 
Solutions  of  mineral  salts,  as  cobalt  ni- 
trate in  water,  cobalt  nitrate  in  alcohol  or 
FIG.  365.  GLASS  Box  FOR      concentrated  HC1;   potassium  permanga- 
ABSORPTION  SPECTRA.         nate. 


638 


EMISSION  SPECTRA 


[Cn.  XV 


Didymium  salts,  such  as  crude  didymium  nitrate  or  the  pure 
neodidymiurn,  praseodidymium,  erbinurn,  and  other  rare  earth  salts. 

The  following  substances  will  show  general  absorption  at  one 
end  of  the  spectrum  and  should  be  shown  in  comparison  with  the 
spectrum  of  white  light : 

Amber  glass,  green  glass. 

Copper  sulphate,  Ferric  chloride,  Nickel  nitrate,  Potassium 
chromate  and  dichrorrate,  Chrome  alum. 


FIG.  366.     COMPARISON  OF  SPECTRA. 
C     Condenser. 

Specimen     This  covers  only  a  part  of  the  slit. 
Slit     Longitudinal  view  of  slit. 

Through  the  rest  of  the  slit  passes  white  light  or  light  traversing  another 
specimen.  The  two  spectra  then  appear  side  by  side. 

EMISSION  SPECTRA 

§  903.  Besides  the  projection  of  absorption  spectra,  the  optical 
system  as  described  above  will  serve  also  for  the  projection  of 
emission  spectra.  In  this  case  the  arc  is  both  the  specimen  and 
the  source  of  light. 

We  will  suppose  that  it  is  desired  to  project  the  spectrum  of  a 
"yellow  flame"  carbon,  this  being  about  the  easiest  and  most 
satisfactory  to  begin  with.  The  apparatus  is  set  up. as  shown  in 
fig.  367 .  L  is  a  vertical  arc,  the  lower  electrode  of  which  is  a  yellow 
flame  arc  carbon  or  a  hollow  carbon  filled  with  a  sodium,  potassium 
or  other  salt.  The  upper  electrode  is  a  carbon  about  13  mm.  (jA 
inch)  in  diameter.  The  carbon  holder  may  be  of  the  hand-feed 
type  or  an  automatic  lamp  may  be  used. 

§  904.    Automatic  lamp    for  use  in  projection  of    spectra. — 

When  using  certain  materials  in  the  arc,  the  arc  goes  out  frequently 


CH.  XV] 


EMISSION  SPECTRA 


639 


and  it  is  desirable  to  have  an  auto- 
matic machine  to  relight  the  arc 
again  instantly.  A  very  conven- 
ient device  for  this  purpose  is  an 
enclosed  arc  lamp  mechanism  for 
shunt  circuits.  The  wiring  of  the 
arc  will  need  to  be  slightly  modi- 
fied to  adapt  it  to  the  heavy 
currents  (15  amps.)  required. 
This  is  done  by  connecting  the 
wires  to  the  rheostat  of  the  lamp 
at  A,  B,  C  and  D  (fig.  367)  and  by 
putting  a  german  silver  wire  be- 
tween E  and  F  of  the  "series 
magnet"  so  that  this  magnet  will 
not  overheat  and  at  the  same  time 
will  not  lift  the  upper  carbon 
too  suddenly. 

§  905.  Current  to  use. — For 
the  projection  of  arc  spectra  the 
current  to  use  will  depend  upon 
the  substance  in  the  arc.  When 
treated  carbon  electrodes  are  used, 
either  alternating  or  direct  cur- 

FIG.  367.     AUTOMATIC   ARC   FOR  THE 

PROJECTION  OF  ARC  SPECTRA. 
The  mechanism  is  that  of  an  enclosed 
shunt,  direct  current  arc.  In  order  to 
get  sufficient  current  the  wire  is  connec- 
ted to  the  resistor  in  the  two  points  A 
and  B.  The  wire  to  the  lifting  magnet 
is  connected  at  the  points  D  and  E. 
This  gives  three  times  the  current  for 
which  the  arc  was  designed,  i.  e.,  about 
15  to  1 8  amperes.  The  lifting  solenoid 
E  F,  must  be  shunted  by  a  suitable 
resistance  easily  found  by  experiment. 
The  clutch  automatically  lifts  the  upper 
carbon  —  whenever  current  is  flowing. 
The  lower  carbon  is  stuffed  with  salts 
and  connected  to  the  positive  wire. 


640  DEMONSTRATION  OF  ULTRA-VIOLET  LIGHT         [Cn.  XV 

rent  may  be  employed,  but  direct  current  is  to  be  preferred. 
About  15  amperes  will  give  the  best  results.  When  direct  current 
is  used  the  lower  carbon,  which  contains  the  salt  to  be  studied,  is 
made  the  positive. 

With  metallic  electrodes  or  with  metallic  oxides  contained  in 
sheet  iron  tubes,  direct  current  only,  can  be  used.  About  6 
amperes  give  the  best  results  in  this  case.  The  metallic  electrode 
is  made  the  lower,  the  upper  electrode  being  carbon.  Most 
metallic  electrodes  will  give  different  results  when  made  the  positive 
than  when  made  the  negative  terminal.  The  magnetite  and 
titanium  ("Luminous  arc")  electrodes  show  the  lines  of  iron  and 
titanium  best  when  connected  to  the  negative  wire. 

Uranium  oxide  contained  in  an  iron  tube  will  give  a  line  spectrum 
when  connected  to  the  negative  wire  but  will  give  a  very  nearly 
continuous  spectrum  when  connected  to  the  positive  wire 
(§  885a). 

DEMONSTRATION  ON  A  SMALL  SCALE;   DEMONSTRATION  OF 
ULTRA-VIOLET  LIGHT  AND  PHOTOGRAPHY 

§  906.  Demonstration  on  a  small  scale. — Spectra  may  be 
projected  on  a  small  scale  for  the  observation  of  a  few  individuals 
with  the  apparatus  shown  in  fig.  368.  This  arrangement  is  similar 
in  every  way  to  that  for  the  projection  of  spectra  on  a  large  screen 


FIG.  368.     PROJECTION  OF  SPECTRA  ON 

SMALL  SCALE. 
L     Radiant. 
C     Condenser. 
S    Slit. 

Oj    Objective  before  the  prism. 
O2   Objective  beyond  the  prism. 
P     Prism. 
R  V    Spectrum. 

For  the  best  definition   there  must   be  an  objective   (02) 
beyond  the  prism  to  focus  the  spectrum  (R  V). 


CH.  XV]  DEMONSTRATION  OF  ULTRA-VIOLET  LIGHT          641 

except  that  the  additional  lens  O2,  brings  the  parallel  rays  of  light  of 
each  wave-length  to  a  focus  in  the  spectrum  R  V.  The  arrange- 
ment in  fig.  368,  with  the  lens  Oi,  giving  a  converging  beam,  will 
not  give  good  results,  and  the  second  lens  O2,  is  required  if  any  fine 
details  in  the  projected  spectrum  are  to  be  shown.  The  two  lenses 
Oi  and  O2,  should  be  achromatic.  The  two  lenses  from  a  symmetri- 
cal photographic  objective  will  give  excellent  results.  Ordinary 
spectacle  lenses  can  be  made  to  answer  if  no  others  are  available. 
The  prism  can  be  of  any  of  the  forms  previously  described.  The 
spectrum  is  received  on  either  a  white  screen  or  one  which  is  coated 
with  anthracene  in  order  to  show  the  ultra-violet. 


FIG.  369.     PROTECTION  OF  SPECTRA  ON  A 
SMALL  SCALE. 

With  this  arrangement,  where  the  distance  from  the 
prism  to  the  spectrum  R  V  is  relatively  short,  as  here 
shown,  the  definition  will  be  poor:     see  fig.  368  for  a  better 
method. 

L     Radiant  (arc  lamp  with  vertical  carbons). 

C    Condenser. 

5     Slit. 

O     Objective. 

P     Prism. 

/      Image  of  the  slit  S  when  no  prism  is  in  place. 

R  V    Spectrum  projected  by  the  prism. 

§  907.  Projection  of  ultra-violet. — Ordinary  glass  prisms  and 
lenses  if  not  noticeably  yellow  or  green  will  transmit  radiation  in  the 
ultra  violet  to  about  .35(1,  which  can  be  observed  by  the  use  of  an 
anthracene  screen  (§  899).  If  the  far  ultra-violet  spectrum  is  to  be 
demonstrated  it  is  necessary  to  use  a  quartz  system,  that  is,  all 
condensers,  lenses,  and  prisms  between  the  source  and  the  screen 
must  be  made  of  quartz,  either  quartz  glass  or  quartz  crystal. 
The  apparatus  is  arranged  as  in  fig.  370. 

The  quartz  prisms  are  usually  made  of  two  30°  prisms,  as  shown 
in  fig.  370,  one  of  which  is  a  right-hand  crystal,  the  other  a  left- 


642          DEMONSTRATION  OF  ULTRA-VIOLET  LIGHT          [Cn.  XV 

hand  crystal.  The  space  between  the  two  prisms  is  filled  with 
glycerin. 

Aside  from  the  material  of  which  the  lenses  and  prisms  are  made 
there  is  but  one  thing  which  is  different  from  the  case  with  a  glass 
system.  By  using  quartz  alone,  no  achromatic  lenses  are  possible 
and  the  spectrum  instead  of  focusing  in  a  line  at  right  angles  to  the 
axis  of  the  beam,  focuses  along  a  line  oblique  to  the  axis.  Thus, 
the  far  ultra-violet,  UV,  focuses  nearer  lens  O2,  than  does  the  visible 
spectrum.  Tilting  the  screen  to  the  position  indicated  will  enable 
one  to  get  all  of  the  spectrum  in  focus  at  once. 

The  anthracene  screen  (§  899)  will  enable  one  to  demonstrate  all 
of  the  lines  of  arc  spectra  which  would  appear  upon  a  photograph 


FIG.  370.     PROJECTION  OF  THE  ULTRA-VIOLET 
ON  A  SMALL  SCALE. 

L     Light  source. 

C    Quartz  condenser. 

5     Slit. 

O,    Plano-convex  quartz  lens. 

P  Quartz  prism,  preferably  made  of  two 
prisms  r  and  /  from  right-and  left-handed  quartz 
crystals  cemented  together  with  glycerin. 

02  Plano-convex  quartz  lens.  Turn  the  convex  sides  of  the  lenses  towards 
the  prism. 

U  V,  V  R     Focus  of  the  spectrum. 

Anthracene  screen,  fluoresces  to  ultra-violet.     Note  its  oblique  position. 

made  by  the  use  of  a  quartz  spectrograph,  the  ultra-violet  lines  of 
the  aluminum  arc  at  .217(1  being  easily  seen.  Demonstration  of 
fluorescence  of  other  substances  to  ultra-violet  may  be  shown  by 
substituting  them  for  the  anthracene. 

The  demonstration  of  the  far  ultra-violet  on  a  large  scale  is 
hardly  possible  owing  to  the  small  intensity  of  the  light  emitted  in 
this  region. 


CH.  XV]  PHOTOGRAPHY  OF  SPECTRA  643 

USE  IN  PHOTOGRAPHY 

§  908.  Apparatus. — Slit,  prism,  grating,  symmetrical  photo- 
graphic objective,  camera  bellows,  bromide  paper,  photographic 
plate. 

The  systems  described  above  for  the  demonstration  of  spectra  on 
a  small  scale  (fig.  368,  371),  may  be  employed  for  the  photography 
of  spectra.  Such  a  system  can  be  used,  for  example,  to  determine 
the  wave-lengths  of  the  radiation  to  which  bromide  paper  is 
sensitive.  If  the  bromide  paper  is  held  firmly  against  a  rigid  sup- 
port so  that  the  spectrum  of  a  right  angle  arc  may  fall  upon  it,  the 

o,  P 


FIG.  371.     ARRANGEMENT  FOR  THE  PHO- 
TOGRAPHY OF  SPECTRA. 

S     Slit. 

Ot    Front  combination  of  a  photographic 
objective. 

O2    Back  combination  of  the  objective. 

P     Prism  between  the  two  combinations 
of  the  photographic  objective. 

R  V    The  spectrum  in  focus  on  the  photographic 
plate. 

Plate     The  sensitive    photographic  plate   in    the 


paper  will  be  found  to  be  blackened  where  blue  and  ultra-violet 
light  struck  it,  but  the  red  and  green  will  show  no  action  at  all. 
If  dry  plates,  however,  are  used  a  more  complete  system  of  shield- 
ing from  the  light  will  be  required.  The  second  lens  O2,  may  be 
held  in  a  camera  box  as  shown  in  figure  371.  A  more  elaborate 
system  for  making  several  exposures  on  the  same  plate  is  not  here 
described  because,  while  good  results  may  be  obtained  with  such 
apparatus  by  sufficient  labor,  it  is  more  satisfactory  to  use  one  of 
the  regular  spectrographic  cameras. 


644  ABBE  DIFFRACTION  THEORY  [Cn.  XV 

DEMONSTRATION  OF  ABBE  DIFFRACTION  THEORY  OF  MICROSCOPIC 

VISION 

§  909.     Apparatus.— Condenser;   Pinhole;   Slit. 

Convex  lens  of  one  meter  focal  length  (spectacle  lens  of  i  diopter, 
§  356a). 

Grating,  photographic  line  screen  (100  to  200  lines  to  the  inch,) 
fine  wire  gauze  (100  mesh),  fine  bolting  cloth. 

Telescope.     The  eye-piece  should  be  of  high  power. 

Diaphragms  to  remove  portions  of  the  diffraction  image. 


-1.5M 


.A- - 4. 


UM- 


FIG.  372.     LENS  SYSTEM  AND  ARRANGEMENT  FOR  SHOWING  THE  ABBE 
DIFFRACTION  THEORY  OF  IMAGE  FORMATION. 

L     Right-angled  arc  lamp  with  small  carbons  (5  mm.) 

C    Condenser  used  temporarily  for  focusing. 

G  Grating  with  coarse  lines.  A  halftoning,  line  screen  or  a  fine  wire  net  will 
answer. 

0  Spectacle  lens  of  I  diopter  (i  meter  focus)  for  projecting  the  image  of  the 
lamp. 

DI     Image  of  the  arc  lamp  L,  projected  by  the  objective  0. 

When  the  grating  (G)  is  in  place,  there  is  formed  at  this  point  a  diffraction 
pattern.  Various  shaped  diaphragms  placed  at  this  point  modify  the  screen 
image  of  the  grating  at  /. 

/     Screen  image  of  the  grating  (G). 

The  lines  and  numbers  above  and  below  indicate  the  approximate  distances 
between  the  different  parts  of  the  system  which  have  been  found  to  give  satis- 
factory results. 

An  interesting  phenomenon  connected  with  the  Abbe  Diffraction 
Theory  of  image  formation  can  be  demonstrated  by  one  of  the 
combinations  described  below.  The  simplest  is  shown  in  fig.  372. 
Suppose  the  test  object  to  be  a  diffraction  grating  with  equidistant 
•lines  vsuch  as  a  fine  wire  gauze  or  a  line  screen  such  as  photo- 
engravers  use  in  making  half  tones. 

§  910.  The  Abbe  diffraction  theory.  Image  formation  with 
directed  light. — In  microscopic  work  and  with  all  transparency 


CH.  XV]  ABBE  DIFFRACTION  THEORY  645 

projection,  objects  are  not  self-luminous  but  are  illuminated  by  a 
-narrow  beam  of  directed  light  which,  were  no  object  present,  would 
pass  through  the  center  of  the  objective.  When  an  object,  a 
diffraction  grating  for  example,  is  illuminated  with  a  narrow  cone 
of  light,  the  light  is  spread  out  into  a  diffraction  pattern.  The 
finer  the  details  of  the  object,  the  larger  will  be  the  diffraction 
pattern.  The  objective  will  unite  the  light  scattered  from  the 
object  by  diffraction  just  as  it  would  light  which  was  spread  out  by 
reflection  from  a  white  surface.  Now  -according  to  the  Abbe 
diffraction  theory,  the  closeness  with  which  the  image  will  corre- 
spond to  the  object  will  depend  upon  the  completeness  with  which 
the  light  from  the  entire  diffraction  pattern  is  collected  to  form  the 
image.  If  the  entire  diffraction  pattern  is  not  united  to  form  the 
image,  but  part  of  it  is  intercepted,  the  image  will  be  that  of  such 
an  object  as  would  produce  a  diffraction  pattern  like  that  part  of 
the  diffraction  pattern  which  is  collected  to  form  the  image. 

§  911.  Lens  system  for  showing  diffraction  images. — The  lens 
system  shown  in  figure  372  will  show  this  phenomenon.  The  arc 
lamp  L,  with  5  mm.  carbons,  three  to  five  amperes  direct  current 
used  as  a  point  source,  is  set  up  six  to  eight  meters  from  the  screen 
at  I.  The  condenser  C  is  used  temporarily  to  illuminate  the 
objective  lens  O.  This  objective  lens  O,  is  of  one  meter  focal 
length  (an  ordinary  convex  spectacle  lens  of  i  diopter  will  answer) . 
It  is  placed  3M.  (10  ft.)  from  the  screen.  The  grating  G,  is  placed 
between  the  source  L,  and  the  objective  O,  so  as  to  be  in  focus  on  the 
screen  at  I.  The  condenser  C  is  now  removed.  The  image  I,  will 
remain  as  before.  At  DI,  would  be  found  an  image  of  the  source 
cast  by  the  lens  O,  but  it  will  be  spread  out  into  a  diffraction  pat- 
tern by  the  grating. 

If  a  vertical  slit  is  placed  at  DI,  so  as  to  remove  all  but  a  vertical 
line  of  images,  the  appearance  will  be  of  parallel  horizontal  lines. 
A  diagonal  slit  will  give  the  appearance  of  diagonal  lines,  no  vertical 
or  horizontal  lines  being  seen.  If  a  vertical  rod  is  put  in  so  as  to 
remove  the  central  row  of  images,  the  diffraction  pattern  will  be 
that  of  a  grating  with  fine  vertical  lines,  twice  as  close  together  as 
the  coarse  horizontal  lines,  and  the  image  at  I,  will  have  heavy 


646  ABBE  DIFFRACTION  THEORY  [Cn.  XV 

horizontal  lines  and  fine  vertical  lines  very  close  together.  Dia- 
phragms cut  from  black  paper  of  various  shapes,  will  give  many 
curious  and  beautiful  appearances  at  I.  A  small  diaphragm 
placed  so  as  to  remove  all  but  the  central  image  of  the  pattern,  or 
one  of  the  lateral  images,  will  allow  light  to  fall  on  the  screen  but 
no  detail  can  be  seen. 

For  lecture  purposes,  where  one  requires  considerably  more  light 
than  for  a  small  class  demonstration,  one  can  use  a  vertical  slit 
with  a  condenser  as  the  source  instead  of  the  arc  lamp  (fig.  373). 
See  §  891  and  §  900,  figure  364.  Use  line  gratings  with  the  lines 


FIG.  373.  DEMONSTRATION  OF  ABBE  DIFFRACTION  THEORY  TO  A  LARGE 
AUDIENCE,  USING  A  SLIT. 

L     Arc  lamp. 

CT    Condenser. 

5     Pinhole  or  Slit. 

C2  Condenser,  preferably  an  achromatic  combination. 

G     Grating. 

0  Objective.  The  diffraction  pattern  is  formed  at  the  face  of  the  objective. 
Diaphragms  are  used  at  this  point  to  modify  the  image. 

/     Image  of  grating. 

vertical.  The  phenomena  shown  are  not  as  interesting  as  when 
using  a  point  source.  By  using  slits  or  rods  to  intercept  part  of 
the  diffraction  pattern,  the  image  on  the  screen  can  be  made  to 
appear  as  if  it  were  of  a  grating  having  finer  lines  than  the  grating 
which  is  actually  used. 

If  this  phenomenon  is  projected  it  will  probably  be  desired  also 
to  demonstrate  it  individually  to  a  few  of  the  observers.  This  may 
be  done  by  the  use  of  a  telescope  t,  (fig.  374).  to  observe  the  grating. 
The  eyepiece  of  the  telescope  should  be  of  a  high  power.  The 
condenser  C2  focuses  the  image  of  the  pinhole  just  in  front  of  the 
telescope  objective.  When  the  grating  g,  is  in  place,  the  diffraction 
images  will  appear  on  both  sides  of  the  pinhole  image.  If  the 
grating  is  viewed  by  the  telescope  it  will  appear  normal  but  if  part 
of  the  diffraction  pattern  is  stopped  out  by  diaphragms,  the  grating 


CH.  XV]  DARK  GROUND  ILLUMINATION  647 

will  appear  changed  as  in  the  case  of  projection.  The  sharpness  of 
the  pattern,  and  the  intricacy  of  design  are  however  much  finer 
than  it  is  possible  to  project. 


FIG.  374.  DEMONSTRATION  OF  THE  ABBE  DIFFRACTION  THEORY  TO  A  SINGLE 

OBSERVER. 

L    Arc. 

Cj    Condenser. 

5     Pinhole  or  Slit. 

C2  Condenser,  preferably  an  achromatic  combination. 

G     Grating. 

T    Telescope  with  high  power  eyepiece. 

The  telescope  is  focused  on  the  grating  and  the  diffraction  pattern  is  focused 
just  in  front  of  the  telescope  objective.  By  placing  suitably  shaped  dia- 
phragms at  this  point,  the  image  as  seen  in  the  eyepiece  will  be  modified. 


DARK  GROUND  ILLUMINATION  :     METHOD  OF  STRIAE 

§  912.  Many  beautiful  experiments  in  Physics  and  Chemistry 
can  be  shown  by  what  is  best  known  as  the  Schliren-methode  of 
Toepler.  This  method  will  yield  results  almost  as  striking  as  those 
obtained  by  polarized  light. 

See  Wiedemann  Annallen,  CXXXI,  p.  33. 

The  use  of  this  method  enables  one  to  demonstrate  any  slight 
lack  in  homogeneity  of  a  medium  which  is  sufficient  to  deviate  a 
beam  of  light. 

To  adapt  this  method  to  projection  the  following  apparatus  can 
be  used : 

§  913.     Apparatus  for  the  experiments  with  striae. — 

(1)  Magic  lantern  with  the  usual  equipment  of  arc  lamp, 
projection  objective,  and  the  first  element  of  the  large  condenser. 

(2)  A  special  condensing  lens  or  combination.     This  need  not 
be  of  especially  large  diameter  or  short  focus  (5  cm.  diameter,  20 
cm.  focus  will  answer),  but  it  should  be  as  free  as  possible  from 
spherical  and  chromatic  aberration,  and  must  have  no  scratches 
and  be  kept  perfectly  clean. 


648 


DARK  GROUND  ILLUMINATION 


[Cn.  XV 


(3)  Diaphragms  to  shut  off  the  direct  light  of 
the  lantern.     These  may  be  simply  sheets  of  tin. 

(4)  Glass  cells  with  parallel  faces. 

§  914.  Method. — Light  from  the  arc  L,  is  ren- 
dered nearly  parallel  by  the  lantern  condenser  Ci. 
The  diaphragm  DI,  cuts  off  the  lower  half  of  this 
beam,  the  other  half  serving  to  illuminate  the  speci- 
men S,  in  the  glass  cell.  The  distance  between  the 
condenser  and  the  specimen  should  be  from  50  to 
100  cm.  (2  to  4  feet).  Either  before  or  after  pas- 
sing through  the  specimen  S,  (preferably  before,  as 
in  fig.  375)  this  light  strikes  the  special  condenser 
C2,  which  brings  the  diaphragm  Db  to  a  focus  at  D2. 
At  this  point  is  placed  the  diaphragm  D2,  which  is  so 
arranged  as  to  just  cut  off  the  remainder  of  the  light, 
its  edge  coming  to  the  edge  of  the  image  of  the  dia- 
phragm DI.  The  objective  O,  is  focused  to  bring 
the  specimen  S,  to  a  sharp  focus  on  the  screen  before 
the  diaphragm  D2,  is  in  place.  With  the  apparatus 
thus  arranged  the  screen  will  be  perfectly  dark,  all 
light  not  intercepted  by  the  first  diaphragm  being 
stopped  by  the  second.  If,  now,  the  liquid  in  the 
cell  S,  is  not  quite  homogeneous  but  is  cordy,  as 
when  glycerine  and  water  are  first  mixed  or  when  a 
crystal  of  salt  is  dissolving,  the  image  of  Db  will  not 

FIG.  375.  DARK  GROUND  ILLUMINATION;  TOEPLER  METHOD 
OF  STRIAE. 

L     Arc. 

Ct     First  part  of  the  magic  lantern  condenser. 

DI     Diaphragm. 

C2  Condenser  of  long  focus.  It  must  be  as  perfect  a  lens 
as  can  be  found. 

S     Specimen,  with  slight  inhomogeneity. 

a  An  inhomogeneity  in  the  specimen  which  deviates  the 
light.  o . 

D2  Diaphragm  intercepting  direct  light  from  the  lantern. 

O     Objective. 

a1     Image  of  a. 

Note  that  with  the  objective  on  the  axis  only  the  upper  por- 
tion of  the  objective  is  used. 


CH.  XV 


DARK  GROUND  ILLUMINATION 


649 


be  quite  sharp  and  some  light  will  escape  the  edge 
of  the  diaphragm  D2,  and  reach  the  screen. 

The  result  is  very  striking  as  even  a  slight  inhomo- 
geneity  of  the  medium  in  the  glass  cell  will  deviate 
light  sufficiently  to  pass  the  second  diaphragm  and 
thus  be  seen. 

Suppose  a  slight  cord  of  a  substance  of  different 
refractive  index  from  its  surroundings  to  exist  at  a1 
in  fig.  376.  This  cord  will  scatter  the  light.  A  ray 
which  would  normally  strike  D2,  and  be  intercep- 
ted, will  spread  out  in  all  directions.  The  part  of 
this  light  which  strikes  the  objective  will  go  to  form 
a  screen  image  of  the  cord  at  a1  (fig.  376). 

The  sensitiveness  of  this  method  depends  upon  the 
sharpness  of  the  image  of  the  diaphragm  DI,  and  the 
closeness  of  adjustment  of  D2,  so  as  to  encroach  as 
little  as  possible  upon  it.  With  a  very  sharp  im- 
age, it  is  possible  to  detect  the  minutest  striae  and 
inhomogeneities  in  the  specimen. 

The  image  sharpness  may  be  disturbed  as  much  by 
imperfections  in  the  condensing  lens  C2,  as  by  an  in- 
homogeneity  of  the  specimen,  hence  these  imperfec- 
tions, if  present,  will  show  distinctly  on  the  screen. 
In  fact,  the  method  is  as  well  designed  to  show  the 


FIG.  376. 


DARK  GROUND  ILLUMINATION; 
METHOD  OF  STRIAE. 


TOEPLER 


L     Arc. 

Ct     First  part  of  the  magic  lantern  condenser. 

D^    Diaphragm. 

C2  Condenser  of  long  focus.  It  must  be  as  perfect  a  lens 
as  can  be  found. 

5     Specimen  with  slight  inhomogeneity. 

a  An  inhomogeneity  in  the  specimen  which  deviates  the 
light. 

D2  Diaphragm  intercepting  direct  light  from  the  lantern. 

0     Objective. 

a1  Image  of  a.  Note  that  a  small  objective  above  the  axis 
is  used.  The  dotted  lines  show  the  course  of  the  rays  which 
are  slightly  deviated  from  their  original  path  by  the  inhomo- 
geneity of  the  specimen. 


650 


DARK  GROUND  ILLUMINATION 


[CH.  XV 


imperfections  of  the  second  condenser  C2,  as  to  show 
the  specimen.  The  difficulty  can,  of  course,  be  les- 
sened by  drawing  the  condenser  face  C^,  out  of  the  focus 
of  the  objective  O.  Dust,  fingermarks,  etc.,  will  then 
produce  a  general  blur,  rather  than  a  distinct  image. 

§  915.  Foucault's  method. — A  slight  modification 
of  the  old  method  of  Foucault  for  testing  telescope  ob- 
jectives also  gives  good  results.  This  method  (fig.  377) 
dispenses  with  the  use  of  the  first  diaphragm  Db  the 
crater  of  the  arc  being  the  first  diaphragm  in  this  case. 
Instead  of  the  ordinary  condenser,  is  substituted  a  lens 
or  set  of  lenses  which  are  to  form  a  very  sharp  image  of 
the  crater.  Just  in  front  or  behind  the  objective 
(wherever  the  sharp  image  of  the  crater  is  formed)  is  a 
diaphragm  which  just  covers  up  the  crater  image.  Such 
a  diaphragm  may  be  made  by  fastening  a  round 
piece  of  black  paper  to  a  piece  of  plate  glass.  The  ob- 
jective brings  the  specimen  to  a  focus  upon  the  screen 
in  the  usual  way.  Any  inhomogeneities  in  the  speci- 
men scatter  the  light  so  that  some  of  it  gets  by  the 
central  stop  at  D2.  It  is  this  scattered  light  which 
serves  to  form  the  image  on  the  screen  at  a1. 

If   the   inhomogeneities  of  the  specimen  are  great  ° 
enough,  the  specimen  may  be  projected  by  the  method 
just  described,  (see  fig.   377),  except  that  instead  of 
using  a  central  stop,  the  lens  is  provided  with  an  iris 
diaphragm,  and  the  image  of   the  arc   is   focused  on 


FIG.   377.     DARK   GROUND   ILLUMINATION, 
FOUCAULT'S   METHOD. 


Arc. 


Condenser  of  long  focus,  as  perfect  as  can  be  found. 
Specimen. 

Inhompgeneity  in  the  specimen  which  deviates  the  light. 
Objective. 

Z>2  Central  stop  to  intercept  the  direct  light  from  the  arc. 
a1     Image  of  a,  projected  on  the  screen. 

The  dotted  lines  show  the  course  of  the  rays  deviated  by  an 
inhomogeneity  in  the  specimen  which  pass  to  one  side  of  the  cen- 
tral stop  and  reach  the  screen. 


CH.  XV]  DARK  GROUND  ILLUMINATION  651 

the  center  of  this  opening.  If,  now,  light  is  slightly  deviated  it  will 
not  get  through  the  small  opening  in  the  diaphragm  and  the 
inhomogeneity  will  appear  as  a  dark  shadow  on  a  light  background. 

EXPERIMENTS     ILLUSTRATING    NORMAL    VISION    AND     SIMPLE, 
REFRACTIVE  EYE  DEFECTS 

§  916.     Apparatus  needed  for  the  demonstrations: 

Suitable  room  for  projection;  White  screen  70  to  100  centi- 
meters (28  to  40  inches)  square;  Arc  lamp  and  magic  lantern  con- 
denser with  lamp-house,  fig.  378-379;  Optical  bench  with  a  range 
beyond  the  condenser  of  at  least  40  cm.  (16  in.) ;  fig.  159,  378-379; 
Lantern-slide  carrier  and  lens  support,  fig.  159,  378-381;  Metal 
holder  for  four  trial  lenses,  fig.  380;  Oculists'  trial  lenses  as  shown 
in  fig.  382 ;  Discs  of  tin  or  sheet-iron  the  size  of  trial  lenses,  and 
with  holes  for  the  pupil  and  a  stenopaeic  slit,  fig.  399,  A,  B.; 
Lantern  slides  (4)  for  illustrating  accommodation,  astigmatism 
and  anisometropia,  myopia,  etc.,  fig.  383,  391-392,  401,  (§  9i6ab). 


§  916a.     The  cost  of  the  special  apparatus  needed  for  the  demonstrations 
in  normal  and  defective  vision : — 

Metal  lens  holder  for  4  trial  lenses $2.25 

Trial  lenses  in  trial  rings  (14  at  20  cts.) 2.80 

Double  trial  lenses  for  unlike  eyes i  .50 

Lantern  slides  (4  at  35  cts.)  1 .40 


$7-95 

To  this  amount  should  be  added  the  cost  of  the  object  and  lens  blocks  and 
the  vertical  pieces  for  carrying  the  lens  holder  and  the  slide-carrier. 

The  screen  of  white  cardboard  and  the  lengthening  rods  for  the  optical 
bench  are  also  extra,  but  all  of  these  should  not  make  a  total  outlay  of  over 
$10.00  in  addition  to  the  magic  lantern.  As  will  be  seen  in  the  appendix,  magic 
lanterns  cost  all  the  waj"  from  $20  to  $500. 

The  trial  lenses,  lens  holder  and  double  lenses  may  be  obtained  through  a 
local  optician,  or  they  can  be  got  direct  from  a  manufacturer  of  spectacles,  etc., 
for  example:  Aloe  &  Co.,  St.  Louis,  Mo.;  Bausch  &  Lomb  Optical  Co., 
Rochester,  N.  Y.,  New  York  City,  Washington,  D.  C.,  Chicago,  111.,  San 
Francisco,  Cal.;  Geneva  Optical  Co.,  Geneva,  N.  Y.,  and  Chicago,  111.; 
Hardy  &  Co.,  New  York  City,  Chicago,  111.,  Denver,  Col.,  Atlanta,  Ga., 
Dallas,  Tex.;  Lloyd  &  Co.,  Boston,  Mass.;  E.  B.  Meyrowitz,  New  York  City, 
Minneapolis  and  St.  Paul,  Minn.;  Williams  Brown  &  Earle,  and  Joseph 
Zentmayer,  Philadelphia,  Pa.,  and  many  others. 

§  916b.  The  authors  feel  greatly  indebted  to  Dr.  Melvin  Dresbach  and  to 
Dr.  Albert  C.  Durand  for  suggestions  and  criticism  in  the  preparation  of  the 
manuscript  for  these  experiments  in  normal  and  defective  vision. 


DEMONSTRATIONS  OF  NORMAL  VISION 


[Cn.  XV 


DEMONSTRATIONS     REPRESENTING     NORMAL    AND     DEFECTIVE 

VISION 

§  917.  Source  of  light. — For  the  most  successful  demonstrations 
of  vision  and  its  refractive  defects  it  is  necessary  to  have  a  right- 
angle  arc  lamp  and  a  direct  electric  current.  However,  for  all  the 
experiments  except  the  one  to  show  unequal  refraction  in  the  two 
eyes,  the  other  sources  of  light  mentioned  in  this  book  can  be  used. 
If  the  large  sources  are  used  it  is  desirable  to  have  a  shield  with  an 


Condenser 


FIG.    378.     PROJECTION   APAPRATUS    WITH   THREE-LENS   CONDENSER   AND 
OPTICAL  BENCH  FOR  DEMONSTRATIONS  REPRESENTING  VISION. 

Commencing  at  the  left : 

W1  Wl  Supply  wire  to  the  knife-switch  and  from  the  switch  through  the 
rheostat  to  the  upper  or  positive  carbon  of  the  arc  lamp. 

W2  W2  Supply  wire  to  the  knife-switch,  and  from  the  switch  to  the  lower 
carbon  of  the  arc  lamp. 

L     The  source  of  light  (crater  of  the  upper  carbon) . 

Lamp  Block  The  block  for  supporting  the  arc  lamp  and  by  which  it  can  be 
moved  back  and  forth  on  the  optical  bench. 

Base  Board  The  board  on  which  are  the  tracks  of  the  optical  bench  (see 
fig.  159). 

1 Condenser2  The  triple-lens  condenser.  The  second  element  of  the  con- 
denser (2)  should  have  a  focus  of  30  to  40  centimeters  (12  to  15  inches). 

W    The  water-cell  to  absorb  the  radiant  heat. 

Obj.     The  object  (lantern  slide  of  radial  lines,  etc.,  fig.  383-393). 

Lens     The  trial  lens  serving  to  project  the  image. 

Block  i     The  block  supporting  the  condenser  and  water-cell. 

Block  2  The  block  serving  as  a  support  for  the  lantern  slide,  and  by  means 
of  which  the  slide  can  be  moved  back  and  forth  on  the  optical  bench. 

Block  3  The  block  supporting  the  lens  carrier,  and  by  means  of  which  the 
lens  can  be  moved  back  and  forth  on  the  optical  bench. 


These  experiments,  with  the  accompanying  explanation,  have  been  com- 
piled from  lectures  and  demonstrations  given  by  the  senior  author  before  the 
Sixth  District  Branch  of  the  Medical  Society  of  the  State  of  New  York, 
October,  1913;  The  Conference  of  Veterinarians  at  the  New  York  State  Vet- 
erinary College,  December,  1913;  and  before  the  Cornell  University  Summer 
School,  July,  1914. 


CH.  XV 


DEMONSTRATIONS  OF  NORMAL  VISION 


653 


opening  a  little  smaller  than  the  trial  lenses  to  put  just  beyond  the 
trial  lens.  This  cuts  off  any  stray  light  falling  outside  the  lens. 
If  large  sources  only  can  be  used,  then  it  would  be  an  advantage  to 
have  lenses  of  greater  diameter  than  the  trial  lenses  so  that  all 
the  light  could  be  utilized  and  thus  give  a  brighter  screen  image. 
The  trial  lenses  answer  admirably  for  the  arc  light,  however. 

§  918.  Centering  along  one  axis. — As  with  the  magic  lantern 
and  the  projection  microscope,  it  is  necessary  to  have  all  of  the 
elements  of  the  projection  outfit  on  one  axis  (§  51-58). 

Also  as  the  light  is  liable  to  get  out  of  the  axis,  it  is  a  great  advan- 
tage to  have  fine  adjustments  on  the  arc  lamp  to  bring  it  back  in 


FIG.  379. 


PROJECTION  APPARATUS  WITH  TWO-LENS  CONDENSER  FOR 
DEMONSTRATIONS  REPRESENTING  VISION. 


Radiant.  The  arc  lamp  in  the  lamp-house.  It  has  fine-adjustment  screws 
(L  A,  V  A),  and  feeding  screws  for  the  carbons  (F  S)  and  the  source  of  light  in 
the  crater  of  the  upper  carbon  (L). 

Lamp  House,   V    The  lamp-house  and  its  ventilator. 

Rods  The  rods  of  this  form  of  lantern.  These  serve  as  a  kind  of  optical 
bench  along  which  the  different  parts  can  be  moved.  They  should  be  long 
enough  to  permit  of  a  separation  of  the  lens  and  the  condenser  of  at  least  40 
cm.  (16  in.). 

Cond,  i  2  The  two  lenses  of  the  condenser.  Lens  2  should  be  of  relatively 
long  focus,  25  to  30  cm.  (10  to  12  inches). 

Object  Block  The  block  supporting  the  object,  and  by  means  of  which  the 
object  can  be  moved  back  and  forth  along  the  rods. 

Lens  Block  The  block  supporting  the  lens.  It  can  be  moved  back  and 
forth  along  the  rods. 

Image  screen  The  white  screen  for  the  image.  It  is  5  meters  (16  ft.)  from 
the  projection  lens. 

37  Centimeters  The  distance  between  the  lens  and  the  object  for  a  3  diopter 
lens. 

28  Centimeters     The  distance  between  the  object  and  a  4  diopter  lens. 

Ind  i,  Ind  2  A  white  strip  of  cardboard  to  serve  as  an  indicator  so  that  the 
spectators  can  see  when  the  object  is  moved  toward  or  from  the  lens. 


654 


DEMONSTRATIONS  OF  NORMAL  VISION 


[Cn.  XV 


position  (fig.  3).  The  vertical  boards  holding  the  lantern  slide  and 
the  trial  lens  holder  should  have  means  of  centering  them  accur- 
ately. This  is  provided  for  by  the  U-shaped  opening  at  the  lower 
B  end  of  the  uprights  where  the  set-screw  is  inser- 

ted to  hold   the   uprights  against  the  movable 
blocks  (fig.  159,  381). 

FIG.  380.  METAL  LENS  HOLDER  FOR  FOUR  TRIAL  LENSES. 
(Half  Natural  Size) 

A  Side  view  of  the  lens  holder  showing  the  stem  by 
which  it  is  held  in  place  in  the  lens  support  (fig.  378). 

B  Face  view  of  the  lens  holder  showing  the  four  grooves 
for  containing  the  lenses,  and  in  which  they  can  be  rotated. 

§  919.  Lenses,  lens  holder  and  white  screen. — For  the  refrac- 
tive part  of  the  eye  (cornea  and  crystalline  lens),  the  trial  lenses 
used  by  oculists  answer  very  well.  To  hold  these  and  to  permit 
of  their  rotation  it  is  necessary  to  have  a  metal  lens  holder  (fig. 
380).  A  lens  holder  with  grooves  for  four  lenses  is  very  desirable. 

To  represent  the  retina  of  the  eye  there  is  needed  a  white  screen 
about  one  meter  (3  feet)  square.  This  screen  is  kept  at  the  con- 
stant distance  of  5  meters  (16  ft.)  from  the  lens  in  all  the  experi- 
ments. 

§  920.  Demonstration  of  normal  vision. — While  men  have 
always  known  that  the  eyes  were  for  seeing  the  things  in  external 
nature,  the  knowledge  that  the  eye  acts  like  an  optical  instrument 
and  produces  an  inverted,  real  image  upon  the  retina  came  only 
when  Kepler  in  1604  demonstrated,  in  the  clearest  possible  manner, 


FIG.  381.     SLIDE-CARRIER  FOR  THE  TEST  SLIDES. 

This  consists  of: 

An  object  block  sliding  on  the  optical  bench  or  the  rods 
(fig.  378-379). 

A  vertical  board  I  cm.  (y£  in.)  thick  with  a  U-shaped 
opening  in  the  lower  end.  A  thumb-screw  and  washer 
serve  to  hold  the  board  in  any  position  against  the  ob- 
ject block. 

The  lantern-slide  carrier  consists  of  a  thin  board  or  ^ 
piece  of  cardboard  with  an  opening  of  the  proper  size  and 
height  (object)  attached  to  the  vertical  board.  The  try- 
square  shaped  piece  (H  H)  is  to  hold  the  test  slide  (G  SI) 
in  position.  (See  also  fig.  159,  b4). 


Object  Block 


CH.  XV] 


DEMONSTRATIONS  OF  NORMAL  VISION 


655 


Kepler,  Retinal  Image, 

Accommo- 
dation 

1604 


Myopia 


Hyperopia 


Astigmia  1800-1825 


Effect  of  the  Pupil 

3) 


Two  Eyes    Different 


FIG.  382.     OUTLINES  OF  THE  TRIAL  LENSES  NEEDED  FOR  SIMPLE 
EXPERIMENTS  IN  NORMAL  AND  DEFECTIVE  VISION. 

(About  one-third  size) 

/     A  3  diopter  convex  lens  (+3).     For  diopter  see  §  356a. 

2     A3  and  a  4  diopter  convex  lens  (+3,  +4). 

j     A  4  diopter  convex  lens  (+4),  and  a  I  diopter  concave  lens  ( — i). 

4  A  2  diopter  and  a  i  diopter  convex  lens  (+2,  +i). 

5  A  3  diopter  and  a  i  diopter  convex  lens  (+3,  -j-i). 

6  A  4  diopter  convex  lens  (+4),  a  0.5  convex  cylinder,  (+0.5  cyl.)  and  a 
0.5  concave  cylinder,  ( — 0.5  cyl.). 

7  A  4  diopter  convex  lens  (+4),  and  a  0.5  convex  or  concave  cylinder 
(0.5  cyl.). 


656 


DEMONSTRATIONvS  OF  NORMAL  VISION 


[Cn.  XV 


8  A  half  convex  lens  of  4  diopters,  and  a  half  convex  lens  of  3  diopters 
(+4,  +3)  in  the  same  trial  frame. 

A  half  convex  lens  of  I  diopter  (  +  i)  and  a  half  circle  of  plane  glass  (0),  in 
the  same  trial  frame  to  serve  as  a  correcting  spectacle.* 

that  whenever  an  object  is  seen,  there  must  be  formed  an  image  on 
the  retina,  and  that  this  image,  following  the  laws  of  optics,  must 
be  inverted.  A  few  years  later,  (1619),  Schemer  showed  by  actual 
experiment  with  the  eyes  of  animals  that  such  an  inverted,  real 
image  is  formed  on  the  retina ;  and  in  the  year  1 62  5 ,  he  showed  that 
the  same  is  true  of  the  human  eye. 


1604 

Kepler: 

Retinal  Image, 

Inversion, 
Accommodation. 


FIG.  383.     LANTERN  SLIDE  FOR  THE  EX- 
PERIMENTS IN  ACCOMMODATION. 


FIG.  384  TRIAL  LENS  FOR  KEP- 
LER'S EXPERIMENTS. 


For  the  demonstration  of  normal  vision  with  the  special  projector 
(fig.  378-379)  are  needed: 

(1)  A3  diopter  convex,  trial  lens  (fig.  384).     For  the  meaning 
of  diopter,  see  §  356a. 

(2)  A  lantern  slide  for  object  (fig.  383). 

The  screen  representing  the  retina  should  be  at  a  distance  of 
5  meters  (16  ft.)  from  the  lens  and  the  lens  should  be  40  centimeters 
(15.5  in.)  from  the  condenser  and  the  object  36  to  37  cm.  distant 
from  the  object  (fig.  379). 

*Of  the  ordinary  trial  lenses,  this  calls  for  three,  3  diopter  convex  lenses;  four,  4  diopter 
convex  lenses;  one,  2  diopter  convex  lens;  two  i  diopter  convex  lenses;  and  one,  i  diopter 
concave  lens.  In  addition  there  are  two,  0.5  diopter  convex  cylinders  and  one  0.5  diopter 
concave  cylinder,  making  14  trial  lenses.  These  cost  about  20  cents  each.  It  would  be  possible 
to  get  along  with  7  different  lenses  by  using  the  same  ones  over  and  over,  but  for  ease  and 
certainty  in  demonstration  it  is  better  to  have  each  step  in  the  demonstration  complete. 

The  authors  have  found  it  convenient  to  have  all  the  lenses  for  each  experiment  in  a 
separate  box,  and  set  edgewise,  then  they  can  be  grasped  quickly  and  surely,  thus  avoiding 
errors  and  loss  of  time. 


CH.  XV]  DEMONSTRATIONS  OF  NORMAL  VISION  657 

The  object  should  be  right  side  up  and  face  the  condenser.  If 
the  arc  lamp  is  lighted  there  will  be  projected  on  the  screen  a  sharp 
image  of  the  lantern  slide.  This  image  will  be  wrong  side  up  as  it 
is  in  the  eye.  In  order  to  have  the  image  erect  on  the  screen  the 
object  must  be  wrong  side  up  in  the  slide-carrier  as  in  magic  lantern 
projection  (fig.  8,  §  35). 

For  the  remainder  of  the  experiments  it  is  desirable  to  have  the 
screen  image  appear  erect  so  that  there  may  be  no  distraction  from 
the  special  points  to  be  shown.  It  is  worth  while  remembering 
though,  that  when  the  image  is  wrong  side  up  on  the  screen  it  will 
be  right  side  up  in  the  eyes  of  the  observers,  and  when  it  is  right 
side  up  on  the  screen  it  will  be  wrong  side  up  in  the  eyes  of  the 
observers.  Objects  appear  right  side  up  to  a  person  only  when  the 
image  is  wrong  side  up  on  his  retina. 

§  921.  Demonstration  of  the  need  of  accommodation  of  the 
eye  for  different  distances  of  the  object. — It  was  pointed  out  by 
Kepler  in  his  discussion  of  vision  that  the  eye  as  an  optical  instru- 
ment could  have  a  sharp  image  on  the  retina  only  in  one  position 
of  the  object,  unless  some  change  took  place  in  the  eye.  Every- 
one with  normal  eyes  knows  that  objects  at  all  distances  from  10  to 
15  centimeters  up  to  infinity  can  be  seen  with  equal  clearness. 
Kepler  thought  that  the  power  to  see  objects  at  different  distances 
was  due  to  the  possibility  of  changing  the  relative  position  of  the 
crystalline  lens  and  the  retina  by  the  elongation  and  shortening 
of  the  eye-ball. 

To  demonstrate  Kepler's  hypothesis  of  accommodation  there  are 
needed : 

(1)  A3  diopter,  convex,  trial  lens  (fig.  384). 

(2)  A  lantern  slide  (fig.  383). 

(3)  A  white  cardboard  screen  about  half  a  meter  (15  to  20  in.) 
square  to  hold  in  the  hands. 

If  now  the  arc  lamp  is  lighted  and  the  lantern  slide  placed  36  to 
3  7  centimeters  from  the  lens  a  sharp  image  will  be  projected  upon 
the  5  meter  screen.  Now  move  the  object  to  about  40  centimeters 
from  the  lens;  the  image  will  not  be  clear,  but  much  blurred.  To 
find  the  position  of  the  sharp  image,  take  the  small  white  screen  in 


658  DEMONSTRATIONS  OF  NORMAL  VISION  [Cn.  XV 

the  hands  and  hold  it  in  the  path  of  the  light  from  the  lens.  It 
will  be  found  at  a  point  between  two  and  three  meters  from  the 
lens.  This  shows  that  if  the  object  is  farther  from  the  lens,  the 
image  will  be  nearer  to  it.  Conversely,  if  the  object  is  brought  up 
toward  the  lens  the  image  will  move  farther  off.  Kepler  thought 
that  following  the  changes  in  the  position  of  the  sharp  image 
with  change  in  the  object,  that  for  a  near  object  the  eyeball  elon- 
gated to  bring  the  retina  in  the  most  favorable  position,  and  that 
when  the  object  was  far  off  the  eyeball  shortened  to  bring  the  retina 
up  to  the  point  where  the  sharp  image  was  formed.  Such  a  method 
of  accommodation  for  objects  at  different  distances  would  be 
effective,  as  everyone  knows  who  uses  a  photographic  camera,  but 
as  is  now  known  it  is  not  the  method  used  by  the  eye  of  the  higher 
animals  and  man. 

§  922.    Demonstration  of  Scheiner's  theory  of  accommodation. 

— Scheiner  admitted  that  the  method  of  accommodation  proposed 
by  Kepler  would  be  effective,  but  he  thought  that  the  eyeball 
remained  unchanged  in  shape,  and  the  crystalline  lens  changed  its 
shape,  being  more  convex  for  near  objects  and  less  convex  for 
distant  objects.  He  put  it  thus :  "The  crystalline  lens  of  the  eye 
is  equal  to  many  glass  lenses." 

There  are  needed  for  demonstrating  Schemer's  theory  : 

(1)  A  convex,  trial  lens  of  3  diopters  (fig.  382,  385). 

(2)  A  convex,  trial  lens  of  4  diopters. 

(3)  A  lantern  slide  of  fig.  383. 

Put  the  three  diopter  lens  in  the  lens  holder  and  light  the  arc 
lamp.  'When  the  lens  is  36  to  37  cm.  from  the  lens  a  sharp  image 
will  be  projected  on  the  5  meter  screen.  Now  move  the  object  up 

to  27  or  28  cm.  from  the  lens.     The 

image  will  be  much  blurred.  Remove 
the  3  diopter  lens  and  put  in  its  place 
the  4  diopter  lens.  The  screen  image 
will  be  sharp  again.  This  shows  that 
if  the  crystalline  can  become  more 

and  less  convex,  depending:  upon  the      ^I(i-  385-   TRIAL  LENSES  FOR 

SCHEINER 's  ACCOMMODA- 
position  of  the  object,  the  screen  image  TION  EXPERIMENT. 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        659 

will  be  sharp  without  changing  the  position  of  the  screen.  And 
this  is  now  known  to  be  what  happens  in  the  accommodation  of 
the  eyes  in  the  higher  animals  and  in  man.  Furthermore,  it 
has  been  found  that  for  near  objects  there  must  be  a  muscular 
effort  to  make  the  crystalline  more  convex,  while  if  the  object  is 
distant,  the  eye  forms  a  perfect  image  without  effort. 

REFRACTIVE  EYE  DEFECTS 

§  923.  For  a  person  with  normal  eyes  it  is  almost  impossible 
to  understand  the  difficulties  under  which  one  labors  if  the  eyes  are 
defective.  The  difficulties  become  especially  trying  for  those  who 
must  do  close  work  in  the  trades  or  in  school  work,  and  in  exacting 
professional  work. 

From  the  examination  of  tens  of  thousands  of  school  children 
in  our  own  and  other  lands  it  is  found  that  over  10%  of  them  have 
eye  defects  of  some  kind.  And  in  a  careful  examination  of  5,000 
college  students  45%  to  50%  had  ocular  defects  of  a  kind  that 
made  the  use  of  spectacles  desirable,  and  for  many  of  them  abso- 
lutely necessary. 

It  is  believed  that  if  those  with  normal  sight  had  anything  like  a 
proper  realization  of  the  difficulties  of  those  with  eye  defects  every 
effort  would  be  made  to  give  relief. 

It  is  hoped  that  these  demonstrations,  which  are  so  easily  made 
arid  show  so  strikingly  the  simpler  refractive  eye  defects,  will  be  of 
service  in  helping  to  give  an  understanding  of  the  facts  and  the 
means  for  relief. 

Great  care  has  been  exercised  in  selecting  demonstrations  which 
shall  show  the  common  defects,  and  those  of  moderate  severity, 
not  the  unusually  severe  or  rare.  From  the  personal  experience  of 
the  senior  author,  it  is  known  that  the  appearances  shown  for 
presbyopia  are  not  exaggerated;  and  friends  with  the  other  eye 
defects  have  assured  us  that  the  appearances  given  in  the  demon- 
strations are  not  uncommon. 


§  923a.  For  a  discussion  of  the  eye  defects  in  school  children  see:  Hermann 
Cohn,  Die  Sehleistungen  von  50,000  Breslauer  Schulkindern,  1899;  Dr.  M. 
Dresbach,  Examinations  of  the  Eyes  of  College  Students,  The  Medical  Record, 
Aug.  3,  1912,  also  in  the  Educational  Review,  Dec.,  1913.  In  Dr.  Dresbach's 
papers  are  many  references  to  the  work  of  others. 


66o       DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        [Cn.  XV 


FIG.  386.     TRIAL  LENSES 
FOR  MYOPIA. 


§  924.  Myopia,  or  short  sight. — My- 
opia is  due  to  any  condition  in  the  eye 
by  which  the  image  of  distant  objects  is 
formed  in  front  of  the  retina.  The  retina 
is  too  far  away,  hence  the  retinal  image  is 
blurred.  Persons  with  this  eye  defect 
are  able  to  get  clear  images  only  when 


the  object  is  quite  close  to  the  eyes. 

As  the  effort  of  accommodation  only  aids  in  seeing  near  objects, 
there  is  no  way  by  which  short  sighted  persons  can  see  distant 
objects  clearly  without  the  use  of  a  telescope  or  of  concave  spec- 
tacle«r 

For  demonstrating  myopia  and  its  remedy  are  needed: 

(1)  A  convex,  trial  lens  of  4  diopters  (fig.  386). 

(2)  A  concave,  trial  lens  of  i  diopter. 

(3)  A  lantern  slide  of  fig.  383. 

The  4  diopter  lens  is  to  represent  the  refractive  power  of  the 
myopic  eye.  It  is  placed  in  the  metal  lens  holder,  and  the  object 
"is  brought  up  to  a  point  27  to  28  centimeters  from  it.  Then  the 
image  will  be  sharp  and  clear  on  the  5  meter  screen. 

Move  the  slide  back  until  the  distance  between  it  and  the  lens 
is  36  to  37  centimeters.  The  image  on  the  5  meter  screen  will  be 
much  blurred;  it  is  too  far  off.  One  can  prove  this  by  taking  the 
.white  cardboard  in  the  hands  and  finding  the  position  of  the  sharp 
image  as  in  §  92 1 .  Now  to  get  a  sharp  image  on  the  5  meter  screen 
it  is  necessary  to  reduce  the  curvature  of  the  4  diopter  lens.  Do 
this  by  adding  the  i  diopter  concave  lens.  This  reduces  the  4  to  a 
3  diopter  convex  lens,  and  now  the  image  is  sharp  and  clear  on  the 
,5-  meter  screen. 

.In  the  same  way  the  short  sighted 
person  can  use  concave  spectacles  which 
will  reduce  the  refractive  power  of  the 
cornea  and  the  crystalline  lens,  and  hence 
the  image  will  be  formed  farther  away. 
If  the  right  spectacles  are  used,  the  image  T 

of  distant  objects  will  be  clear  and  sharp  '  FOR  HYPEROPIA. 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        66 1 

on  the  retina.  If  the  person  wishes  to  look  at  near  objects  the  eye 
is  accommodated  to  make  the  crystalline  more  convex  and  the 
diverging  rays  from  near  objects  are  brought  to  a  focus  on  the 
retina  as  with  persons  having  normal  eyes  and  not  using  spectacles. 

§  925.  Hyperopia,  or  long  sight. — This  eye  defect  is  due  to  any 
condition  in  which  distant  objects  have  their  images  formed 
behind  the  retina.  The  retina  is  too  close  to  the  crystalline  lens, 
hence  there  is  a  blurred  image  formed  on  it  even  with  parallel  rays, 
unless  there  is  active  accommodation  and  the  crystalline  lens  is 
made  more  convex.  That  is,  with  hyperopic  eyes,  no  object  can 
be  seen  without  effort . 

For  the  demonstration  of  hyperopia  there  are  required:* 

(1)  A  convex,  trial  lens  of  2  diopters  (fig.  387). 

(2)  A  convex,  trial  lens  of  i  diopter. 

(3)  A  lantern  slide  of  fig.  283. 

The  lens  of  2  diopters  is  to  represent  the  refractive  power  of  the 
hyperopic  eye.  Place  the  lens  in  the  metal  holder,  and  light  the 
arc  lamp.  Move  the  object  near  to  and  distant  from  the  lens,  and 
no  place  within  the  range  will  be  found  where  a  clear  image  will  be 
formed  on  the  5  meter  screen.  The  screen  is  too  near  the  lens  and 
the  sharp  image  is  formed  somewhere  behind  it.  If  the  room  is 
long  enough  the  place  where  the  image  is  sharp  can  be  located. 

Now  move  the  object  until  it  is  36  to  37  centimeters  from  the 
lens  and  add  the  i  diopter  convex  lens.  This  will  add  its  strength 
and  the  refractive  power  will  be  equal  to  3  diopters,  and  now  the 
image  will  be  clear  and  sharp  on  the  5  meter  screen. 

With  the  proper  convex  spectacles,  the  long  sighted  person  can 
see  distant  objects  without  effort,  then  when  he  wishes  to  see  near 
objects  clearly  the  crystalline  is  made  more  convex  as  with  normal 
eyes,  thus  making  the  entire  range  of 
vision  normal. 

§  926.    Presbyopia,  or  old  age  sight.— 

This  comes  gradually  to  every  one  with 
advancing   years,  until  finally,  for  most 

people  after  60  or  65  years  of  age,  the 
FIG.  388.     TRIAL  LENSES 

FOR  PRESBYOPIA.  crystalline  lens  has  lost  its  elasticity  so 


662        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS       [Cn.  XV 

9Q° 

6«\ 
& 


FIG.  389.     THE  RADIAL  LINES  SHOWING  THE  SIZE  OF  THE  IMAGE  WITHOUT 
SPECTACLES,  AND  WITH  CONCAVE  AND  CO-NVEX  SPECTACLES. 

A  shows  the  size  of  the  image  without  spectacles. 

B  shows  the  diminished  size  of  the  image  when  a  spectacle  lens  of  — .5  diopter 
is  put  with  the  5.5  diopter  lens.  (This  reduces  the  5.5  to  a  5  diopter  lens). 

C  shows  the  increased  size  of  the  image  when  a  spectacle  lens  of  +.5  diopter 
is  added  to  the  5.5  diopter  lens.  (This  increases  the  5.5  diopter  lens  to  6 
diopters). 

The  photographs  were  made  by  fixing  the  camera  so  that  the  objective  and 
sensitive  plate  were  at  a  constant  distance,  then  the  image  was  focused  by 
moving  the  object  farther  off  for  B  and  nearer  to  the  lens  for  C,  just  as  in  the 
projection  experiments  (§  922). 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        663 

that  it  cannot  be  made  more  convex  no  matter  how  great  the  effort 
at  accommodation.  If  the  eyes  were  originally  normal,  the  images 
of  distant  objects  are  still  clear,  but  the  diverging  rays  of  near 
objects  can  no  longer  be  brought  to  a  focus  on  the  retina,  without 
artificial  aid. 

For  illustrating  presbyopia  there  are  needed : 

(1)  A  convex  trial  lens  of  3  diopters  (fig.  388). 

(2)  A  convex  trial  lens  of  i  diopter. 

(3)  A  lantern  slide  of  fig.  383. 

Place  the  3  diopter  lens  in  the  metal  holder  and  the  lantern  slide 
in  the  slide-carrier,  then  move  the  slide  up  till  it  is  36  to  37  centi- 
meters from  the  lens.  Light  the  arc  lamp  and  there  will  be  a  sharp 
image  on  the  screen.  This  represents  the  appearance  for  distant 
objects. 

Now  move  the  object  up  to  27  or  28  centimeters  from  the  lens  to 
represent  a  near  object.  The  image  on  the  screen  is  much  blurred, 
something  as  the  ordinary  print  of  a  newspaper  looks  to  an  old 
man  without  spectacles.  Put  the  i  diopter  lens  with  the  3  diopter 
lens  making  a  refracting  medium  equal  to  4  diopters,  and  the  image 
on  the  screen  will  become  clear  and  sharp,  just  as  the  print  of  the 
newspaper  becomes  clear  and  sharp  to  the  old  man  when  he  puts 
on  the  proper  spectacles. 

§  927.  Astigmatism,  Astigmia,  or  unequal  curvature  of  a 
refracting  surface. — This  is  a  common  defect  in  the  eye,  and  is 
found  very  frequently  in  the  cornea.  Roughly  speaking  an 
astigmatic  curve  is  like  the  bowl  of  a  spoon  or  a  hen's  egg,  the 
curve  being  greater  in  one  direction  than  in  the  direction  at  right 
angles. 

The  greater  curvature  will,  of  course,  bring  rays  of  light  to  a 
focus  sooner  than  the  lesser  curvature ;  and  with  such  a  refracting 
surface  not  all  points  in  a  circle  can  be  focused  sharply  in  any  posi- 
tion. Usually  objects  at  right  angles  can  be  sharply  focused  by 
changing  the  position  of  the  objects,  bringing  them  nearer  for  the 
greater  curvature  and  moving  them  farther  away  for  the  lesser 
curvature.  With  a  radial  disc  like  fig.  3 89,  391,  392,  if  the  vertical 
lines  are  sharp  in  one  position,  the  horizontal  lines  will  be  sharp  in 


664        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        [Cn.  XV 

a  different  position  of  the  object.  The  intermediate  lines  cannot 
be  made  perfectly  sharp  in  any  position. 

With  the  eye  when  it  is  accommodated  for  vertical  lines,  the 
horizontal  lines  will  be  blurred,  and  when  the  horizontal  lines  are 
sharp  and  clear  the  vertical  lines  will  be  blurred.  All  the  inter- 
mediate lines  will  be  more  or  less  blurred  all  the  time. 

To  correct  astigmatism  it  is  necessary  to  do  away  with  the 
inequality  of  the  curvature  of  the  refracting  surface.  This  can  be 
done  either  by  increasing  the  lesser  curvature  or  by  reducing  the 
greater  curvature  sufficiently  to  make  the  refracting  surface 
uniform. 

To  demonstrate  astigmatism  there  are  needed: 

(1)  A  convex  lens  of  3  or  of  4  diopters 
(fig.  390). 

(2)  A   convex  cylindrical  lens  of  0.5 
diopter. 

(3)  A  concave  cylindrical  lens  of  0.5 

diopter. 

,  \      A1  1.1       ,.  ^,      !  .  ,  .      FIG.  390.     TRIAL  LENSES 

(4)  A  lantern  slide  of  the  history  of  F£|  ASTIGMATISM. 

astigmatism  (fig.  393). 

(5)  A  lantern  slide  of  the  radial  lines  (fig.  391). 

Put  the  3  or  the  4  diopter  lens  in  the  lens  holder,  and  the  lantern 
slide  of  the  radial  lines  (fig.  391)  in  the  slide-carrier.  Move  the 
slide  until  the  radial  lines  are  as  sharp  as  possible  on  the  screen. 

Put  with  the  projecting  lens  the  0.5  diopter  convex  cylinder  and 
turn  it  so  that  the  axis  of  the  cylinder  is  vertical.  The  horizontal 
lines  will  remain  sharp,  and  the  vertical  lines  will  be  most  blurred. 

The  addition  of  the  0.5  convex  cylinder  produced  anunsymmet- 
rically  curved  refracting  surface.  Along  the  axis  of  the  cylinder 
no  change  is  made  in  the  refractive  power,  hence  light  rays,  from 
points  in  the  horizontal  lines,  which  are  in  planes  parallel  with  the 
axis  of  the  cylinder,  are  brought  to  a  focus  at  the  same  distance  as 
if  the  cylinder  were  absent,  but  rays  in  any  plane  oblique  to  this 
axis  are  affected  by  the  curvature  of  the  cylinder,  and  are 
not  brought  to  the  same  focus,  hence  only  horizontal  lines  appear 
sharp. 


CH.  XV]       DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        665 


180 


FIG.  391-392.     RADIAL  LINES  IN  BLACK  AND  WHITE  FOR  DETERMINING  THE 
PRESENCE  OF  ASTIGMATISM. 

A  lantern  slide  of  the  radial  lines  is  very  desirable  for  the  demonstrations  on 
astigmatism. 

The  black  lines  on  a  white  ground  have  the  advantage  that  the  lantern  slide 
is  less  liable  to  break  than  the  white  lines  on  a  black  ground  (§  852).  It  will 
be  noticed  that,  by  contrast,  the  central  white  circle  seems  lighter  than  the 
white  spaces  between  the  radial  lines.  In  like  manner  the  central  black  circle 
seems  blacker  than  the  black  spaces  between  the  radial  lines.  These  are 
optical  illusions,  for  the  white  is  uniform  and  so  is  the  black. 

Now  place  the  concave  cylinder  in  front  of  the  convex  cylinder 
and  make  their  axes  parallel  (fig.  390).  All  the  lines  will  become 
sharp  again.  This  is  because  the  concave  and  the  convex  cylinders 
with  their  axes  parallel  just  balance  each  other  and  then  act  like  a 
piece  of  plane  glass.  To  compare  the  effect  of  astigmatism  on 
printed  matter  with  its  effect  on  the  radial  lines,  remove  the  cylin- 
ders and  focus  sharply  the  lantern  slide  of  the  history  of  astigma- 
tism, (fig.  393).  Now  add  the  0.5  diopter  convex  cylinder  and 
make  the  axis  vertical.  The  horizontal  lines  in  the  print  will  be 
sharp,  but  the  others  blurred  (fig.  394-395).  Now  rotate  the 
cylinder  until  its  axis  is  horizontal,  and  the  vertical  lines  will  be 
sharp  and  clear  (fig.  396-397).  It  is  to  be  noted  that  with  these 
Gothic  letters,  it  is  easier  to  read  the  words  when  the  vertical  lines 
are  clear,  because  vertical  lines  preponderate. 


666        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        [Cn.  XV 

As  shown  with  the  glass  lenses  it  is  possible  to  do  away  with 
astigmatism  by  rendering  the  inequality  of  curvature  uniform, 
hence  a  person  with  astigmatism  can  be  given  normal  vision  by  the 
use  of  the  proper  spectacles. 

§  928.  Correct  position  of  spectacles.  —  It  is  extremely  import- 
ant that  glasses  to  correct  astigmatism  should  be  correctly  adjusted 
to  the  eyes.  The  necessity  of  this  can  be  strikingly  shown  by 
making  the  axes  of  the  two  cylinders  in  the  last  experiment  some- 
what oblique  .  When  the  axes  are 

oblique  the  confusion  is  greater  ASTIGMATISM 

than  when  the  correcting  lens  is 

removed  entirely.   Not  only  must          Thomas    Young,  1800 
the  correct  spectacle  be  used,  but         Philoso^yCat 
it   must   be    correctly    adjusted.         Pfr3i 

Furthermore  ,  it  should  be  known  'ens. 

that   the   axis    Of    astigmatism    in  P-  57.  astigmatism  of  the  cornea. 

Correction  by  obliquity  of  the  spectacle*. 

the  eye  sometimes    changes    so 


that  the  spectacles  which    gave 

perfect  vision  at  one  time  would  George  B.  Airy,  1825 

not   do     SO    at    a    later    time.       In  Cambridge  Philos.  Trans.  Vol.  II  (1827) 

such  a  case  new  spectacles  with         *T^tt*«ZSZXZ?~ 

axes  arranged  to  meet  the  changed 

FIG.  393.   THE  DISCOVERERS  OF  As- 

conditions  in  the  eyes  are  neces-         TIGMATISM  AND  THE  MEANS  OF 

CORRECTING  IT. 


§  929.  The  two  foci  of  astigmatic  lenses,  and  the  correction  of 
astigmatism  with  cylinders  having  the  same  form  (both  convex 
or  both  concave).  —  Use  the  same  outfit  as  for  §  927. 

Focus  sharply  the  image  of  the  radial  lines.  Add  the  0.5  diopter 
convex  cylinder  and  make  its  axis  vertical.  The  horizontal  lines 
will  be  sharp.  The  vertical  and  intermediate  lines  will  be  blurred. 
Now  move  the  object  up  towards  the  astigmatic  combination  and 
soon  the  vertical  lines  will  become  sharp  and  the  horizontal  and 
intermediate  lines  dim.  In  this  position  the  focus  is  for  the  original 
projection  lens  (3  or  4  diopters)  with  the  added  curvature  (0.5 
diopter)  of  the  cylinder.  That  is  the  greatest  curvature  is  now 
acting  to  focus  the  image  of  the  vertical  lines.  With  the  horizontal 
lines  in  focus,  it  was  the  least  curvature  which  was  acting. 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        667 


=T-       —       -        S 


FIG.  394,  395.     FIGURES  SHOWING  THE  APPEARANCE  OF  THE  RADIAL  LINES 
AND  OF  PRINTED  MATTER  WHEN  AN  ASTIGMATIC  LENS  is  IN  Focus  FOR 
HORIZONTAL  LINES. 


[Ml 


ASTIGMAII15M 

YMIIIUI,   IMOO 


l'lillii«ii|ilil«Ml   li.tM«.n  Ilium  n|  HIM 
U.iy.,1   'nn.li.lv.  IMIII 

!'(.     111    -111    H*ll||IIIMlUinnl    tllH  |.|»«lMlllllM 


\\l   MllU'iialUiH 


M.  Aity,  IIUM» 


IH      Vlll      l|        III     ' 


FIG.  396,  397.     FIGURES  SHOWING  THE  APPEARANCE  OF  THE  RADIAL  LINES 
AND  PRINTED  MATTER  WHEN  THE  ASTIGMATIC  LENS  is  IN  Focus  FOR 

VERTICAL  LINES. 

Figures  394-397  were  made  by  adding  a  +.5  cylindrical  lens  to  a  +5.5 
diopter  photographic  objective,  and  the  cylinder  was  placed  with  its  axis 
vertical  in  fig.  394-395,  and  horizontal  in  fig.  396  and  397;  no  change  was 
made  in  the  focus  of  the  objective.  The  aperture  of  the  objective  was  F/i6 
when  the  photograph  was  made. 

As  shown  in  figures  395-397,  Gothic  print  seen  through  an  astigmatic  lens  is 
clearer  when  the  axis  is  such  that  the  vertical  lines  are  in  focus.  This  is 
because  the  vertical  lines  are  more  numerous  than  the  horizontal  lines. 


668       DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        [Cn.  XV 

With  the  vertical  lines  in  focus,  add  another  convex  cylinder  of 
0.5  diopter  and  arrange  the  axes  of  the  two  cylinders  at  right 
angles  (cross  the  cylinders).  All  the  lines  will  now  be  sharp,  for 
the  added  convex  cylinder  increases  the  curvature  where  it  was 
lacking,  and  thus  gives  the  combination  a  symmetrical  curvature. 
It  is  to  be  noted  that  when  convex  cylinders  are  crossed  in  this  way 
they  add  to  the  original  lens  the  dioptry  of  the  cylinders.  In  this 
case  0.5  diopter,  and  the  image  is  increased  in  size  (fig.  389  C). 

Two  concave  cylinders  can  be  used  in  the  same  way,  but  with 
concave  cylinders  the  entire  system  is  reduced  in  dioptry  the 
amount  of  the  cylinders.  In  this  case  it  would  reduce  the  dioptry 
half  a  diopter  and  hence  the  image  would  be  smaller  (fig. 
389  B). 

§  930.  Correction  of  astigmatism  by  the  obliquity  of  the 
spectacles. — It  was  pointed  out  by  Young  (1800),  that  astigmatism 
might  be  corrected  by  making  the  spectacles  sufficiently  oblique  to 
neutralize  the  defect.  This  can  be  demonstrated  very  strikingly 
as  follows : 

Use  the  same  outfit  as  in  §  927.  Make  the  image  of  the  radial 
lines  sharp  on  the  screen  and  add  the  +0.5  diopter  cylinder  with 
the  axis  vertical  (fig.  390).  Now  put  a  convex  lens  of  i  diopter  in 
front  of  the  cylinder  and  focus  for  the  lines  parallel  to  the  axis  of 
the  cylinder  (vertical  in  this  case).  Tip  the  convex  lens  up  or 
down,  i.  e.,  across  the  axis  of  the  cylinder,  and  when  the  right 
obliquity  is  reached  the  lines  will  all  be  sharp.  This  is  because  the 
tipped  lens  introduces  the  curvature  lacking  in  the  cylinder.  This 
can  be  shown  by  removing  the  cylinder  and  the  horizontal  lines  will 
be  sharp  showing  that  the  vertical  meridian  is  unchanged  but  the 
horizontal  meridian  has  been  increased  in  curvature. 

Use  the  same  cylinder  but  a  concave  lens  of  i  diopter  instead 
of  the  convex  lens ;  focus  the  combination  until  the  horizontal  lines 
are  sharp,  then  rotate  the  concave  lens  sidewise  (i.  e.,  parallel  with 
the  axis  of  the  cylinder) ,  and  when  at  the  right  obliquity  the  radial 
lines  will  all  be  sharp.  This  is  because  the  oblique,  concave  lens 
neutralizes  the  greater  curvature  of  the  +0.5  cylinder.  In  a  word, 
the  oblique  position  of  the  spectacle  makes  it  act  like  a  cylinder  in 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS       669 


addition  to  its  magnifying  or  reducing  power.  Acting  as  a 
cylinder  it  follows  the  law  of  the  cylinder  as  given  in  §  929. 

If  one  keeps  in  mind  the  effect  of  oblique  lenses  it  will  help  to 
appreciate  the  necessity  of  having  the  spectacles  properly  adjusted. 

§  931.    Effect  of  the  aperture  of   the  pupil  in  vision.  —  As  a 

general  statement  it  may  be  said  that  the  larger  the  aperture  of  the 
pupil  the  more  brilliant  will  be  the  image  as  more  light  is  admitted. 
On  the  other  hand  the  larger  the  pupil  the  more  strongly  do  eye 
defects  deteriorate  the  retinal  image. 

When  the  aperture  of  the  pupil  is  small,  only  a  small  part  of  the 
refracting  surface  produces  the  image,  and  consequently  any 
defects  of  curvature  are  minimized  ;  but  the  small  aperture  makes 
the  image  less  brilliant  as  only  a  limited  amount  of  light  goes  to 
form  it,  and  furthermore  it  requires  muscular  effort  to  contract  the 
iris  to  make  the  pupil  small.  With  a  small  pupil,  objects  can  be 
seen  clearly  only  when  they  are  in  a  brilliant  light,  hence  eye 
defects  cannot  be  compensated  for  in  a  dimly  lighted  place  by 
closing  the  pupil. 

For  demonstrating  the  effect  of  the  pupillary  aperture  there  are 
needed  : 


FIG.  398.    TRIAL  LEN-     FIG.  399.     Discs  WITH  PUPILS  OF  LARGE  AND  SMALL 

SES     TO    SHOW     THE  APERTURE;  STENOP/EIC   SLIT.       (%  size) 

EFFECT  OF    THE  A     Black  metal  discs  of  the  size  of  trial  lenses,  one 

with  a  pupillary  aperture  of  2.5  mm.  and  the  other 

B     Stenopaeic  slit. 

(1)  A  3  or  4  diopter  convex  projection  trial  lens  (fig.  398). 

(2)  A  0.5  diopter  concave  or  convex  cylinder. 

(3)  Two  black  discs  the  size  of  trial  lenses  and  with  apertures, 
one  of  2.5  mm.,  and  one  of  7.5  mm. 

(4)  A  lantern  slide  of  the  radial  lines. 

(5)  A  black  disc  with  a  Stenopaeic  slit. 


6 70        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        [Cn.  XV 

Put  the  3  or  4  diopter  lens  in  place  in  the  metal  holder,  and  the 
lantern  slide  of  the  radial  lines  (fig.  391)  in  the  slide-carrier. 
Light  the  lamp  and  focus  the  slide  by  moving  it  toward  or  from 
the  projection  lens.  Now  introduce  the  0.5  cylinder.  Only  the 
horizontal  lines  will  be  sharp  with  the  axis  vertical  or  only  the  verti- 
cal lines  if  the  axis  is  horizontal.  Put  in  front  of  the  projection 
lens  the  black  disc  with  an  aperture  of  7.5  mm.  The  image  will 
be  much  improved.  Remove  this  and  put  in  place  the  disc  with  a 
pupil  of  2.5  mm.  If  now  the  light  is  well  centered  the  entire  circle 
of  the  radial  lines  will  be  fairly  good.  The  image  will  be  rather 
dim,  however. 

Remove  the  small  pupil  and  put  in  place  the  stenopaeic  slit  (fig. 
3996) .  Place  the  slit  parallel  with  the  axis  of  the  cylinder  and  the 
lines  will  all  appear  sharp.  This  is  because  the  slit  allows  the 
light  to  pass  only  along  a  line,  thus  eliminating  most  of  the  disturb- 
ing rays  from  the  unequal  curvature.  People  with  astigmatism 
can  partly  overcome  the  trouble  by  narrowing  the  pupil  and  partly 
closing  the  eye-lids  so  that  objects  are  seen  through  a  slit  something 
as  in  the  experiment  (§  931  a). 

§  932.    Anisometropia  or  unlike  refraction  in  the  two  eyes.— 

This  is  not  a  rare  defect.  One  eye  may  be  normal  and  one  astig- 
matic, one  with  myopia  and  the  other  long  sighted  or  normal,  etc. 
Where  the  two  eyes  are  different,  the  efforts  to  get  a  correct  image 
are  greatly  hampered,  for  an  accommodation  which  would  give  a 
correct  image  in  one  eye  will  make  the  image  of  the  other  eye  more 
confused. 

When  the  differences  in  the  two  eyes  are  considerable,  the  image 
of  one  eye  is  discarded,  or  the  poor  eye  is  turned  aside  (squinted) 
to  get  it  out  of  the  way,  and  one  gets  along  with  monocular 
vision. 

To  make  this  demonstration  in  the  most  perfect  manner  there 
should  be  two  lanterns  side  by  side,  each  projecting  an  image  at  the 

§  93 la.  Preparation  of  the  pupils  and  slit. — These  are  easily  made  by 
cutting  out  pieces  of  thin  tin  or  other  metal  the  size  of  the  trial  lenses  and 
boring  the  holes  and  cutting  the  slit.  Metal  is  recommended  because  the  im- 
age of  the  crater  must  be  focused  on  the  pupil  or  slit,  and  paper  or  wood  would 
be  burned  by  the  absorbed  energy  (§  852.) 


CH.  XV]        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS        671 

same  time.     If,  however,  direct  current  is  available  the  demon- 
stration is  successful  with  one  projector. 
There  are  needed : 

(1)  A  trial  frame  with  half  the  lens  of  4  diopters  and  half  of 
3  diopters  (fig.  400). 

(2)  A  trial  frame  with  half  the  lens  of  i  diopter  and  the  other 
half  of  plane  glass. 

(3)  A  lantern  slide  of  fig.  401. 

For  the  demonstration  the  arc  lamp  and  the  lens  should  be  so 
related  that  the  image  of  the  source  of  light  is  rather  large  as  shown 


Left 

Eye  Eye 

FIG.   401.     LANTERN  SLIDE   FOR  THE 
FIG.  400.     DOUBLE  TRIAL  DEMONSTRATION  OF  UNLIKE  REFRAC- 

LENSES  FOR  UNLIKE  RE-  TI°*  '*  THE  Two  EYES- 

FRACTION  IN  THE  TWO 
EYES. 

by  the  concentric  circles  in  fig.  400.  The  light  must  be  accurately 
centered  also. 

Put  the  lens  in  the  metal  holder  and  the  special  lantern  slide  in 
its  carrier  and  move  the  slide  up  to  a  point  27  to  28  centimeters 
from  the  lens.  The  image  of  the  right  eye  (4  diopter  lens)  will  be 
sharp,  and  that  of  the  left  eye  will  be  blurred.  Now  pull  the  slide 
back  to  a  distance  of  36-37  centimeters  from  the  lens  and  the  left 
eye  image  will  be  sharp  and  the  right  eye  blurred.  This  is  com- 
parable to  a  defect  of  myopia  in  one  eye  and  hyperopia  in  the 
other — one  eye  is  short  sighted  and  one  long  sighted.  Put  the 
slide  back  in  position  for  the  4  diopter  lens  so  that  the  right  eye 
will  be  in  focus.  Now  put  in  front  of  the  lens  the  correcting  lens 
of  i  diopter  for  the  left  half.  This  will  make  both  sides  of  the  lens 
4  diopters  and  both  images  will  be  sharp  as  in  normal  vision  (fig. 
402,  A.B.). 

It  will  be  seen  that  the  blurred,  left-eye  image  (fig.  402  A)  is 
smaller  than  the  sharp  right-eye  image.  This  is  because  the  3 


672        DEMONSTRATING  REFRACTIVE  EYE  DEFECTS       [Cn.  XV 

diopter  lens  gives  a  smaller  image  than  the  4  diopter  lens  (see  also 
fig.  389  B.  C.). 


B 


Lot! 
Er« 

Right 
Eye 

Left 
Eye 

Right 
Eye 

FIG.  402.  APPEARANCE  OF  THE  SCREEN  IMAGE  WITH  UNLIKE  REFRACTION 
IN  THE  Two  EYES  AND  WITH  LIKE  REFRACTION. 

A  Image  of  a  3  diopter  left  eye  and  a  4  diopter  right  eye;  the  4  diopter 
eye  is  in  focus. 

B  Image  when  a  I  diopter  convex  spectacle  is  added  to  the  3  diopter  left 
eye,  making  it  like  the  right  eye. 


§  932a.  In  the  experiment  showing  anisometropia  each  half  of  the  double 
lens  projects  both  images,  but  when  the  light  is  properly  centered  and  in  the 
correct  position  to  give  the  large  illumination  on  the  lens  (fig.  400),  each  half 
lens  projects  a  much  more  brilliant  image  of  its  own  side,  hence  the  fainter 
image  of  the  opposite  side  is  overwhelmed  and  overlaid  so  that  only  one  image 
shows  on  each  side.  If  the  light  is  not  in  a  good  position,  both  images  show 
and  that  spoils  the  effect. 

This  demonstration  with  two  half  lenses  was  fully  successful  only  when  a 
right-angle,  direct  current  arc  lamp  was  used  as  a  source  of  light. 

By  using  a  I  diopter  concave  lens  to  reduce  the  4  diopter  half  lens  to  a  3 
diopter  power,  the  right  eye  image  can  be  made  sharp  when  the  lantern  slide 
is  in  position  to  make  the  left  eye  image  sharp,  and  the  right  eye  image  blurred. 
It  is  a  little  more  satisfactory  to  work  with  the  4  diopter  lens,  however,  and  to 
add  the  i  diopter  convex  lens  to  the  3  diopter  left  lens. 


BRIEF  HISTORICAL  SUMMARY 

In  dealing  with  the  historical  development  of  projection  three  forms  of 
apparatus  must  be  considered: 

I.     NATURAL   CAMERA   OBSCURA 

The  formation  of  images  in  a  dark  place,  the  light  from  the  brilliantly  illum- 
inated objects  or  scenes  being  admitted  through  a  small  opening,  is  a  perfectly 
natural  phenomenon  and  entirely  independent  of  man's  invention  or  control. 
This  is  represented  by  images  of  the  sky  with  its  clouds  and  the  brilliant  scenes 
of  nature  pictured  on  the  walls  of  caves  facing  the  scenes,  and  the  images  of 
the  sun  admitted  through  chinks  between  the  leaves,  etc. 

In  rooms  of  man's  construction  such  images  are  often  seen  if  light  enters 
through  a  chance  hole  in  the  right  position.  General  Waterhouse,  from  his 
own  observation,  says  it  is  a  common  occurrence  in  the  bungalows  of  India, 
and  the  writers  have  often  seen  the  same  in  America. 


*It  was  our  intention  when  this  work  was  undertaken  to  include  a  somewhat 
extended  account  of  the  discoveries  and  inventions  relating  to  vision,  including 
spectacles,  general  optics,  and  optical  instruments,  especially  the  telescope,  the 
microscope  and  projection  apparatus  of  all  kinds.  As  the  book  has  already 
exceeded  its  limit  in  size,  this  extended  account  must  wait  for  a  special  work. 
We  have  thought  it  best,  however,  to  add  a  brief  summary  of  the  more  per- 
tinent points,  and  a  historical  bibliography  which  will  put  those  interested  on 
track  of  the  special  and  early  sources  of  information. 

Our  appreciation  is  great  for  the  aid  we  have  received  from  many  sources. 
First  of  all  to  the  Library  of  Cornell  University  for  its  magnificent  collection 
of  works  bearing  on  the  history  of  science,  for  the  purchase  of  rare  and  costly 
works,  and  for  the  trouble  taken  to  borrow  from  other  libraries,  rare  works  for 
our  use.  Among  the  other  libraries  drawn  upon  we  mention  in  the  first  place 
that  of  the  Surgeon  General's  Office  in  Washington,  D.  C.  Those  of  Columbia, 
Chicago,  Harvard  and  the  University  of  Pennsylvania  also  loaned  us  many 
works. 

Among  the  individuals  who  gave  us  special  aid  are: 

Professor  George  L.  Burr,  for  securing  the  portrait  of  Scheiner,  (fig.  407). 

Professor  E.  Lavasseur  of  the  College  of  France  who  supplied  the  photograph 
for  the  portrait  of  Marey  (fig.  412). 

Mr.  Augustus  J.  Loos  of  Philadelphia  for  securing  information  concerning 
the  Langenheim  brothers  who  were  the  first  to  make  photographic  lantern 
slides  by  the  albumen  process  (1850). 

Mr.  Edward  Pennock  of  Philadelphia  for  putting  us  in  communication  with 
Mr.  C.  W.  Briggs  of  that  city.  Mr.  Briggs  gave  us  much  valuable  information 
concerning  his  father,  Dr.  Daniel  H.  Briggs,  who  made  the  first  photographic 
lantern  slides  by  the  collodion  process  (1851-1852). 

Effie  Alberta  Read,  Ph.D.,  M.D.,  for  looking  up  references  and  verifying 
quotations  in  the  libraries  of  Washington,  D.  C. 

Theodore  Stanton  for  aid  in  securing  the  photograph  of  Marey,  (fig.  412). 

And  finally  to  Dr.  A.  C.  White  of  the  Cornell  University  Library  for  transla- 
tions from  the  Greek  and  Latin  works  of  the  old  writers,  in  which  some  of  the 
earliest  information  on  our  subject  is  to  be  found. 

673 


674  OPTIC  PROJECTION 

II.     ARTIFICIAL   CAMERA   OBSCURA 

No  one  knows  who  first  designedly  arranged  a  darkened  room  with  a  white 
wall  or  screen  oh  one  side,  and  on  the  other  a  small  opening  facing  some  object 
or  scene  that  could  be  brightly  illuminated.  All  we  know  is  that  the  earliest 
accounts  of  the  pictures  in  a  dark  place  are  in  connection  with  the  explanation 
of  some  other  phenomenon,  and  not  to  show  that  such  pictures  were  possible. 
It  was  also  recognized  in  the  first  statements,  as  in  the  works  of  Aristotle  and 
of  Euclid,  that  as  light  rays  extend  in  straight  lines,  that  those  from  an  object 
must  cross  in  passing  through  a  small  hole,  and  hence  the  images  beyond  the 
hole  in  the  dark  place  must  be  inverted,  the  top  being  below  and  the  right  being 
left. 

According  to  Wiedemann  and  Werner,  the  Arabians,  Iban  Al  Haitem  (1039 
A.D.),  and  Levi  Ben  Gersen  (1321-1344),  gave  descriptions  which  clearly 
belong  to  the  camera  obscura.  However,  that  may  be,  we  have  the  illustrated 
manuscripts  of  Leonardo  da  Vinci,  which  not  only  describe  the  phenomena 
of  the  camera  obscura,  but  give  pictures  which  are  unmistakable.  The  pic- 
tures and  descriptions  are  in  connection  with  his  explanation  of  vision.  As 
Leonardo  died  in  1519,  these  manuscripts  arc  of  an  earlier  date,  probably  before 
1500  A.D.  (See  especially  folio  8  of  Ms.  D.) 

Also  in  the  accounts  of  eclipses,  etc.,  of  the  astronomers  Reinhold,  Frisius 
and  Moestlin,  they  very  clearly  describe  and  give  figures  of  the  arrangement  of 
the  dark  room  pictures  (1540-1545) ;  and  in  the  quaint  old  volume  of  Cardanus 
(De  Subtilitate,  1550),  there  is  a  very  graphic  description  of  the  means  of 
getting  dark  room  pictures  and  of  their  appearance.  Baptista  Porta,  in  1558, 
in  his  Natural  Magic,  also  gives  a  good  description.  Porta  is  credited  in  the 
popular  mind  with  the  invention  of  the  camera  obscura,  but  as  seen  from  the 
above,  it  is  a  natural  thing,  and  man  had  got  camera  pictures  by  design  before 
Porta  was  born.  The  Natural  Magic  of  Porta  was  very  popular  in  its  day, 
and  was  translated  from  the  Latin  into  most  modern  languages,  hence  it  is 
intelligible  that  people  thought  him  the  inventor,  as  he  gave  credit  to  no  one, 
and  gave  out  that  many  of  the  things  had  never  been  known  before.  To  credit 
him  with  the  discovery  of  the  marvelous  things  he  describes  would  be  like 
making  the  modern  magazine  writer  the  inventor  or  discoverer  of  the  wonder- 
ful things  he  describes.  In  justice  to  Porta,  it  must  be  said  that  he  states  in 
the  preface  to  his  book  that  he  has  consulted  all  libraries,  and  has  visited  many 
skillful  artisans  to  find  out  all  the  secrets. 

It  may  be  stated  in  passing,  that  the  name  "Camera  Obscura"  was  not  used 
by  Porta,  nor  the  others  mentioned  above.  They  used  expressions  like  these: 
cubiculum  obscurum,  cubiculum  tenebricosum,  conclave  obscurum,  locus 
obscurus,  etc.  The  first  occurrence  of  the  name  "Camera  Obscura"  found  by 
us  is  in  the  Paralipomena  of  Kepler,  (1604),  p.  209  of  the  original,  p.  261  in 
the  Opera  Omnia,  vol.  ii.  Kepler  also  uses  the  expression,  "camera  clausa," 
vol.  ii,  p.  1 60. 


BRIEF  HISTORICAL  SUMMARY 


675 


While  mirrors  had  been  used  in  the  camera  obscura  for  changing  the  position 
or  causing  the  images  to  appear  erect,  so  far  as  known  at  present,  no  one  used  a 
projection  lens  in  the  aperture  of  the  dark  room  until  1568.  In  that  year  was 
published  the  work  on  perspective  by  Daniel  Barbara,  and  on  p.  192,  Ch.  V,  he 
directs  that  to  make  the  image  more  brilliant,  a  convex  spectacle  glass  be  put 
in  the  aperture,  and  that  a  white  paper  screen  be  moved  back  and  forth  until 
the  picture  shows  most  clearly,  then  it  can  be  traced.  From  this  time  onward 
a  projection  objective  has  always  been  used,  except  for  experiments,  such  as 
with  pin-hole  photographic  cameras,  etc. 

In  the  camera  obscura  considered  above,  the  observers  were  in  the  room 
where  the  picture  was  formed.  For  a  small,  movable  camera,  something  like 
the  photographic  cameras  of  the  present,  where  the  observer  is  outside  the 
camera  box,  the  first  description  found  by  us  is  the  one  of  Robert  Boyle,  and 
dates  from  1669.  He  called  it  a  "A  Portable  Darkened  Room,"  and  says  that 
it  had  already  been  exhibited  to  many  friends  several  years  before  the  paper 
was  written. 


—  ' 

A 
B 

C 

10 

JO 

B 

rs 

FIG.  403.     WALGENSTEN'S  MAGIC  LANTERN  (1665). 
(From  Milliet  de  Chales,  Mundus  s.  Cursus  Mathematicus,  1674,  vol.  ii,  p.  666) 

Here  is  a  naked  light  with  a  reflector  behind  it.  There  is  no  condenser. 
The  object  is  put  in  the  proper  inverted  position  before  the  objective,  and  the 
image  appears  erect  and  enlarged  on  the  screen. 


m.     PROJECTION   INSTRUMENTS 

The  third  form  of  projection  apparatus  consists  of  a  relatively  small  instru- 
ment in  which  a  small  object  is  brilliantly  illuminated,  and  the  light  from  it 
extends  out  through  a  projection  lens  or  objective  and  forms  a  relatively  large 
image  on  a  white  wall  or  screen  in  a  dark  place. 

The  third  form  is  the  converse  or  conjugate  so  to  speak  of  the  camera  obscura 
where  the  object  is  large  and  the  image  small. 


676 


OPTIC  PROJECTION 


Projection  instruments  of  the  third  class  can  be  properly  divided  into  three 
groups:  i,  the  Magic  Lantern;  2,  the  Projection  Microscope,  and  3,  the 
Moving  Picture  Machine. 

1.     The  Magic  Lantern 

It  is  not  certainly  known  who  first  produced  a  workable  magic  lantern. 
The  first  figure  and  description  we  have  found  is  the  one  of  a  Danish  mathe- 
matician (Walgensten).  The  figure  and  description  occur  in  the  mathematical 
treatise  of  Milliet  de  Chales  (1674),  where  it  states  that  "in  the  year  1665  there 
came  to  Lyons  a  learned  Dane  well  versed  in  dioptrics.  Among  other  things 
he  exhibited  a  magic  lantern.  .  .  In  the  first  place  the  greater  the  distance 


dldlt8 


FIG.  404.     THE  MAGIC  LANTERN  OF  KIRCHER. 

(From  the  Ars  Lucis  et  Umbrae,  1671,  p.  768} 

The  lamp  is  a  naked  flame  with  a  concave  reflector  behind  it.  The  lantern 
slide  is  a  long  strip  with  many  pictures  which  can  be  shown  one  after  the  other. 
The  lantern  slide  appears  at  the  wrong  end  of  the  projection  objective, 
making  it  difficult  to  see  how  any  image  could  be  projected.  At  the  bottom 
of  the  picture  is  a  part  of  the  text  in  which  the  better  form  of  Walgensten  '& 
lantern  is  conceded. 


BRIEF  HISTORICAL  SUMMARY  677 

of  the  wall  upon  which  the  image  was  exhibited  the  larger  was  the  image.  .  . 
In  the  third  place,  the  little  image  in  the  lantern  was  inverted  in  order  to 
exhibit  its  figure  erect  upon  the  opposite  wall.  If  the  object  was  removed  there 
appeared  only  a  circle  of  light"  (vol.  ii,  p.  655;  2d  ed.,  vol.  iii,  p.  680); 

Figure  403  is  a  facsimile  of  the  lantern  of  Walgensten  which  he  exhibited  at 
Lyons  in  1665.  A  glance  at  it  will  show  any  one  that  it  is  in  all  essential  par- 
ticulars like  the  modern  magic  lantern.  Indeed  such  lanterns  are  much  in 
vogue  for  Christmas  presents  at  the  present  time,  differing  only  in  having  a 
kerosene  lamp  with  a  chimne}'  instead  of  the  naked  flame  as  shown  in  the 
original. 

Kircher  himself  in  the  second  edition  of  his  work,  (Ars  Magna  Lucis  et 
Umbrae,  1671,  p.  768-769),  claims  that  the  lantern  of  the  Dane  is  merely  a 


.  2  82. 


FIG.  405.     MOLYNEUX'S  MAGIC  LANTERN  WITH  A  CONDENSING  LENS  BEFORE 

THE  OBJECT. 

(From  Molyneux's  Dioptrica  Nova,  i6g2] 

This  is  the  first  picture  of  a  magic  lantern  with  a  condensingllens  that  we 
have  found. 


slight  modification  of  the  one  described  by  him,  but  he  admits  that  Walgen- 
sten's  instrument  is  in  better  form  and  has  many  pictures  on  a  single  slide 
painted  in  transparent  colors  that  can  be  shown  one  after  the  other. 

Kircher  figures  his  magic  lantern,  which  is  here  reproduced  in  facsimile  (fig. 
404).  As  pointed  out  by  Neuhauss,  it  is  difficult  to  see  how  a  picture  could  be 
projected  by  the  arrangement  shown  by  Kircher.  The  text  describes  the  lan- 
tern as  here  shown,  so  both  text  and  figure  agree.  In  Kircher's  lantern  as 
figured  and  described  by  himself,  the  object  is  put  at  the  wrong  end  of  the 
projection  objective;  or  if  the  tube  and  glass  shown  represent  a  condenser, 
which  he  does  not  claim,  then  in  that  case  there  is  no  projection  objective.  In 
either  case  no  image  could  be  projected. 


678 


OPTIC  PROJECTION 


So  far  as  the  evidence  goes  then,  it  was  not  Kircher,  but  Walgensten  who 
exhibited  the  first  workable  magic  lantern,  and  the  date  was  1665. 


FIG.  406.     JOHANNES  KEPLER,  1571-1630. 

(From  the  Library  of  Original  Sources   Vol.   V) 

Astronomer.  Father  of  Modern  Dioptrics,  Keplerian  Telescope  and  Micro- 
scope. The  Amplifier  and  the  Telo- Photo  Combination.  Inverted  Retinal 
Image. 


\ 


BRIEF  HISTORICAL  SUMMARY 
2.    Projection  Microscope 


679 


As  pointed  out  in  the  text  (p.  221),  the  projection  microscope  is  only  a  magic 
lantern  with  a  relatively  short  focus  projection  objective.  The  screen  image  is 
therefore  correspondingly  larger  than  with  the  magic  lantern. 

The  first  magic  lantern  described  (1665)  was  recognized  as  a  kind  of  micro- 
scope by  Milliet  de  Chales,  for  he  says  in  the  description:  "rricroscopium 


FIG.  407.     CHRISTOPHORO  SCHEINER,  1573-1650. 

Astronomer  and  Inventor 
(From  the  biography  by  Anton  von  Braunmuehl,  i8pi) 

Projection  Apparatus  for  Drawing  Sun  Spots.     Demonstrated  the  Retinal 
Image.     Said  the  Crystalline  Lens  of  the  Eye  is  Equal  to  Many  Glass  Lenses. 
Invented  the  Pantograph. 


habes  in  hujusmodi  machina,"  vol.  ii,  p.  667.  A  few  years  later  (1685),  Zahn 
in  his  work  on  all  kinds  of  optical  instruments  (Oculus  Artificialis) ,  says  on  p. 
255,  "Lucerna  magica  est  species  microscopii."  Both  also  point  out  that  this 
kind  of  a  microscope  is  preferable  to  the  ordinary  one  as  many  can  see  at  the 
same  time. 


68o  OPTIC  PROJECTION 

If  any  individual  should  be  mentioned  in  connection  with  the  projection 
microscope,  it  is  Kepler,  for  in  his  Dioptrics,  1611,  he  showed  the  advantage  of 
adding  an  amplifier  in  projection,  and  also  a  second  convex  lens  (ocular),  to 
magnify  the  real  image  of  the  objective,  and  also  at  the  same  time  to  render  it 
erect.  See  Opera  Omnia,  vol.  ii,  pp.  549-550,  555. 

3.    Moving  Pictures 

Moving  picture  projection  is  like  micro-projection  when  no  ocular  is  used. 
The  screen  distance  is  usually  rather  great  and  the  many  slightly  differing 
pictures  are  changed  so  rapidly  that  the  successive  screen  images  seem  to  fuse 
together  and  thus  give  the  appearance  of  motion. 

The  first  step  in  getting  moving  pictures  was  an  investigation  of  persistence 
of  vision  by  momentary  glimpses  of  similar  moving  objects.  The  men  inves- 
tigating the  matter  were  all  physicists,  and  the  results  of  their  observations 
were  given  in  scientific  papers.  See  in  the  bibliography  papers  by  Fara- 
day, Plateau,  Homer  and  Stampfer.  The  paper  on  the  magic  disc  by  Plateau 
was  dated  Jan.,  1833,  and  the  paper  of  Homer  on  the  daedaleum  (zoetrope) 
was  dated  1834,  as  was  also  the  paper  of  Stampfer  on  the  magic  disc.  Both 
the  magic  disc  (fig.  408)  and  the  zoetrope  (fig.  409)  give  the  appearance  of 
movement  with  great  satisfaction. 

As  the  instruments  were  for  one  or  at  most  for  very  few  observers,  the  magic 
lantern  was  called  in  to  give  screen  images  so  that  many  could  see  at  the  same 
time.  The  magic  lantern  was  used  successfully  by  Uchatius  in  1853.  He 
used  several  (as  many  as  12)  slightly  differing  transparencies,  each  transparency 
having  its  own  projection  objective.  The  objectives  were  all  directed  toward 
the  same  point  on  the  screen,  hence  the  images  all  appeared  in  the  same  place. 
A  lime  light  and  condenser  were  attached  to  a  crank,  and  moved  from  picture 
to  picture  in  rapid  succession,  and  the  projected  images  gave  the  appearance 
of  movement  as  perfectly  as  did  the  magic  disc. 

It  was  also  natural  that  the  new  art  of  photography  should  be  called  upon 
to  depict  the  various  phases  of  a  moving  body  for  use  in  place  of  the  drawings 
which  had  been  previously  used;  this  was  suggested  by  Plateau  about  1848. 
In  1870  Heyl  realized  this  possibility  by  arranging  a  series  of  photographic 
transparencies  of  posed  motion,  and  projecting  them  on  the  screen.  The 
transparencies  were  arranged  on  the  edge  of  a  large  disc,  and  by  the  step  by 
step  movement  of  the  disc  the  successive  transparencies  were  brought  in  the 
axis  of  the  magic  lantern.  To  prevent  the  blur  while  the  pictures  were  changed, 
a  two  wing  shutter  was  used  to  cut  off  the  view.  This  method  of  projecting 
was  very  successful  and  required  only  one  projection  objective,  consequently 
the  number  of  pictures  was  limited  only  by  the  practicable  size  of  the  rotating 
disc. 

Up  to  1872  the  pictures  used  were  either  drawings  or  photographic  trans- 
parencies of  posed  movements,  not  photographs  of  movement  in  continuous 
change  as  at  present. 


BRIEF  HISTORICAL  SUMMARY  68 1 

From  1872  onward  there  have  been  three  epoch  making  periods  in  reaching 
approximate  perfection  in  moving  pictures. 

The  first  period  is  represented  by  the  work  of  Eadweard  Muybridge,  who 
first  made  successful  analyses  of  rapid  movement  in  1872-1881.  In  1879  he 
arranged  the  successive  stages  of  a  movement  on  a  glass  disc  and  projected  the 


FIG.  408.     PLATEAU'S  MAGIC  Disc  (PHENAKISTOSCOPE). 
(From  the  Correspondance  Mathematique  et  Physique,   Tome  VII,  1832) 
Notches  were  cut  around  the  edge  as  indicated  by  the  dark  terminations  of 
the  radii.     A  pin  is  put  in  the  center,  the  figures  turned  toward  a  well  lighted 
mirror,  and  the  disc  rotated.     By  the  momentary  glimpses  through  the  radial 
slits  the  figure  seems  to  go  through  the  movements  of  the  dance.     The  back 
of  the  disc  should  be  black,  and  the  figures  show  better  if  the  outlines  are  made 
heavier  than  in  the  picture. 


682 


OPTIC  PROJECTION 


same  by  means  of  a  magic  lantern,  and  synthesized  or  recombined  the  move- 
ment on  the  screen  as  he  had  previously  done  in  the  zoetrope.  From  1883- 
1885,  under  the  auspices  of  the  University  of  Pennsylvania,  over  one  hundred 
thousand  (100,000)  pictures  of  movements  of  men  and  all  kinds  of  animals  were 
made.  These  were  published  in  several  folio  volumes  in  1887. 


FIG.  409.     THE  D^D ALBUM  OF  HORNER  (ZOETROPE). 

(From  Marey,  Movement,  1895} 

In  this  instrument  figures  or  photographs  can  be  arranged  in  a  band  around 
the  inside  of  the  cylinder,  or,  as  in  this,  case  models  of  a  moving  animal  can  be 
arranged  in  order.  When  the  instrument  is  revolved  the  images  or  models 
seem  to  perform  their  natural  movements  of  walking,  flying,  etc. 


The  second  period  is  represented  by  the  making  of  the  gelatino-bromide 
process  of  photography  practical  by  Maddox  in  1871,  and  by  making  this 
process  exceedingly  rapid  by  heating  or  boiling  the  emulsion  (Bennett  and 
others  in  1878  and  later). 

The  third  epoch  making  period  was  inaugurated  by  the  Rev.  Hannibal 
Goodwin  when  he  worked  out  a  practical  method  of  making  a  solution  and  then 
a  film  of  transparent,  tough,  flexible  cellulose  which  was  unaffected  by  the 
chemicals  and  liquids  used  in  photography. 

His  application  for  a  patent  was  filed  in  1887,  and  the  patent  granted  in 
1898,  and  the  validity  of  the  patent  finally  confirmed  by  the  United  States 


BRIEF  HISTORICAL  SUMMARY 


683 


District  Court  of  New  York  in  1913,  and  this  decision  confirmed  by  the  United 
States  Circuit  Court  of  Appeals  of  New  York  in  1914.  (See  in  the  Bibliog- 
raphy). 

Muybridge's  first  pictures  were  made  by  the  wet  collodion  process,  but  his 
Philadelphia  work  was  done  with  the  new,  rapid  gelatino-bromide  plates.  He 
used  many  cameras,  sometimes  24  in  a  row  to  get  different  phases  of  a  motion, 
and  sometimes  the  cameras  were  arranged  in  groups  to  get  the  movement 
simultaneously  from  different  points  of  view. 

In  1 88 1  he  gave  demonstrations  of  his  pictures  in  Europe,  and  projected 
the  synthesis  on  the  screen  with  the  lantern,  the  first  demonstrations  being  in 
the  physiological  lecture  room  of  Marey,  the  French  master  of  investigating 


FIG.  410.     THE  MOVING  PICTURE  PROJECTOR  OF  UCHATIUS. 
(From  the  Sitz.  Berichte  d.  k.  Akad.  Wiss.,  z.  Wien.  Math.  Natur.  Cl.,  Vol.  X, 

1853} 

This  shows  some  of  the  pictures  with  the  individual  objectives  directed  to 
the  same  point.  The  lime  light  and  condenser  and  the  crank  for  moving  them 
from  picture  to  picture  are  also  shown. 


animal  movement  by  the  graphic  method.  From  that  time  on  Marey  took 
hold  of  the  photographic  method  for  the  analysis  and  synthesis  of  animal 
motion  with  the  greatest  enthusiasm.  Instead  of  the  battery  of  cameras  used 
by  Muybridge,  he  adopted  the  system  of  the  French  astronomer,  Janssen, 
using  a  single  camera  and  objective,  but  taking  many  pictures  on  a  single  plate. 
In  1887,  he  used  the  roller  films  on  paper,  and  immediately  that  they  were 
available,  the  celluloid  films  devised  by  Goodwin.  In  this  way  pictures  could 
be  made  in  a  long  series.  Not  only  did  Marey  use  the  ribbon  films  but  he 
devised  a  special  camera  for  doing  so,  and  a  projector  for  showing  the  ribbon 
pictures  on  the  screen. 


684 


OPTIC  PROJECTION 


FIG.  411.     EADWEARD  MUYBRIDGE,   1830-1904. 

(From  Animals  in   Motion,   1890) 

Photographic  Analysis  and  Synthesis  of  Animal  Motion,  Commencing  in 
1872. 


BRIEF  HISTORICAL  SUMMARY 


685 


FIG.  412.     JULES  ETIENNE  MAREY,   1830-1910. 

College  de  France 

(From  a  photograph  furnished  by  Professor  E.  Lavasseur,   College  de  France) 
Graphic  Method  in  Physiology;    Photographic  Analysis  and  Synthesis  of 
Animal  Motion,  1881-1910. 


686  OPTIC  PROJECTION 

In  perfecting  cameras  to  make  ribbon  pictures,  and  projectors  for  exhibiting 
ribbon  transparencies  of  these  pictures  on  the  screen,  many  inventors  have 
taken  part.  Among  these  should  be  mentioned  Marey  and  his  assistant, 
Demney,  and  the  Lumieres  in  France;  Green  and  Evans,  Donisthrope  and 
Crofts  in  England;  Jenkins  and  Edison  in  America.  These  were  among  the 
first  to  work  out  practical  apparatus  that  made  moving  pictures  possible  and 
practical.  For  the  present  perfection  of  cameras,  films,  and  projectors,  and 
the  general  methods  employed,  the  number  of  manufacturers  and  inventors  is 
legion. 

The  first  light  used  was  sunlight,  and  that  remains  the  most  brilliant  of  all. 
Animal  and  vegetable  oils  were  burned  in  lamps  without  a  chimney  (fig.  403- 
405),  and  very  recently  mineral  oil  (kerosene)  has  £>een  used  in  lamps  with  a 
chimney  (fig.  65-67). 


FIG.  413.     DAVY'S  CARDON  ARC. 
(From  Davy's  Collected  Works,  vol.  iv,  pi.  in,  fig.  17} 

See  p.  no  of  vol.  iv  for  a  discussion  of  the  carbon  arc.     The  carbons  are 
horizontal,  and  the  arc  arches  upward  hence  the  name  arc. 


The  lime  light,  the  most  brilliant  after  sunlight  and  the  arc  light,  came  in 
with  the  discovery  by  Hare  in  1802  that  the  oxyhydrogen  flame  when  blown 
against  lime,  etc.,  gave  a  dazzling  light.  This  was  applied  to  projection  by 
Birkbeck  in  1 824  for  the  magic  lantern ;  and  in  the  same  year  by  Woodward  for 
the  phantasmagoria.  (Goring  and  Pritchard's  Micrographia,  pp.  170-171; 
also  the  Microscopical  Journal  and  Structural  Record,  Vol.  I,  1841).  This 
light  is  still  much  used  for  all  forms  of  projection.  For  the  oxygen  ether  lime 
light,  see  Ives,  in  the  Bibliography. 

The  electric  light.  This  most  satisfactory  and  powerful  artificial  light  yet 
devised,  was  first  shown  by  Humphrey  Davy  in  Sept.,  1800,  and  recorded  in 
Nicholson's  Journal  of  October  in  that  year  (See  Cantor  Lectures  of  Silvanus 
P.  Thompson  on  the  arc  light,  Journal  of  the  Royal  Society  of  Arts,  Oct.  25, 
1895,  and  fig.  413  for  Davy's  carbon  arc).  According  to  the  same  lecturer, 
W.  E.  Straite  devised  the  first  automatic  electric  lamp  in  1846. 

The  first  arc  lamps  were  for  direct  current.  As  it  was  not  desirable  to  have 
the  carbons  burn  off  unequally  with  the  Jablochoff  lamp  where  the  carbons 
were  parallel  and  close  together,  alternating  currents  were  used  (1877).  (S.  P. 


BRIEF  HISTORICAL  SUMMARY  687 

Thompson,  p.  953-954) .  While  this  works  well  for  general  lighting,  it  is  shown 
in  the  preceding  pages  (553-566)  that  alternating  current  is  far  inferior  to 
direct  current  for  projection  purposes. 

At  first  the  carbons  were  both  horizontal  (fig.  413),  then  they  were  made 
vertical,  and  later  at  various  angles  of  inclination.  In  order  to  keep  the  crater 
of  the  positive  carbon  constantly  in  the  optic  axis,  Mr.  Albert  T.  Thompson  of 
Boston  manufactured  and  used,  especially  for  projection  purposes,  an  arc  lamp 
in  which  the  carbons  are  at  right  angles,  the  positive  carbon  being  horizontal 
and  hence  constantly  in  line  with  the  axis  of  the  projection  instrument.  This 
was  in  1894."  From  that  time  onward  the  advantage  of  this  position  has 
become  more  and  more  appreciated,  and  the  superiority  for  projection  purposes 
is  shown  graphically  in  the  curve  given  in  Chapter  XIII  (fig.  302). 


The  following  is  the  statement  of  Mr.  Albert  T.  Thompson  concerning  the 
90°  arrangement  of  the  carbons  in  an  arc  lamp  for  the  magic  lantern: 

BOSTON,  Dec.  6,  1907. 

"Replying  to  your  valued  communication  of  the  2d,  I  will  state  that  I  first 
manufactured  the  90°  arc  lamps  in  1894  and  a  careful  search  of  all  arc  lamp 
and  stereopticon  catalogs  published  about  that  period,  fails  to  show  arc  lamps 
of  the  90°  construction.  . 

"I  did  not  patent  the  lamp,  for  at  that  time  there  was  no  demand  for  them, 
and  of  course  it  was  difficult  to  look  into  the  future  and  realize  that  in  a  few 
years  thousands  and  thousands  would  be  sold. 

"The  facts  to  the  best  of  my  knowledge  and  belief  were  never  published  in 
any  scientific  journal. 

Yours  very  truly, 

A.  T.  THOMPSON." 


SOME  MANUFACTURERS  AND  DEALERS   IN   OPTICAL 
AND  PROJECTION  APPARATUS  AND  SUPPLIES 

Within  recent  years  there  has  been  great  improvement  in  projection  appara- 
tus and  all  the  necessary  accessories,  and  many  optical  manufacturing  houses 
have  taken  hold  of  the  work  in  earnest,  so  that  now  one  can  find  in  the  open 
market  practically  everything  required  at  reasonable  prices.  Furthermore,  if 
special  apparatus  or  combinations  are  desired,  or  if  a  person  has  notions  of  his 
own,  it  is  not  difficult  to  obtain  the  optical  and  electrical  apparatus  needed  of 
the  manufacturers,  and  only  a  small  amount  of  special  construction  will  be 
needed  to  adapt  the  apparatus  to  the  special  individual  or  the  special  purpose. 

It  is  hoped  that  the  special  apparatus  described  in  this  volume,  for  example, 
the  projection  microscope  and  the  projection  outfit  for  showing  normal  and 
defective  vision,  and  for  some  special  demonstrations  in  physics,  will  give  sug- 
gestions which  will  open  the  way  for  those  who  do  not  find  the  apparatus  in 
the  open  market  quite  suitable  to  their  needs. 

As  models  of  instruments  are  constantly  changing  and  new  forms  are  being 
produced,  the  authors  advise  that  any  one  desirous  of  installing  projection 
apparatus  of  any  kind  should  get  the  catalogues  of  several  manufacturers  and 
select  that  which  best  suits  his  needs  and  means.  It  may  be  stated  in  passing, 
that  the  most  expensive  apparatus  is  not  necessarily  the  best  adapted  for  a 
given  case.  Often  apparatus  of  moderate  price  is  easier  to  manage  and  more 
effective.  Naturally  the  manufacturers  prefer  to  install  an  expensive  outfit, 
but  if  the  needs  are  clearly  stated,  and  the  sum  available,  the  manufacturer 
will  give  most  excellent  advice  as  to  the  outfit  required. 

The  dealers  in  lantern  slides  have  a  system  of  rental  by  which  one  can  get  for 
a  moderate  fee  a  set  of  slides  to  illustrate  some  special  or  general  subject.  Of 
course,  slides  of  any  number  or  grouping  can  also  be  purchased,  but  often  a 
special  lecture  on  a  country  or  a  period  can  be  greatly  helped  by  a  good  selec- 
tion of  lantern  slides,  the  use  of  which  will  cost  but  a  small  sum. 

For  those  interested  in  moving  picture  cameras,  the  development  of  exposed 
films,  etc.,  the  advertising  pages  of  the  Moving  Picture  World  will  give  the 
names  of  the  firms  who  can  give  the  information  or  the  help  needed. 

The  following  list  of  manufacturers  and  dealers  is  arranged  alphabetically, 
and  from  our  experience  with  them  we  know  that  they  try  to  be  of  real  service 
to  their  customers.  Of  course  there  are  many  others  who  are  equally  reliable ; 
and  new  manufacturers  and  dealers  are  constantly  coming  into  the  field.  One 
can  get  on  track  of  them  by  consulting  the  advertising  pages  of  standard 
periodicals  as:  Science,  the  Scientific  American,  the  Moving  Picture  World, 
Journals  in  Electrical  and  Illuminating  Engineering. 

688 


MANUFACTURERS  AND  DEALERS          689 

After  each  name  in  this  list  are  given  the  text  figure  or  figures  taken  from 
the  publications  of  the  given  manufacturer,  or  the  section  (§)  in  which  the 
apparatus  or  material  is  considered: 

American  Theater  ^Curtain  and  Supply  Company,  105  North  Main  Street, 
St.  Louis,  Mo.  'Radium,  gold  fiber  screens,  §  629. 

The  Bausch  &  Lomb  Optical  Company,  Rochester,  New  York.  Photographic 
objectives,  microscopes,  projection  apparatus,  spectacles  and  all  laboratory 
supplies,  §916,  fig.  17,  33-34,  7°,  100-101,  104-107,  123,  131,  136,  169-173, 

200-2OI,   223-224. 

R.  &  J.  Beck,  Limited,  68,  Cornhill,  London,  England.  Microscopes,  photo- 
graphic objectives,  projection  apparatus. 

John  A.  Brashear  Company,  Limited,  Pittsburg,  N.  S.,  Pa.  Optical,  physical, 
astrophysical  and  astronomical  instruments,  including  heliostats.  Ch.  VI. 

Chas.  Beseler  Company,  no  East  23d  St.,  New  York.  Projection  apparatus 
and  lantern  slides,  fig.  55,  §  598.  Small  automatic  arc  lamps. 

C.  W.  Briggs,  628  Callowhill  Street,  Philadelphia,  Pa.  Magic  lantern  slide 
manufacturer.  Mr.  C.  W.  Briggs  is  the  son  of  Dr.  Daniel  H.  Briggs  who 
made  the  first  photographic  lantern  slides  by  the  collodion  process.  The 
beautiful  lantern  slides  made  by  the  son  are  made  by  the  same  collodion 
process  used  by  the  father  before  1855.  Ch.  VIII. 

Brown  &  Sharpe  Manufacturing  Co.,  Providence,  Rhode  Island.  Manu- 
facturers of  fine  tools.  -See  their  wire  gauge,  p.  502. 

Century  Manufacturing  Company,  272  West  Genesee  St.,  Buffalo,  N.  Y. 
Manufacturers  of  "Sanitary  paint"  including  "Artists'  Scenic  White"  for 
image  screens,  §  6250. 

Conrad  Lantern  Slide  and  Projection  Company,  4028  Jackson  Boulevard, 
Chicago,  111.  Lantern  slides  for  science  teachers  and  lecturers.  Ch.  VIII. 

Detroit  Engine  Works,  Detroit,  Michigan.  Direct  current  electric  lighting 
outfits  for  projection  and  moving  pictures.  Kerosene  engines  for  power 
§682. 

Detroit  Motor  Car  Supply  Company,  Detroit,  Michigan.  Sandow  moving 
picture  electric  light  plant,  using  a  Sandow  kerosene  stationary  engine,  §682. 

Dolby  &  Company,  3613  Woodland  Ave.,  Philadelphia,  Pa.  Importer  and 
dealer  in  microscopes  and  optical  apparatus,  lantern  slides,  laboratory 
supplies. 

Eastman  Kodak  Co.,  Rochester,  New  York.  Photographic  outfits  and  sup- 
plies, including  moving  picture  film,  fig.  119,  §  3332,  451. 

Edison  Manufacturing  Company,  Orange,  N.  J.  Moving  picture  machines 
and  films,  the  home  kinetoscope,  etc.,  etc.,  fig.  63,  221,  224,  233-236. 

Education  Department,  Division  of  Visual  Instruction,  State  of  New  York, 
Albany,  N.  Y.  Many  series  of  lantern  slides  for  use  throughout  the  state, 
(Ch.  VIII). 

Enterprise  Optical  Manufacturing  Co.,  564-572  West  Randolph  St.,  Chicago, 
111.  Moving  picture  machines,  Calcium  gas  outfit  etc.,  fig.  56. 

The  Ernon  Camera  Shop,  18  West  27th  St.,  New  York.  Moving  picture 
camera. 

Folmer  &  Schwing  Manufacturing  Co.,  Manufacturers  of  enlarging,  reducing 
and  tilting  cameras.  With  Eastman  Kodak  Co.,  Rochester,  N.  Y.,  fig.  119. 


690  OPTIC  PROJECTION 

Foos  Gas  Engine  Co.,  Springfield,  Ohio.     Oil,  gas  and  gasoline  engines  for 
supplying  the  power  for  a  private  electric  lighting  plant  and  for  projection. 
§683. 

Fort  Wayne  Electric  Works  of  the  General  Electric  Co.  Compensarc  instead 
of  a  rheostat,  §  736. 

R.  Fuess,  Steglitz  bei  Berlin,  Germany.  Optical  instruments,  projection 
apparatus,  heliostats,  etc.,  fig.  79,  84. 

General  Electric  Company,  Schenectady,  New  York.  Electric  apparatus  of 
all  kinds,  generator  sets,  mercury  arc  rectifiers,  mazda  concentrated  filament 
lamps,  and  nitrogen  lamps  for  the  magic  lantern,  etc.,  fig.  258-264,  §  754. 

General  Film  Company,  200  Fifth  Ave.,  New  York.  Educational  films  for 
the  moving  picture  machine,  Ch.  XI. 

General  Specialty  Company,  St.  Louis,  Mo.  Indirect  and  semi-indirect 
lighting  fixtures,  §  606". 

J.  H.  Gentner  Co.,  Newburgh,  N.  Y.     Mirroroide  screens  §  629. 

Gregory  Electric  Company,  16  &  Lincoln  Sts.,  Chicago,  111.  Electric  supplies, 
generators,  motors,  etc.,  Ch.  XIII. 

Gundlach-Manhattan  Optical  Co.,  Rochester,  New  York.  Photographic 
objectives  and  cameras,  microscopes,  projection  objectives  for  moving  pic- 
tures, etc.,  fig.  229. 

J.  H.  Halberg,  36  East  23d  St.,  New  York.  Moving  picture  machines  and 
supplies  of  all  kinds. 

Hartford  Machine  Screw  Company,  Hartford,  Conn.,  fig.  161. 

Harvey  Hubbell,  Inc.,  Bridgeport,  Conn.  Manufacturers  of  Machinery,  tools, 
Electrical  specialties,  fig.  48-50,  268-269. 

P.  Keller  &  Co.,  Successors  to  J.  B.  Colt  Co.,  465  Greenwich  St.,  New  York. 
Projection  apparatus  and  accessories,  fig.  36. 

Kleine  Optical  Company,  166  North  State  Street,  Chicago,  111.  Motion  pic- 
ture apparatus  and  supplies;  theater  supplies. 

Max  Kohl,  Chemnitz,  Germany.  Projection  apparatus  and  accessories, 
chemical  and  physical  apparatus,  fig.  68,  81. 

F.  Koristka,  Milano,  2  Via  G.  Revere.  Italy.  Microscopes  and  projection 
apparatus,  fig.  181. 

Ward  Leonard  Electric  Company,  Bronxville,  N.  Y.  Rheostats,  circuit 
breakers,  theater  dimmers,  etc.,  fig.  147,  183,  186-187,  §  723. 

List  of  Electrical  Fittings.  Published  by  the  National  Board  of  Fire  Under- 
writers, 135  William  St.,  N.  Y.,  §  691.  Manufacturers  of  standard  fittings 
and  supplies. 

Ernst  Leitz,  Wetzlar,  Germany,  30  East  i8th  St.,  New  York.     Microscopes, 

photographic  objectives,  projection  apparatus,  fig.  41,  96,  123,  163,  202-205. 
T.  H.  McAllister  Company,  49  Nassau  St.,  New  York.     Projection  apparatus 

and  lantern  slides,  fig.  89. 
Mclntosh  Stereopticon  Company,  35  and  37  Randolph  Sts.,  Chicago,  111. 

Projection  apparatus  and  lantern  slides,  fig.  66,  166. 
Motion  Picture  Camera  Company,  5  West  i4th  St.,  New  York.     Cameras 

and  projectors  for  moving  pictures. 

Motion  Picture  Screen  Company,  Shelbyville,  Indiana.  Mirror  screens,  §  629, 
6297,  630. 


MANUFACTURERS  AND  DEALERS          691 

National  Electric  Supply  Company,  Chicago,  111.     Rheostats,  etc.,  fig.  138, 

193,  196. 
National  X-Ray  Reflector  Company,  236  Jackson  Boulevard,   Chicago,   111. 

Eye-Comfort  Illumination  from  concealed  sources,  fig.  237,  §  606. 
New  York  State  Education   Department,   Division  of  Visual  Instruction, 

Albany,  N.  Y.     Lantern  slides  for  use  throughout  the  state,  Ch.  VIII. 
Newton  &  Co.,  3  Fleet  St.,  London,  England.     Projection  apparatus  and 

lantern  slides,  fig.  67. 
Edward  Pennock,  3609  Woodland  Ave.,  Philadelphia,  Pa.     Microscopes  and 

supplies,  photographic  objectives  and  cameras.      Lantern  slides,  etc. 
Pennsylvania  Flexible  Metallic  Tubing  Company,  Broad  &  Race  Sts.,  Phila- 
delphia, Pa.     See  fig.  60. 
Phantoscope  Manufacturing  Company,  Washington,  "D.  C.     Motion  picture 

cameras  and  motion  picture  projectors  of  C.  F.  Jenkins  for  the  house  lighting 

system,  §  598. 

Picture  Theater  Equipment  Company,  21  East  I4th  St.,  New  York. 
Nicholas  Power  Company,  90  Gold  Street,   New  York.     Manufacturer  of 

Powers  Cameragraph,  electrical  appliances  for  motion  picture  machines. 

New   dissolving    stereopticon,    "bill-splitter"   current    ballast,    §   736,   fig. 

222-223,  227,  232. 
Prest-O-Lite  Company,   Indianapolis,   Indiana.     Compressed  acetylene,  see 

fig.  71. 

C.  Reichert,  Optische  Werke,  Vienna,  Austria.     Projection  apparatus,  micro- 
scopes, etc.,  fig.  43-44.  54- 

Ross,  Limited,  3  North  Side,  Clapham  Common,  London,  England.     Projec- 
tion apparatus,  microscopes,  etc. 
Franz  Schmidt  &  Haensch,  Berlin,  Germany.     Projection  apparatus,  etc.,  etc., 

fig.  57,  69. 
Alfred  L.  Simpson,  131  West  132  St.,  New  York.     Simpson's  solar  screen  for 

receiving  the  projected  image  of  the  magic  lantern,  moving  picture  machine, 

etc.,   §  629. 
Slingerland  Lantern  Slides.    Lantern  slides,  plain  and  colored  of  insects,  birds, 

trees,  fruits  and  other  nature-study  subjects.     Manufactured  by  Mrs.  Mark 

V.  Slingerland,  Ithaca,  N.  Y. 
Spencer   Lens   Company,   Buffalo,   New  York.     Microscopes,   photographic 

objectives,    projection   apparatus   and   accessories.     Laboratory   supplies, 

fig.  38,  108-111,  130,  149,  174,  198-199. 
L.  S.  Starrett  Co.,  Athol,  Mass.     Starrett  Tools,  fig.  160. 
C.  H.  Stcelting  Co.,  121  North  Green  St.,  Chicago,  111.     Projection  apparatus, 

laboratory  apparatus  and  supplies  of  all  kinds,  fig.  16,  75,  102-103,  l67- 
The  Chas.  A.  Strelinger  Co.,  Detroit,  Michigan.     The  Brush  electric  lighting 

set.     This  consists  of  an  engine  for  gas,  gasoline  or  kerosene  and  a  proper 

dynamo  for  direct  current,  §  683. 
Arthur  H.  Thomas  Company,  1200  Walnut  St.,  Philadelphia,  Pa.     Dealer  and 

importer  in  microscopes  and  other  optical  apparatus  and  all  laboratory  sup- 
plies. 
A.  T.  Thompson  &  Company,  15  Tremont  Place,  Boston,  Mass.     Projection 

apparatus  of  all  kinds.      Inventor  of  the  right-angle  arc  lamp,  fig.  97,  168, 

1 86. 


692  OPTIC  PROJECTION 

Underwood  &  Underwood,  3  and  5  West  igth  Street,  New  York.  Magic  lan- 
terns, lantern  slides  showing  tours  of  the  world. 

Valentine  &  Company,  456  Fourth  Ave.,  New  York.  Valspar  varnish  for 
making  glass  boxes,  etc.,  §  3940. 

Voigtlander  &  Sohn,  A.  G.,  Optical  Works,  Braunschweig,  Germany.  Micro- 
scopes, photographic  objectives  and  cameras,  projection  apparatus,  fig.  124, 
142. 

W.  Watson  &  Sons,  313  High  Holborn,  London,  England.  Microscopes, 
Projection  apparatus  and  accessories. 

Westinghouse  Electric  Manufacturing  Company,  Pittsburg,  Pa.  Rectifiers, 
transformers,  balance  coils,  motion  picture,  motor-generator  set,  etc.,  etc., 
§  681,  723,  736,  739. 

Weston  Electric  Instalment  Company,  Newark,  N.  J.  Voltmeters  and 
ammeters,  etc.,  fig.  133,  145,  272-273,  §  662,  664,  666,  700,  7020. 

Whyte  Whitman  Company,  36  East  23d  St.,  New  York.  Moving  picture 
cameras. 

Williams,  Brown  &  Earl,  918  Chestnut  Street,  Philadelphia,  Pa.  Microscopes 
and  accessories,  Laboratory  supplies,  Projection  apparatus  and  moving 
picture  machines  and  lantern  slides,  fig.  32,  52,  59,  72-73,  98-99,  164-165, 
§  598. 

Carl  Zeiss  Optische  Werkstaette,  Jena,  Germany.  All  kinds  of  optical  appara- 
tus; Microscopes  and  projection  apparatus,  fig.  95,  123,  128-129,  156,  217- 
219. 

Joseph  Zentmayer,  manufacturing  optician,  microscopes,  spectacles  and 
lenses  of  all  descriptions  etc.,  226-228  South  I5th  St.,  Philadelphia,  Pa. 

See  also  the  list  of  spectacle  manufacturers,  p.  651. 


I.  BIBLIOGRAPHY 

Arrhenius,  Svante  August.     Lehrbuch  der  kosmischen  Physik.     1026  p.     304 

fig.     3  Plates.     Leipzig,  1903.     Price  40  marks. 
Ayrton,  Mrs.  Hertha.     The  electric  arc.     479  p.     146  fig.     "The  Electrician." 

Printing  and  Pub.  Co.,  Ltd.,  Salisbury  Court,  Fleet  St.,  London,  E.  C., 

1902.     Price  I2s,  6d. 
Ball,  Sir  Robert  Stawell.     Elements  of  Astronomy,  New  ed.     459  p.     136  fig. 

Longmans,  Green  &  Co.,  London,  New  York,   15  East  i6th  St.,   1886. 

Price  $2.00. 
Barnard,    J.    Edwin.     Practical    Photo-micrography.     322    p.     79    fig.     10 

Plates.     E.  Arnold,  London,  1911.     Price,  155. 
Barrows,  William  Edward.     Electrical  illuminating  engineering.     212  p.     135 

fig.     McGraw  Pub.  Co.,  New  York,  1908.     Price  $2.00. 
Bayley,  R.  Child.     The  Complete  Photographer.     410  p.     65  Plates.     32  fig. 

McClure,  Phillips  &•  Co.,  New  York,  1907.     Price  $3.00. 
Bayley,  R.  Child.     Modern  Magic  Lanterns,  a  Guide  to  the  Management  of 

the  Optical  Lantern  for  the  use  of  entertainers,  lecturers,  photographers, 

teachers  and  others,     no   +    15  p.     73  fig.     L.   Upcott  Gill,   London; 

Charles  Scribner's  Sons,  153-157,  Fifth  Ave.,  New  York.     Price  $.50. 
Beck,  Conrad,  and  Andrews,  Herbert.     Photographic  Lenses.     7th  edition 

completely  revised.     287  p.     163  fig.     44  plates.     R.  &  J.  Beck,  Limited. 

68  Cornhill,  London,  England.     Price  i  shilling.     Full  discussion  of  modern 

objectives  for  photography  and  for  projection. 
Cohn,  Hermann  (Ludwig).     Die  Sehleistungen  von  50,000  Breslauer  Schulkin- 

dern,    nebst   Anleitung   zu   ahnlichen    Untersuchungen   fur   Aerzte   und 

Lehrer.     Schlesische   Buchdruckerei,    Kunst-und   Verlags   Anstalt   v.    S. 

Schottlaender.     Breslau,    1899.     J48   P-     26   Plates.     5   fig.     Price   3M, 

Geb.  4  M. 
Cole,  Aaron  Hodgman.     Manual  of  Biological  Projection  and  anesthesia  of 

Animals.     200  p.    28  fig.    Neeves  Stationery  Co.,  543  W.  63d  St.,  Chicago 

111.,  1907.     Price  $1.50. 
Cyclopedia  of  Motion  Picture  Work.     2  vols.     Vol.  I.  Stereopticon,  motion 

head  projecting  machine,  talking  pictures.     206  p.     119  fig.,  many  plates. 

List  of  moving  picture  makers.     Vol.  II.  Photography,  motography,  photo- 
plays, motion  picture  theater.     311  p.     no  fig.,  many  plates.     Published 

by  the  American  School  of  Correspondence.     Chicago,  111.,  1911. 
Dolbear,  A.  E.     The  Art  of  Projecting;    a  manual  of  experimentation  in 

Chysics,  chemistry,  and  natural  history  with  the  port-lumiere  and  magic 
intern.     158  p.     112  fig.     Lee  &  Shepard,  Boston  and  New  York,  1877. 

Price  $2.00. 

Donaldson,  Leonard.     The  cinematograph  and  natural  science,  the  achieve- 
ments and  possibilities  of  cinematography  as  an  aid  to  scientific  research. 

120  p.     23  fig.     Ganes  Ltd.,  85  Shaftesbury  Ave.,  London,  1912.     Price 

2s,   6d. 
Donders,   Franciscus   Cornelis.     On  the  anomalies  of  accommodation  and 

refraction  of  the  eye,  with  a  preliminary  essay  on  physiological  dioptrics. 

Translated  by  W.  D.  Moore,     xviii  +  i  +  635  p.     175  fig.     New  Syden- 

ham  Society,  (Publications  22).     London,  1864. 

693 


694  OPTIC  PROJECTION 

Fourtier,  H.  La  pratique  des  projections.  2  v.  in  i.  i,  146  p.,  66  fig.  ii,  142 
p.,  67  fig.  Gauthier-Villars  et  Fils,  55  Quai  des  Grandes-Augustus,  Paris, 
1892-93.  Price,  4  fr. 

Fourtier,  H.,  et  Molteni,  A.  Les  projections  scientifiques;  etude  des 
appareils,  accessoires  et  manipulations  diverses  pour  1'enseignement 
scientifique  par  les  projections.  292  p.,  113  fig.  A.  Molteni,  44  Rue  du 
Chateau-d'  Eau,  Gauthier-Villars  &  Fils,  55  Quai  des  Grandes  Augustus. 
Paris,  1894.  Price  3  fr.,  SQC. 

Gould,  George  M.  The  New  Ophthalmology  and  its  relation  to  General 
Medicine,  Biology  and  Sociology,  Congress  of  Arts  and  Science,  Universal 
Exposition,  St.  Louis,  1904,  Vol.  VI,  pp.  422-445.  Discusses  among  other 
things  the  relation  of  general  health  and  truancy,  etc.,  of  children  to  defec- 
tive eyes. 

Halberg,  J.  H.  Motion  picture  electricity.  299  p.,  125  fig.  Published  by 
the  Moving  Picture  World,  17  Madison  Ave.,  N.  Y.  Price,  $2.50. 

Harrison,  Newton.  Electric- Wiring,  Diagrams  and  Switchboards.  272  p. 
105  fig.  The  Norman  W.  Henley  Pub.  Co.  New  York,  1909.  Price  $1.50 

Hassack,  Karl,  and  Karl  Rosenberg.  Die  Projektionsapparate,  Laternbilder 
und  Projektionsversuche  in  ihren  Verwendungen  in  Unterrichte  von  K. 
Hassack  und  K.  Rosenberg.  336  p.  308  fig.  A  Pichlers  Witwe  &  Sohn, 
Wien  und  Leipzig,  1907.  Price  6  M. 

Helmholtz,  Hermann  (Ludwig  Ferdinand),  von.  Handbuch  der  physiologischen 
Optik.  3d  ed.  564  p.  81  text  fig.,  6  plates,  1910.  Leopold  Voss,  Ham- 
burg, 1909-11.  3v.  I  Die  Dioptrik  des  Auges,  xvi  -f-  376  p.  146  fig. 
Price  14  M,  geb.  16  M.  II  Die  Lehre  von  den  Gesichtsempfindungen. 
viii  -f  391  +  17  p.  80  fig.  3  Plates.  Price  16  M,  geb.  18  M.  Ill  Die 
Lehre  von  den  Gesichtswahrnehmungen  564  p.  81  text  fig.,  6  plates, 
1910.  Price,  26.50  marks. 

Hep  worth,  Cecil  M.  Animated  Photography,  the  A  B  C  of  the  Cinemato- 
graph. Hazell,  Watson  &  Viney,  Ltd.  i ,  Creed  Lane,  Ludgate  Hill,  London 
E.  C.  1900.  128  p.  31  fig.  Price  $.50.  Amateur  Photographer's 
Library,  No.  14. 

Hep  worth,  Thomas  Cradock.  The  Book  of  the  lantern;  a  guide  to  the  work- 
ing of  the  optical  or  magic  lantern,  with  directions  for  making  and  colouring 
lantern  pictures.  278  p.  75  fig.  Wyman  &  Sons,  Great  Queen  St., 
Lincoln's-Inn  Fields,  London,  W.  C.  1888.  Price  35,  6d. 

Hopkins,  Albert  A.  Magic,  stage  illusions  and  scientific  diversions  including 
trick  photography.  556  p.,  400  fig.  Munn  &  Co.,  Scientific  American 
Office,  New  York.  1898.  Price  $2.50.  12  pages  of  references  to  books  on 
magic. 

Jenkins,  C.  F.,  and  O.  B.  Depue.  Handbook  for  motion  pictures  and  stereopti- 
con  operators.  132  p.  20  fig.  The  Kenega  Co.,  Inc.  Washington, 
D.  C.,  1908.  Price  $2.50. 

Index  Catalogue  of  the  Library  of  the  Surgeon  General's  Office  of  the  United 
States  Army.     Government  Printing  Office,   Washington,   D.   C.     First 
series,  vol.  i-xvi,  1880-1895.     Second  series,  vol.  i-xviii  +  1896-1913  +. 
Book  and  periodical  literature;    subjects  and  authors  in  one  continuous 
alphabetical  list.     Full  lists  of  current  literature. 

Journal  of  the  Royal  Microscopical  Society.  1878  +.  Published  by  the 
Society  at  20  Hanover  Square,  London,  W.  England.  6  numbers  per 
year;  subscription,  price  37  shillings  6  d. 

Lambert,  Rev.  F.  C.  Lantern  Slide  Making.  140  p.  27  fig.  Hazell, 
Watson  &  Viney,  Ltd.,  52  Long  Acre.  London,  W.  C.  1907.  Price,  $.50. 
The  Amateur  Photographer's  Library,  No.  22. 


BIBLIOGRAPHY  695 

Leiss,  C.  Die  Optischen  Instrumente  der  Firma  R.  Fuess,  deren  Beschreiburig, 
Justierung  und  Anwendung.  Wilhelm  Engelmann,  Leipzig,  1899.  xiv  + 
387  +  i  p.  3  Plates.  233  fig.  Price  n  M.,  geb.  12  M. 

Lummer,  O.,  and  Silvanus,  Thompson.     Contributions  to  photographic  optics. 

135  P-»  55  %•     MacMillan  &  Co.,  New  York,  1900.     Price  $1.75.     Dis- 
cusses the  different  forms  of  photographic  objectives. 
"National  Electrical  Code."     Rules  and  requirements  of  the  National  Board 

of  Fire  Underwriters  for  Electric  Wiring  and  Apparatus  as  recommended 

by  the  National  Fire  Protection  Association,  Ed.  of  1913  or  later.     Fur- 
nished by  the  National  Board  of  Fire  Underwriters,  135  William  St.,  N.  Y. 

See  §  691. 
Nichols,  E.  L.     The  Outlines  of  Physics.     Pp.  452;    414  figures.     The  Mac- 

millan  Co.     New  York,  1897.     Price  $1.40. 
Nicholas  Power  Company.     Hints  to  [Moving  Picture]  Operators.     17  figures, 

96  pages.     Published  by  the  Nicholas  Power  Co.,  88-90  Gold  St.,  N.  Y. 

1914. 
Norris,  Henry  H.     An  introduction  to  the  study  of  Electrical  Engineering. 

Ithaca,  N.  Y.,  1912.     224  pages,  26  plates,  167  figures.     Price  $1.50. 
Norton,  C.  Goodwin.     The  Lantern  and  How  to  Use  It.      Hazell,  Watson  & 

Viney,  Ltd.  i,  Creed  Lane,  Ludgate  Hill,  London,  E.  C.,  1901.     152  p. 

74  fig.     Price  $.50.     The  Amateur  Photographer's  Library,  No.  10. 
Nutting,  Perley  Gilman.     Outlines  of  applied  optics.     P.  Blakiston's  Son  & 

Co.,  1012  Walnut  St.,  Philadelphia,  1912.    ix  +  234  p.    73  fig.    Price  $2.00. 
The  Moving  Picture  World.     The  Chalmers  Publishing  Co.,  17  Madison  Ave. 

New  York  City.     Published  weekly.     Subscription  price  $3.00. 
Pringle,  Andrew.     The  Optical  Lantern  for  Instruction  and  Amusement. 

149  p.     72  fig.     Hampton  &  Co.,  13  Cursitor  St.     London,  E.  C.,   1899. 

Price  45,  6d. 
Richardson,  F.  H.     Motion  Picture  Handbook,  a  Guide  for  Managers  and 

Operators  of  Motion  Picture  Theaters.     432  p.     176  fig.     Moving  Picture. 

World,  Pullman  Building,  17  Madison  Ave.,  New  York  City.     Price  $2.50. 
Scientific  American,  1845  +  and  Scientific  American  Supplement,    1876    -f 

Published  by  Munn  &  Co.,  361  Broadway,  New  York.     Weekly.     Sub- 
scription, $3.00,  and  for  the  supplement,  $5.00. 
Talbot,  Frederick  A.     Moving  pictures,  how  they  are  made  and  worked. 

340  p.,  132  fig.     J.  B.  Lippincott  Co.,  Philadelphia,  1912.     Price  $1.50. 
Tennant,  John  A.,  Ed.     Lantern  Slides.     51  p.,  8  fig.     Tennant  &  Ward,  122 

East  25th  St.,  New  York,  The  Photo-Miniature,  Vol.   i,  No.  9,   1899. 

Price  $.25. 
Tennant,  John  A.     Coloring  Lantern  Slides.     48  p.,   n  fig.     Tennant  and 

Ward,  122  East  25th  St.,  New  York,  The  Photo-Miniature,  Vol.  7,  No.  83, 

1907.     Price,  $.25. 
Trutat,  Eugene.     Traite  general  des  projections.     391  -f-  276  p.     185  -+-  137 

fig.     Charles  Mendel,  118  et  118  Bis,  Rue  d'Assas,  Paris,  1897.     2  v.  in  i. 

Price  7  fr.,  5oc. 
Tyndall,  John.     Six  lectures  on  Light  delivered  in  America  in  1872-1873. 

275  P-     59  %•     Longmans  Green  &  Co.,  New  York,  1873. 
Wimmer,    Franz    Paul.     Praxis   der   Makro-und   Mikro-Projektion   fur   die 

Lehrzwecke  in  Schule  und  Haus,  sowie  fur  Lichtbildvortrage,  etc.     360  p. 

112  fig.     26  Plates.     Otto  Nemnich,  Leipzig,  1911.     Price  5  M. 


696  OPTIC  PROJECTION 

Wistar  Institute's  Style  Brief.  A  style  brief,  giving  the  typographic  arrange- 
ment and  methods  to  be  followed  in  the  preparation  of  manuscripts  and 
drawings  for  publication  in  the  Journals  published  by  the  Wistar  Institute. 
Sent  free  to  authors  by  the  Wistar  Institute,  36th  and  Woodland  Ave. 
Philadelphia,  Pa. 

Wright,  Lewis.  Light;  a  course  of  experimental  optics,  chiefly  with  the  lan- 
tern. 367  p.,  190  fig.  4  plates.  The  Macmillan  Co.,  New  York,  1882. 
Price,  $2.00. 

Wright,  Lewis.  Optical  projection;  a  treatise  on  the  use  of  the  lantern  in 
exhibition  and  scientific  demonstration.  4th  edition,  450  p.,  247  fig. 
Longmans,  Green  &  Co.,  New  York,  1906.  Price  $2.25. 

Zeitschrift  fur  wissenschaftliche  Mikroskopie  und  fur  mikroskopische  Technik. 
1884  +.  Verlag  von  S.  Hirzel,  Leipzig,  Germany.  Published  quarterly. 
Subscription  to  foreign  countries,  21.60  marks. 

Zeitschrift  fur  Instrumentenkunde,  herausgegeben  unter  mitwirkung  der 
physikalisch-technischen  Reichsanstalt.  Verlag  von  Julias  Springer, 
Berlin,  1881+.  12  numbers  per  year;  subscription,  24"  marks. 


II.    HISTORICAL  BIBLIOGRAPHY 

Adams,  George,  1720-1773.  Micrographia  illustrata;  or,  the  microscope 
explained,  in  several  new  inventions;  likewise  a  natural  history  of  aerial, 
terrestrial  and  aquatic  animals,  etc.,  considered  as  microscopic  objects, 
lix  +  325  p.  72  plates.  Pub.  for  the  author,  London,  1771. 

Adams,  George,  1750-1795.  Essays  on  the  microscope,  containing  a  descrip- 
tion of  the  most  improved  microscopes,  a  history  of  insects,  their  transfor- 
mations, peculiar  habits,  and  economy  with  a  catalogue  of  interesting 
objects.  724  p.  31  plates.  Pub.  for  the  author,  London,  1787. 

Airy,  George  Bid  dell.  On  a  peculiar  defect  in  the  eye  and  a  mode  of  correcting 
it.  Cambridge  Philosophical  Transactions,  Vol.  II,  (1827),  pp.  267-271. 
This  is  a  discussion  of  astigmatism  and  its  correction  by  means  of  cylindrical 
spectacles.  Paper  read  Feb.  5,  1825.  See  Thomas  Young. 

Alhazen.  Opticae  thesaurus  Alhazeni  Arabis,  libri  septem,  nunc  primum  editi, 
ejusdem  liber  de  crepusculis  et  nubium  ascensionibus,  item  Vitellionis 
Thuringopoloni,  libri  X.  omnes  instaurati,  figuris  illustrati  et  aucti,  adjectis 
etiam  in  Alhazenum  commentariis.  A  Frederico  Risnero.  Folio,  many 
figures.  Basileae,  per  Episcopios.  1572. 

Bacon,  Roger.  Opus  Majus,  edited  with  introduction  and  analytical  table  by 
John  Henry  Bridges.  2  volumes  and  supplementary  vol.  Vol.  I,  clxxxvii 
+  440  p.,  23  fig.  Vol.  II,  568  p.,  187  fig.  Supplement,  xv  +  187  p. 
The  Clarendon  Press,  Oxford  England,  1897-1900.  For  modern  optics 
the  part  designated  De  Scientia  Perspectiva  is  most  important.  For  use 
of  convex  lenses  to  aid  the  sight  of  old  men,  see  vol.  ii,p.  157,  and  for  burn- 
ing flasks,  p.  471. 

Bacon,  Roger.  Essays  contributed  by  various  writers  on  the  occasion  of  the 
commemoration  of  the  seventh  centenary  of  his  birth.  Collected  and 
edited  by  A.  G.  Little.  426  p.  Clarendon  Press,  Oxford,  England,  1914. 
Price,  $5.25.  Biography  of  Bacon  and  essays  upon  his  work  in  various 
fields.  List  of  Bacon's  writings. 

Baker,  Henry,  F.R.S.  Of  Microscopes  and  observations  made  thereby. 
2  vol.  New  edition.  442  p.,  17  pi.  Vol.  I  The  microscope  made  easy. 
Projection  microscope.  Vol.  II,  Employment  for  the  microscope.  Lon- 
don, 1785. 

Barbaro,  Daniel.  La  pratica  della  perspettiva  di  Monsignor  Daniel  Barbaro, 
eletto  patriarca  d'  Aquileia.  Opera  molto  utile  a  pittori  a  scultori  &  ad 
architetti.  Con  privilegio.  208  p.  Many  figures.  In  Venetia,  appresso 
Camillo,  &  Rutilio  Borgominieri  fratelli,  al  segno  di  S.  Giorgio.  M.D. 
LXVIII  (1568).  First  known  user  of  a  lens  in  the  camera.  Cap.  V,  p.  192. 

Borellus,  Petrus.  De  vero  Telescopii  inventore,  cum  brevi  omnium  Con- 
spiciliorum  historia.  Ubi  de  eorum  confectione,  ac  usu,  seu  de  effectibus 
agitur,  novaque  quaedam  circa  ca  proponuntur.  Accessit  etiam  centuria 
observationum  microcospicarum.  Authore  Petro  Borello,  regis  christia- 
nissimi  consiliario,  et  medico  ordinario.  Hagae-Comitum,  ex  typographia 
Adriani  Vlaco,  M.D.  CLV.  (1655).  Important  for  the  history  of  optic 
instruments. 

Brewster,  Sir  David.  The  Edinburgh  encyclopaedia.  Optics,  Vol.  14,  pp. 
589-798,  Plates  428-442.  Joseph  and  Edward  Parker,  Philadelphia, 
1832. 

697 


698  OPTIC  PROJECTION 

Boyle,  Honourable  Robert.  "Of  the  systematical  or  cosmical  qualities  of 
things."  Written  in  1669.  To  be  found  in  the  Works  of  Boyle  in  six 
volumes.  See  for  the  Portable  darkened  room.  Vol.  Ill,  Ch.  VI. 

Cardani,  Hieronymi,  Opera.  Lugduni  MDC  LXIII  (1663).  The  reference 
to  pictures  in  a  dark  room  occurs  in:  Tomus  Tertius,  De  Subtilitate 
(1550  A.D.),  Liber  quartus,  p.  426  of  the  left  column. 

Chadwick,  W.  J.  The  magic  lantern  manual.  138  pp.,  100  fig.  Frederick 
Warne  &  Co.,  Bedford  Street  Strand,  London,  1878.  Price  is. 

Davy,  Sir  Humphrey,  Bart.  Collected  works;  edited  by  John  Davy.  12 
Early  Miscellaneous  Papers.  14  Elements  of  Chemical  Philosophy.  15 
Bakerian  Lectures  and  Misc.  Papers.  Smith,  Elder  &  Co.,  Cornhill, 
London,  1839-40.  9  volumes.  IDS,  6d.,  per  Vol..  First  electric  carbon 
arc,  vol.  iv,  pi.  iii,  fig.  17,  p.  no. 

Descartes,  (Lat.  Cartesius)  Rene",  Oeuvres,  Publiees  par  C.  Adam  et  P.  Tannery 
sous  les  auspices  ministere  de  1'instruction  publique  Vol.  i-xii.  Dioptrique, 
Vol.  6,  pp.  87-228,  73  fig.  Leopold  Cerf,  12  Rue  Sainte  Anne,  Paris,  1902. 

Faraday,  Michael.  On  a  peculiar  class  of  optical  deceptions.  Journal  of  the 
Royal  Institution,  Vol.  I,  1831,  pp.  205-223.  Deals  with  the  visual 
appearances  in  looking  at  two  toothed  wheels  revolving  in  opposite  direc- 
tions. 

Foucault,  (J.  B.)  Leon.  Recueil  des  travaux  scientifiques.  4°,  31  -f  592  p. 
31  text  figures.  Atlas,  19  double  plates.  Paris,  1878. 

Gemmae  Frisii,  Medici  et  Mathematici,  De  Radio  Astronomico  et  Geometrico 
Liber.  Basilae  et  Louanii,  1545  (see  p.  31  of  this  work  for  an  account  of  the 
method  of  observing  eclipses  in  a  camera  obscura). 

Goodwin,  Rev.  Hannibal.  United  States  patent  No.  610,861  for  a  film  sup- 
port for  photographic  purposes,  especially  in  connection  with  roller  cameras. 
This  patent  was  applied  for  May  2d,  1887,  and  granted  Sept.  13,  1898,  and 
is  the  fundamental  patent  covering  the  production  of  films  or  ribbons  of 
cellulose  for  taking  the  place  of  glass  and  paper  to  serve  as  the  backing  for 
the  sensitive  coating.  It  is  practically  unaffected  by  the  liquids  and 
chemicals,  used  in  photography.  See  the  opinion  of  Judge  Hazel  in  the 
United  States  District  Court,  of  New  York,  Aug.  14, 1913,  Federal  Reporter 
Vol.  207,  pp.  351-362  in  the  case  of  Goodwin  Film  and  Camera  Co.  versus 
Eastman  Kodak  Co.,  deciding  that  the  patent  is  valid.  See  also  the 
opinion  of  the  Circuit  Court  of  Appeals  (U.  S.  Court),  second  circuit, 
N.  Y.,  March  10,  1914,  federal  Recorder,  Vol.  213,  pp.  231-239  before 
Judges  Lacombe,  Coxe  and  Ward,  Opinion  by  Judge  Coxe.  A  brief  history 
of  the  whole  matter  is  given  in  both  opinions,  and  the  patent  is  held  valid 
in  both.  Every  one  interested  in  the  history  of  photography  should  read 
these  opinions. 

Goring  and  Pritchard.  Micrographia,  containing  practical  essays  on  reflecting 
solar,  oxy-hydrogen  gas  microscopes,  micrometers,  eye-pieces,  etc.,  etc. 
231  p.,  many  figures  in  the  text,  one  plate.  Whittaker  &  Co.,  Ave-Maria- 
Lane,  London,  England,  1837. 

Govi,  Gilberto.  Galileo  the  inventor  of  the  compound  microscope,  Journal  of 
the  Royal  Microscopical  Society,  1889,  pp.  574-598.  Discussion  of  the 
earliest  discoveries  and  inventions  in  optics.  The  compound  microscope 
here  referred  to  as  the  invention  of  Galileo  is  the  Dutch  telescope  used  as  a 
microscope,  i.  e.,  an  instrument  like  the  ordinary  opera  glass  with  a  longer 
tube  for  the  convex  objective  and  concave  ocular. 


HISTORICAL  BIBLIOGRAPHY  699 

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sive  introductio  ad  philosophiam  Newtonianam.  4°.  Auctore  Guilielmo 
Jacob  s'Gravesande.  3d  edition,  2  vol.,  1073  P-t  I27  plates.  Apud 
Johannem  Amoldum  Langerak,  Johannem  et  Hermannum  Verbeek,  Biblio. 
Leidae.  1742.  First  edition,  1719. 
First  clock  driven  heliostat.  Fig.  77,  §  233. 

Hare,  Robert  Jr.  Memoir  on  the  supply  and  application  of  the  oxyhydrogen 
blowpipe.  Philosophical  Magazine.  XIV  (1802),  pp.  238-245;  298-306. 

Harting,  P.  Gebrauch  des  Mikroskopes  und  Behandlung  mikroskopischer 
Objecte.  3  vol.,  1109  p.,  469  fig.  Friedrich  Vieweg  und  Sohn,  Braun- 
schweig, 1866.  Price  $3.50. 

Heyl,  Henry  R.  Contribution  to  the  history  of  the  art  of  photographing  living 
subjects  in  motion,  and  reproducing  the  natural  movements  by  the  lantern. 
Journal  of  the  Franklin  Institute,  CXLV  (1898),  p.  310-311,  Vol.  145. 

Hooke,  Robert.  Animadversions  on  the  Machina  Ccelestis  of  Hevelius.  p.  8. 
Published  in  1674.  It  is  in  this  place  that  Hooke  states  that  for  two  points 
to  be  seen  as  two  the  visual  angle  must  be  one  minute. 

Hop  wood,  Henry  V.  Living  Pictures:  their  History,  Photoproduction  and 
Practical  Working,  with  A  Digest  of  British  Patents  and  Annotated 
Bibliography.  275  +  xxvii,  p.  242  fig.  The  Optician  &  Photographic 
Trades  Review,  123-125  Fleet  St.,  London,  E.  C.  1899.  Price,  £1.25. 

Horner,  W.  E.  On  the  properties  of  the  Daedaleum,  a  new  instrument  of 
optical  illusions.  Philos.  Mag.,  1834,  vol.  iv,  pp.  36-41.  The  Daedaleum 
is  a  hollow  cylinder  with  slits  around  the  edge  and  pictures  in  various  phases 
of  movement  on  the  inside.  It  is  revolved  on  the  long  axis  of  the  cylinder 
and  gives  the  same  appearance  as  the  magic  disc  of  Plateau.  It  is  now 
called  a  zoetrope. 

Ives,  Fred  E.  The  Ether-oxygen  Lime  Light,  Journal  of  the  Franklin  Insti- 
tute, Vol.  125,  1888.  pp.  28-31.  Also  vol.  129,  1890,  pp.  230-234. 
Report  of  a  committee  of  the  Institute  on  the  Ether-oxygen  portable 
lantern,  (see  Ch.  IV,  above). 

Janssen.  Presentation  du  revolver  photographique.  Built,  soc  franc, 
photog.  vol.  xxii  (1876,  p.  100). 

Jenkins,  C.  F.  Picture  Ribbons.  An  exposition  of  the  methods  and  apparatus 
employed  in  the  manufacture  of  the  picture  ribbons  used  in  projecting 
lanterns  to  give  the  appearance  of  objects  in  motion.  56  p.,  many  plates 
and  cuts  unnumbered.  Published  by  the  author.  Washington,  D.  C., 
1897.  Discussion  of  the  origin  and  development  of  moving  pictures. 

Kepler,  Johannes.  Opera  Omnia,  Vol.  II,  Ad  Vitellionem  Paralipomena. 
(De  modo  visionis  et  humorum  oculi  usu).  1604  pp.  226-269,  n  fig. 
Correct  dioptrics  of  the  eye  here  given,  and  also  the  explanation  of  the  effect 
of  convex  and  concave  spectacles.  Dioptrica.  Demonstratio  eorum  quae 
visui  et  visibilibus  propter  conspicilla  non  ita  pridem  inventa  accidunt. 
pp.  519-567,  35  fig.  1611.  The  amplifier,  real  images,  and  erect  images. 
The  Keplerian  microscope  (modern  microscope). 

Kircher,  Athanasius.  Ars  Magna  lucis  et  umbrae  in  decem  libros  digesta.  2d 
edition.  Hermanni  Scheus,  Amsterdami,  1671.  ist  ed.  Romas,  1646. 
isted.  40  +  935  +  17  p.,  26.  ed.,  30  +  810  -f  9  p.  About  650  fig.  34 
Plates.  The  magic  lantern  is  described  in  the  second,  but  not  in  the  first 
edition. 

Libri,  Guillaume.  Histoire  des  mathematiques  en  Italie  depuis  la  renaissance 
des  lettres  jusqu'  a  la  fin  du  dix-septieme  siecle.  4  vol.  8°.  Chez  Jules 
Rennard  et  Cie  Libraires,  Paris,  1838.  In  Vol.  IV,  pp.  303-314  there  is 
discussed  the  invention  of  the  camera  obscura.  Refers  to  Leonardo  da 
Vinci.  Thinks  Porta  reported  what  had  been  known  a  long  time. 


yoo  OPTICAL  PROJECTION 

Langenheim,  W.  Catalogue  of  Langenheim's  colored  photographic  magic 
lantern  pictures.  W.  Langenheim,  722  Chestnut  St.,  Philadelphia,  1861. 
First  edition,  1 850.  The  Langenheims  were  the  first  to  make  photographic 
lantern  slides.  They  used  the  albumen  dry  process,  and  exhibited  their 
slides  at  the  London  World's  Fair  in  1851.  Art  Journal  of  London,  April, 
1851,  p.  106,  Athenaeum,  June,  1851,  p.  631. 

Lenses,  Their  History,  Theory  and  Manufacture.  Bausch  &  Lomb  Optical 
Co.,  Rochester,  N.  Y.,  1906.  47  p.,  34  fig. 

Marey,  E.  J.  Photo-chronographie.  Comptes-Rendus  Acad.  de  Sciences, 
cvii,  (1888),  pp.  607,  643,  677.  Description  of  camera  with  the  band  form 
of  sensitive  surface  for  photography  of  moving  objects. 

Marey,  Etienne  Jules.  La  Chronophotographie.  Nouvelle  methode  pour 
analyser  le  mouvements  dans  les  sciences  physiques  et  naturelles.  Revue 
generales  des  sciences  pures  et  appliqu£es.  Vol.  II,  15  Nov.,  1891,  pp. 
689-719.  The  text  is  accompanied  by  many  figures  including  the  way  the 
ribbons  are  actuated  in  the  chronograph  camera.  There  are  given  pictures 
showing  the  movements  of  men  and  animals  including  insects  and  some 
other  invertebrates.  Some  microscopic  objects  with  their  changing  shapes 
are  also  shown.  Important  for  the  history  of  the  moving  picture. 

Marey,  Etienne  Jules.  Director  of  the  Physiological  Station.  Movement. 
International  Scientific  Series  (No.  73).  318  p.,  200  fig.  D.  Appleton  & 
Co.,  New  York,  1895. 

Marey,  Etienne  Jules.  The  history  of  Chronophotography.  Annual  Report 
of  the  Smithsonian  Institution  for  1901.  pp.  317-340;  42  fig.  IX  pi. 
See  also  his  work.  Movement,  N.  Y.,  1895. 

Matas,  Rudolph,  M.D.  The  cinematograph  as  an  aid  to  medical  education 
and  research.  A  lecture  illustrated  by  moving  pictures  of  ultramicroscopic 
life  in  the  blood  and  tissues,  and  of  surgical  operations.  Presedential  ad- 
dress. Transactions  of  the  Southern  Surgical  and  Gynecological  Associa- 
tion, 1912.  27  pages.  4  plates.  A  bibliography  of  50  publications  given, 
with  special  reference  to  those  in  medicine  and  surgery. 

Mayall,  John,  Jr.  Cantor  Lectures  on  the  Microscope  delivered  before  the 
Royal  vSociety  for  the  encouragement  of  arts,  manufactures  and  commerce, 
Five  lectures,  Nov.,  Dec.,  1885,  97  p.,  103  fig.,  and  two  additional  lectures 
in  1888,  18  pp.,  26  fig.  Published  by  the  Society  at  John  Street,  Adelphi, 
London,  W.  C.,  England.  Price,  2  shillings  6d,  and  i  shilling. 

Melloni,  M.  Memoir  on  the  free  transmission  of  radiant  heat  through  differ- 
ent solid  and  liquid  bodies.  Scientific  Memoirs,  Vol.  I.  Longman,  Brown, 
Green  and  Longmans,  1837.  39  p.  This  paper  shows  the  superior  absorb- 
ing power  of  water  for  radiant  heat.  See  also  Ernest  Nichols. 

Milliet  de  Chales,  Claude  Francois.  Cursus  seu  mundus  mathematicus;  mine 
primum  in  lucem  prodit.  Ex  ofricina  Anissoniana,  Lugduni,  [Lyons],  1674. 
3y.,  folio.  The  second  edition  is  dated  1690  and  has  four  folio  volumes. 
Fig.  403  is  given  on  p.  666  of  vol.  ii  in  the  first  edition,  and  on  p.  697  of  vol. 
iii  of  the  second  edition. 

Molyneux,  William.  Dioptrica  nova,  a  treatise  of  Dioptricks,  in  two  parts, 
wherein  the  various  effects  and  appearances  of  spheric  glasses,  both  convex 
and  concave,  single  and  combined  in  telescopes  and  microscopes,  together 
with  the  usefulness  in  many  concerns  of  human  life  are  explained.  By 
William  Molyneux,  of  Dublin,  Esq.  Fellow  of  the  Royal  Society.  Pre- 
sented to  the  R.  S.,  1690,  printed  1692.  Much  history  and  translations  of 
many  Latin  extracts.  The  first  figure  of  a  magic  lantern  with  condenser 
lens,  see  fig.  404. 


HISTORICAL  BIBLIOGRAPHY  701 

Montucla,  J.  F.  Histoire  des  mathematiques.  New  edition  in  4  quarto  vols. 
Edited  by  J.  de  la  Lande.  1802.  The  progress  of  optics  during  the  i8th 
century  is  given  in  vol.  iii,  pp.  427-605. 

Muybridge,  E.  Animal  locomotion.  The  Muybridge  work  at  the  University 
of  Pennsylvania,  the  method  and  the  result.  Printed  for  the  University  by 
J.  B.  Lippincott  Company,  Philadelphia,  1888.  136  pages,  many  text 
figures  and  diagrams. 

Muybridge,  Eadweard.  Animals  in  Motion.  An  electro-photographic 
investigation  of  consecutive  phases  of  animal  progressive  movements. 
Commenced,  1872,  completed,  1885.  Folio,  264  p.,  many  hundred  figures 
reproduced  from  original  photographs.  Portrait  of  the  author  as  frontis- 
piece. Chapman  &  Hall,  LD.  London,  1899.  In  the  preface  is  given  a 
historical  summary  of  the  author's  work  in  analyzing  and  synthesizing 
animal  movement,  and  in  an  introduction  a  brief  statement  of  the  views 
of  writers  on  animal  locomotion  from  the  earliest  times ;  also  diagrams  and 
descriptions  of  the  methods  used  by  the  author. 

Muybridge,  Eadweard.  Born  1830,  died  1904.  For  biographical  account  see 
the  Dictionary  of  National  Biography,  2d  Supplement,  Vol.  II,  pp. 668-669. 
The  Macmillan  Co.,  New  York,  1912. 

Nichols,  Ernest  F.  A  study  of  the  transmission  spectra  of  certain  substances 
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Pansier,  P.  Histoire  des  Lunettes  par  le  Docteur  P.  Pansier,  d'  Avignon  • 
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mung  fur  all  Freunde  dieses  Instruments.  248  p.,  191  fig.  2  plates.  R. 
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erecto  situ  spectaculis,  necnon  solidis  rationum  momentis  radius  visualis 


702  OPTIC  PROJECTION 

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Scheiner,  Christophorus.  Rosa  Ursina  sive  Sol,  ex  admirando  facularum  et 
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Smith,  Robert,  LL.D.  A  Compleat  system  of  opticks.  pp.  458  +  171  of 
remarks.  63  plates  in  the  text,  20  plates  in  the  remarks.  Cambridge, 
England,  1738. 

Stampfer,  S.  Ueber  die  optischen  Phaenomene  welche  durch  die  stroboskop- 
ischen  Scheiben  hervorgebracht  werden.  Koeniglich-Kaiserliches  poly- 
tech.  Institut.  Wien.  Jahrbucher,  Vol.  XVIII,  1834,  p.  237-.  Describes  a 
magic  disc  like  Plateau's. 

Thompson,  Silvanus  P.  The  arc  light.  Cantor  lectures  delivered  before  the 
Royal  Society  of  Arts,  1895.  Jan.  14,  the  physics  of  the  arc,  pp.  943-960. 
Carbon  arc  by  Davy,  Sept.,  1800.  Jan.  21,  the  optics  of  the  arc,  pp.  961- 
976.  Jan.  28,  the  mechanism  of  arc  lamps,  980-^991.  It  is  stated,  p.  981, 
that  W.  E.  Staite  devised  an  automatic  lamp  in  1846. 

Uchatius,  Franz.  Apparat  zur  Darstellung  beweglicher  Bilder  an  der  Wand. 
Sitzungsberichte  der  kaiserlichen  Akademie  der  Wissenschaften.  Math- 
Natur.  Classe.,  Vol.  X,  Wien.  1853,  pp.  482-484,  one  plate.  Describes  a 
method  of  projecting  moving  pictures  drawn  on  glass  by  means  of  a  lime 
light  and  condenser  moving  from  picture  to  picture.  Each  picture  was 
fixed  in  position  and  had  its  own  projection  objective;  the  axis  of  each 
objective  pointed  to  the  same  place  and  the  pictures  all  appeared  in  the 
same  position. 

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the  best  statement  of  the  case  found.  Many  extracts  from  original  sources 
are  given.  See  also  the  last  edition  of  the  Encyclopedia  Britannica  under 
Camera  obscura,  written  by  General  Waterhouse. 

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of  the  combination  of  a  convex  and  concave  lenses  for  projection,  i.  e., 
the  use  of  an  amplifier. 

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Werner  discusses  the  claims  of  Da  Vinci,  (1500),  and  of  Levi  ben  Gerson, 

1321-1344.     Wiedemann    (which  see)  refers  back  to  Ibn  al  Haitem,  about 

1039. 
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39-40  he  describes  astigmatism  and  shows  that  it  can  be  corrected  by  making 

the  spectacles  oblique,  p.  43.     See  also  Airy.     On  pp.  57-58  is  described  a 

decisive  experiment  to  show  that  the  accommodation  of  the  eye  is  due  to  a 

change  in  the  crystalline  lens. 
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Rucker  und  Puchler,  Berlin.     Part  I,  1838;    Part  II,  1843. 
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methode  explicatum  ac  comprismis-e   triplici  fundamento  physico  seu 

naturali,  mathematica  dioptrico  et  mechanico,  seu  practice  stabilitum; 

opus  curiosum  theorico-practicum   magna  rerum    varietate    adornatum. 

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Norimbergae,    1702. 


INDEX 


PAGE 

Abbe  diffraction  theory 644-647 

apparatus  for 644 

demonstrating  to  an  audience 
and  to  an  individual .  .  .  646-647 

grating  needed  for 644 

light  for 646 

Abbe  substage  condenser 

273,  280,  626 

Abbott,  the  Sun 139 

Aberration,       chromatic       and 

spherical 580-583 

maximum  and  minimum  .582,  589 

Absolute  temperature 547 

Absorption  spectra 637-638 

comparison     spectrum     with 

637-638 

glass  box  for  use  with 637 

substances  for 637-638 

Accommodation  in  vision 

651,  656-659 

Kepler's  theory  of 657 

lantern  slide  for  experiments  656 

necessity   for 657 

produced  by  muscular  effort  659 

Scheiner's  theory 658 

Acetylene,  flame  and  lamp.  .127-130 
amount  in  prestolite  tank.  .  .    129 

position  of  lamp 128 

summary 135 

Achromatic  and  aplanatic  com- 
binations   581-583 

substage  condenser 272,  626 

Adam's  Essays 142 

Adapters   for   small   carbons   in 

large  lamp 80 

Advertising  Magic  Lanterns ....   435 

a-k  on  microscope  slide 361,  387 

Alcohol,  burner 130 

ethyl,  methyl  and  denatured 

supply 130 

Alcohol  lamp 131 

lighting 131 

precautions,  with 133 

putting  out 133 

summary  for  use 137 

Alco-radiant 130 

Alternating  current 475 

advantages 475 


PAGE 

amperage    484 

arc  542 

arc,  form  of  lamp 68 

ballast  with 544 

carbons     for 87,  552 

cycle    486 

disadvantages  with  arc  lamp 

68,  475 

frequency 486 

how  produced 475 

magic  lantern  with 68-77,  87 

micro-projection   with 284 

power  factor  with 485 

radiant  efficiency  of  arc .  .  568-569 
summary   for   use   with   the 

magic  lantern 76 

units  of 484 

voltage    484 

watts  with 485 

wiring 69,  512 

Ambronn's  Handbuch,  Astrono- 

mische  Instrumente 138 

American,  Institute  of  Electrical 

Engineers 5°° 

Journal  of  Science 157 

lantern  slides 200,  201 

Microscopical  Society 383 

Ammeter 479,  510 

alternating  current 73 

connections 480-482 

direct  current 1 1 ,  48 1 

does  not  register 52,  510 

external  shunt 479 

in  projection 236,  481 

need  in  micro-projection.  .  .  .   236 

precautions  for  use 481 

self-contained  shunt 479 

soft-cored 510 

to  measure  current 479 

testing  polarity  with 509,  510 

Amperage 476,  484 

alternating  current 484 

carrying  capacity  of  copper 

wires    500 

cords  and  cables 501 

for  different  forms  of  projec- 
tion       500 

found  by  Ohm's  law 521-522 


706 


OPTIC  PROJECTION 


found  by  wattmeter 483 

varying  with  inductor 532 

Ampere 476 

Amperes  and  volts  in  transform- 
er     533 

Amplifier .  .  .  .  . 229-600 

actual  magnification  and  im- 
age     257 

image  formation  with 600 

magnification  due  to 600 

on  projection  microscope ....   238 

Anatomical  Record 289,  320 

Anatomische  Gesellschaf t 363 

Angle,  closing  of  light 611 

brilliancy  of  image  depend- 
ent on 611-613 

visual 210 

Anisometropia,  unlike  refraction 

in  the  two  eyes 670-672 

Anthony  &  Co 210 

appearance     of     the     screen 

image 672 

source  of  light  for  demonstrat- 
ing   670-672 

Anthracene     screen     for    ultra- 
violet     635 

Aperture,  172,  233,  274,  278,  406,  601, 
609-611,  616 

and  light  losses 601 

effect  of 609,  610-611 

image  formation  and 615 

of  condenser,  to  increase.  ...   616 
of  microscope  objective.  .615-616 
of    projection    objective,    in 
opaque   and   transparency 

projection 172 

plate  of  moving  picture  ma- 
chine    406 

relation    of    condenser    and 

objective 274 

standard  for  moving  pictures  406 

substage  condenser 278 

substage  condenser  to  objec- 
tive       233 

with  directed  light 610 

Apparatus,  adapted  to  frequency  495 
arrangement   with   sun   pro- 
jection     286 

blackened 242 

block  and  its  construction  291-294 

block,  guide  for 292 

block,  size  and  weight 291 

home-made 287-296 


Apparatus     for,     drawing     and 

photography 319 

drawing  with  the  microscope  354 
electric  currents  and  arc  lamps  474 

lantern  slide  making 200 

lantern  with  alternating  cur- 
rent      68 

lantern  with  direct  current .  .       9 

house  lighting  system 78-99 

lime  light 100 

oil,  gas,  acetylene,  alcohol ...    119 
measuring  radiant  efficiency  567 

micro-projection    221 

moving  pictures 390 

opaque  projection 166 

photography 319,  643 

polarized,  light 622 

preparation  of  lantern  slides  200 

projection  microscope 221 

rooms  and  screens 439 

small  current  arc  lamps 78 

sockets  and  stems 292 

spectra    627 

sunlight 138 

ultra-violet  light 627-641 

vision  experiments 651-672 

Apochromatic  objectives.  .  .  583,  600 
compensation  oculars  for ....   600 

Arc 535 

alternating  current 549 

carbon  . 537.539 

cause  of  light  from 546 

characteristics  of 486 

construction  of 535 

direct  current 1 1,  536 

electric    535 

electrical  behavior  of  direct 

current    536 

figure  of,  507,  537-538,  540-543, 
545,  548 

inclined 543,  545,  548 

lamp  for  spectra 628,  639 

length  and  potential 538 

length  and  voltage 538 

parts  of 536-537 

parts  as  source  of  light 547 

right-angled  figures  of 

507,  538,  540-542 
stream  and  bright  violet  lines  546 

vertical 537 

voltage 476,  536,  538,  544 

with  10  and  20  amperes.   249-540 


INDEX 


707 


Arc  lamp 12,  15,  61,  401,  536 

alternating  current 68,  72,  74 

automatic  12,  61,  83,  328,  364,  512 

ballast  for 539,  544 

choke  coil  for 88,  352,  532,  544 

direct  current 

candle-power   553~565 

carbons  for 87,  551 

current  needed 24,  248,  328 

drawing  with 328,  341 

for  spectra 628,  639 

for  micro-projection 236 

for  vision  experiments.. 65 1-672 
fine  adjustments  for .  .  .  12,  72,  237 

going  out  of 48 

hand-feed. 12,62 

house  circuit 80 

inductor  for 88,  352,  544 

installation  of 500-505 

intrinsic  brilliancy  of  crater .  .   564 

lighting 21,24,  88 

"Lilliput"  or  baby  form 81 

managing 24,  414 

material  for  installation 502 

opaque  projection  with 182 

polarity  test .  .84,  506-511 

position  of  carbons  in 

49,  50-51,  6 1,  70,  72,  550,  553 
rheostat  for 

11,70,83,521,539,544 

small 81,82 

small  automatic 83,  364 

small  carbons  for 341 

small  with  clock-work 364 

small  with  drawing 340 

special  dynamo  for 486 

starting 23 

summary  for  small 97 

testing  polarity 506-512 

three- wire,  automatic  238, 263, 512 

three- wire,  supply 514 

turning  off 88 

with  transformer 532~534 

wiring 15,  69,  496-505,  513 

wiring  when  far  from  main 

supply 513 

wiring  for  large  currents ...   513 

Arrhenius 138 

Asbestos-patch  gloves 21,  74 

Astigmatism,    astigmia    or     un- 
equal curvature 663-669 

change  with  age 666 

correct  position  of  spectacles 
for   .  .   666 


correction  by  cylindric  lenses 

665-666 
correction    by    obliquity    of 

spectacles 665 

correction  by   stenopaeic  slit 

669-670 

demonstration   of 663-668 

discovery  of  by  Young  and 

Airy 666 

radial"  lines  for 662-665 

Atmospheric  pressure 103 

Attachment  plugs .  . .  .86-87,  502-504 

Auditorium  for  projection  .  .  395,  439 

Automatic  arc  lamp,  12,  61,  83,  286, 

328,  334,  336,  364,  512,639 

Bausch  and  Lomb  form 512 

Ewon's    548 

Leitz    364 

Nernst 92 

Reichert's 83 

Thompson's 334 

Axis  of  lenses 576-581 

principal,  and  secondary .  576-579 
Ayrton,   Mrs 539 

Balancing  devices 52 1 ,  532 

Ballast,  alternating  current 

69-70,  521,  532,  544 

direct  current n,  521,  539 

moving  pictures 399 

need  with  arc 539,  544 

Nernst  lamp 92 

position  of 512-513 

small   arc 83 

Ball's  Astronomy 139 

Ball-pointed  pen  on  unvarnished 

glass 187,  207,  304,  306 

Balopticon, 

diagrams 188,   189 

Edison   Moving   Picture   at- 
tachment for 405 

large  for  opaque  objects 192 

Nicholas  Power,  moving  pic- 
ture attachment 404 

home 184 

Balopticon,  convertible.  187,  304,  306 

Universal 191,  307 

Barbaro 675 

Barrel,  rheostat 526 

Baseboard,  for  home-made  ap- 
paratus  288-296 

fixing  track  to 290 


OPTIC  PROJECTION 


Bausch  &  Lomb  Optical  Com- 
pany, cuts  loaned  by,  35,  60,  90, 
91,  127,  184,  187-189,  191-192, 
304,  306-308,  356,  358,  404-405 
ref.  in  text,  80,  190,  286,  459, 
512 

Bayley 9,  217 

Bench,  home-made  optical 288 

Beseler,  (Chas.  Beseler  Co.) 94 

Blackboard,   lighting 448~452 

Black  stain  for  tables 289 

Blackened,  apparatus ....  7,  242-246 

objectives 246 

Block  with  lead  sheets 292 

Blocks  for  apparatus 291 

Blondel,  intrinsic  brilliancy  de- 
termination     564-566 

Blood  corpuscles,  demonstration 

of 227 

"Blow"  said  of  a  fuse 83 

Blow-through  jet 105 

Botanical  Gazette 289 

Botanische  Zeitschrift 289 

Boyle's  law  and  camera.  .  104, 129,675 

Box  to  cover  switch 517 

Break  in  circuit 5H 

Briggs'  lantern  slides 673 

Brilliancy,  of  screen  image 610 

intrinsic.  .  .564-566,  613-615,  619 
limit  of,  with  projection  micro- 
scope    618 

reduction   by   substage   con- 
denser, amplifier  and  ocular  618 

sunlight  most  brilliant 619 

British  lantern  slide 200,  203 

British  photographic  society.  ...   217 
Brown  &  Sharp's  wire  gauge.  .  .  .   502 
Bulletin  of  the  Bureau  of  Stand- 
ards   547,  566 

Bunsen   burner 125 

Burner,  blow-through  type 105 

lime  light 105 

mixed  jet 107 

roars  or  hisses 115 

Burning  out  a  fuse 519 

Burning  point  or  focus 578 

Burr,  Geo.  L 673 

Bushings   for   small   carbons   in 
large  lamp 80 

Camera,  adjustable  back 212 

and  plate-holder 382 

enlarging,  reducing,  copying  210 
hinged  baseboard 211 


landscape  drawing 166,  167 

lantern  slide  making 209,  213 

printing  by  projection 378 

spectrographic 643 

tilting    212 

vertical 382 

Camera  obscura,  history  .  .  .673-575 

Candle-power,  and  current 559 

current  and  size  carbons 

87,  248,  551-552 
direct  current  inclined  carbons  556 

formula  for  finding 613 

of  arc  lamps 553-565 

from  kilowats 558 

lime  light,  relative 100 

petroleum    lamp 125 

power  consumption 561-562 

rectifier 55^-559 

rectifier 

inclined  carbons 557 

right-angled   carbons.  .  .  .   558 

sun 138 

variation  with  current.  .  .553-556 
watts,     alternating    current, 

inclined  carbons. 557 

right-angled   carbons 557 

right-angled   carbons 556 

Capacity  of  gas  cylinders  104, 128-129 

Capacity  meter 103 

Carbon  arc  lamp  for  spectra.  .  .     628 

Carbons,  adjusting.  .  . 252 

alternating   and    direct    cur 

rent  with 340 

arc  with  10  and  20  amperes .     249 
arrangement    of    543,    545,    548, 
inclined;  right-angled,  342,  507, 
538, 540-542 

arrangement    in    micro-pro- 
jection     284 

bad  position 50 

composition  and  movement.      72 
converging,  with  alternating 

current 564-565 

cored  and  solid 250 

correct  position 49 

efficiency     of     different     ar- 
rangements   560-561 

electrodes 539 

feeding  with  alternating  cur- 
rent       74 

feeding  on  house  circuit 342 

feeding  with  small  arc 87 

house  system 341 

image  on  screen .   251 


INDEX 


709 


inclined 61,  70 

incorrect  position 51 

not  near  enough 47,  48 

observing  with  moving  pic- 
tures   403,  414-415 

polarity  right  and  wrong. 506-5 12 
position,  72,  284,  507,  538,  540- 

548, 
preparation  for  exhibition  250,  433 

distribution  of  light 565 

size  87,  41 5,  551-552 

size  for  moving  pictures 415 

current,   candle-power...   248 

house  circuit 341 

small  with  alternating  current  342 

small  arc  lamp 79,  87 

small  with  direct  current ....   342 

solid  and  cored 250 

terminals 12,  539 

too   short 47 

why  small  on  house  circuit ...   341 

Cardboard  screen 80,  458 

Cardanus 674 

Carrying   capacity    of   electric 

cords,  how  calculated 501 

and  insulation 500 

flexible  cables 501 

of  copper  wire 500 

Caution  for  lime  light 107 

Celluloid,  films 431 

inflammability  of 43 1 

Center  of  lenses 576,  577 

of  lens  face,  how  to  find ....  40,  41 

Centering 23,39 

alternating  current  lantern .  .      73 
apparatus  for  vision  experi- 
ments   653,  671 

heliostat    150 

mazda  lamp 91 

mechanical  40 

micro-projection.  .  .  .246,  251,  285 
moving  picture  machine. 4 10-4 12 

Nernst  lamp 93 

objective 247 

objective  hood 246 

objective,  vertical .  .  . 45 

optical    test,    condenser   and 

objective 44 

perpendicular  to  axis 41 

radiant  and  condenser 41 

substage  condenser 280 

troubles  if  incorrect 53 

Centimeter  rule 27,  318,  371 

Cerium  iron  gas  lighter 106 


Chadburn 173,  180,  183,  190 

Chadwick,  Hep  worth  and  Wright  173 

Chamot,  Dr.  E.  M 207 

Chemical  polarity  indicators.  ...   510 

Chevalier    322 

Choke-coils 88,  352,  532,  544 

Chromatic  and  spherical  aberra- 
tion   580, 583 

Circuit,  electric 496 

breakers    518 

open  and  closed  by  switches 

5H-5I8 

with  break 514 

with  one  ground 497 

with  two  grounds 498 

Clock-shaft  of  heliostat,  parallel 

with  earth's  axis 147,  160 

Cloth  screens 457 

Cohn,  eye  defects  in  school  child- 
ren  659 

Color,  used  in  moving  pictures .  .   392 

Colored  moving  pictures 392 

Coloring  lantern  slides 217 

Combined  apparatus 356 

Combined  projection,  175,  176,  180, 
182,  184,  186,  189,  193,  194,  295, 
297,  300 
avoiding  contrast  of  images .  .    177 

two  complete  outfits 297 

Comparison  spectrum 637-638 

Compensation  ocular 232 

Concave    lens    for    parallelizing 

light 337 

Condenser 14,  587 

Abbe  substage 273,  626 

aperture  of,   to  increase,   or 

diminish    616-617 

achromatic  substage 272,  626 

aperture  of  substage 278 

centering  substage 280 

diaphragm    opening 337 

distance   from   objective,    in 

microprojection 247 

distance  from  radiant 41 

end  to  be  next  the  radiant 

58,  62,  66 

ends  reversed 58,  62,  66 

first  element  of 65,  588 

focus  of  second  element  for 

the  objective 65,  590-591 

focus  too  long 55 

focus  too  short 55 

for  demonstrations  in  vision 

652-653 


7io 


OPTIC  PROJECTION 


for  lantern  slides 202 

for  sunlight 139,  161 

lamp  with  special 343 

lens  breaking 57 

light  for  different  positions  of 

lamp    42 

lighting  with  main  condenser 

only .   234 

lighting  opaque  objects  with  189 
micro-projection  with.  .  .237,  273 

mounting  of 58,  62 

moving  picture 403 

and  objective,  proportions ...     43 

opaque  projection  with 175 

out  of  center 53 

parabolic    for    micro-projec- 
tion       588 

protecting  by  sheet  mica 

or  glass    608 

reduction  of  brilliancy  by.  .  .   618 
second  element  of,  and  the 

objective 65,  588,  590 

second  element  for  micro- 
projection.  ..  .274-278,   591 

size  for  drawings 331 

size  for  lantern  slides 202 

spherical  aberration  of 585 

substage.  . 232,  347 

substage,  high  power 271 

substage,  K&hler  method  with  278 

sunlight 139,  161 

three-lens 13,  652 

two-lens 10,  653 

with  spherical  aberration  585 
without  spherical  aberra- 
tion      584 

types  of 587 

triple 588 

two  lens 588 

Conditions  for  good  micro-pro- 
jection     309 

Connections,   acetylene 129 

ammeter 480—482 

attachment  cap  of  separable 

plug 86-87,  503-504 

electric  supply 502,  513 

gas  lamp.  .  . 126 

lime  light 106 

to  the  switch 504 

voltmeter    478-482 

wattmeter 482 

Connectors,   metallic 106 

separable   extension 87 

Converging  light,  parallelizing..   273 


Cornell  University 147,  673 

Crater,  increase  in  size  with  cur- 
rent  248,  249,  564 

temperature  of 546-548 

Crystalline  lens  of  the  eye,  change 
in  shape  for  accommodation .  .   658 
in  hyperopia  and  myopia .  660-661 
rigid  in  presbyopia.  .  .661-662 

Cuff  and  Adams 141 

Current,  alternating 475 

amount  compared  with  direct 

73,  553-565 

at  anodes,  oscillogram  of ....   495 
appearance  of  arc.  .  .342,  542,  545 

and  candle-power 553-5^5 

connections   of  ammeter    to 

measure 482 

controlled  by  resistance .  .  542-544 

direct 1 1 ,  474 

direct  for  arc  with  magic  lan- 
tern      24 

for  drawing 338 

for  spectra 628,  640 

for  experiments  in  vision  651-672 

for  micro-projection 248 

increase  in  heat  by  increase 

of  current   248 

increases  size  of  crater 248 

insulation  for  large 513 

lack   of 48 

lamps  for  small 78 

magic  lantern  with  alternating 

68-77 

micro-projection   with   alter- 
nating    284 

moving  pictures  with  alter- 
nating  398,  401,  566 

opaque  projection 182 

oscillogram  of  delivered  and 

supplied    493 

potential  drop  at  arc 536-539 

precaution  for  heavy 1 86 

rectifier 489-492 

screen  distance  and 73 

size     carbons     and     candle- 
power 248,  551-552 

unit  of,  the  ampere 476 

wiring  for  large 513 

Curtains  for  darkening  room ....  446 
Curves  of  light  reflection ....  460-465 
Cycle  with  alternating  current . .  486 


INDEX 


711 


Dark  ground  illumination  . . .  647-650 

apparatus  required . 647 

for  demonstrating  inhomo- 
genieties  in  liquids  or  trans- 
parent solids 648-650 

Foucault's  method 650 

method  of  striae ^47 

Toepler's  "Schlieren-Methode" 

647-650 

Darkening  room,  method 446 

Darkness,  avoiding 112 

in  opaque  projection 175 

of  room  for  sunlight 143 

relative  for  different  projec- 
tion   443-446 

Davids  (Thaddeus  Davids  Co.)      207 

Davy's  Carbon  Arc 686 

Daylight  vision 175 

Decentering,  effects  of 53 

Decoration   of  projection   room 

440-441 

Defective  vision 659-672 

apparatus  required  for.  .  .651-655 
condenser  for  demonstrations 

652-653 

demonstrations  of 659-672 

lantern  slides  for.  .  .  .656-666,  671 

source  of  light  for 670 

trial  lenses  for 651-655 

Delineascopes 

63,  193-196,  310,  312,  355 
Descartes  sine  law  for  refraction  576 

Deschanel's  Physics 212 

Diaphragm,    effect    on    cone   of 

light 279 

of  substage  condenser,  effect 
with  Koehler  method,  and 
with  main  condenser  me- 
thod of  illumination 620 

Diffraction,  gratings 631,  644 

images 632 

pattern    644-645 

Diopter,  definition 229 

in  spectacle  lenses 655 

Direct  current  apparatus 9 

arc,  radiant  efficiency  com- 
pared with  alternating 

69,  474-475,  553-565 

production  of 474 

summary  for  magic  lantern .  64-67 

units    475-478 

use  of 474 

Dirt  on  lenses 55 


Distortion,  how  to  avoid  in  draw- 
ing      338 

of  image 338 

Distribution  of    light,    different 

forms  of  arc 562 

Diverging  light,  to  parallelize.  .  .   274 
Dolbear,  Art  of  projecting 

122,  138,  174,  211,  621 
Double-pole  switch.  .  .  12,  70,  515-517 

Drawing  apparatus 319-389 

attachment  for  ordinary  ma- 
gic lantern  327 

booth 321 

direct  current  arc  lamp  for ...   328 

distance,  varying  for 335 

fastening  letter  to 375 

from  lantern  slides 326 

high  power 335 

horizontal  surface  with  magic 

lantern    331 

with  opaque  lantern 332 

house  lighting  system  for ....   339 

landscapes    166 

large  objects  with  low  powers  333 

lettering  the 374 

microscope  for 344,  385 

microscope  and  lamp  at  right 

angles    348 

microscope  in  a  dark  room .  .  .  346 
microscope  without  ocular  or 

substage  condenser 334 

models 357 

objectives,  16-8  mm 335 

oculars  for 337 

opaque  lantern.  .  . 332 

outfit 350 

house  system 343 

with  inductor 352 

(Koristka)    323 

with  microscope 343~354 

with  small  currents 343 

photographic  camera  for ....  332 
projection  apparatus  for.  ...  319 
projection  apparatus,  early 

forms 372 

projection  microscope  for.  .  .   333 

publication  of 373 

radiants  other  than  the  arc  .  .   329 
range  of  objects  with  projec- 
tion microscope 333 

room  and  curtains 321 

room,  special 320 

shelf 325 

on  projection  table 326 


7I2 


OPTIC   PROJECTION 


size  condenser  to  use 331 

size  and  lettering 376 

small 330 

small  arc  lamp,  for 340 

summary 388 

surface  horizontal 322 

surface  vertical 32 1 

table    324 

with  mirror  attached. 3 2 3-3 2 5 
with  movable  shelf.  .  .325-326 

tracing 374 

troubles  in 384 

varnished  slides  for 205 

wall  diagrams 329 

Dresbach,  Dr.  Melvin 651 

papers  on  eye  defects 659 

Durand,  Dr.  Albert  C 651 

Dust  to  show  light'rays 247 

Dynamo 474 

connected  directly  to  arc  lamp 

486-488 

special  for  arc  lamp 486 

wattmeter  to  measure  power 
delivered  by 482 

Eastman  Kodak  Co.  .  .  .212,  215,  319 

Eclipse  voltmeters  and  ammeters  510 

Ecliptic 163 

Edinger 345 

apparatus,  erect  images  with  373 

large  apparatus 360-362 

outfit  for  small  arc  lamp  and 

ordinary  microscope 363 

vertical   apparatus 360 

Edison,  Thomas  A.,  figures  loaned 
by  and  text  references 

113,  400,  406,  418,  429,  435,  686 

"Effects"  with  multiple  lanterns     34 
with  single  lanterns . 36 

Efficiencies  of   current  and  car- 
bon arrangement 560-562 

Electric,  apparatus,  list  of.  .474,  499 

arc  535-538 

circuit 496 

fan  for  drawing  room 320 

flashlight 14 

measurements    475-486 

supply,  connections  to ...  502-505 
units    475-478 

Electrical  behavior  of  arc .  .  .  536,  539 
World    553 

Electrodes,  of  carbon 539 

and  potential  drop  at  arc  536-539 
for  spectra 627,  638 


stuffed    with    various     sub- 
stances     638,  639 

Electromotive  force,  unit  of.  ...   476 

Emission  spectra 638-643 

arrangement  of  the  electrodes 

for    640 

automatic  arc  lamp  for ...  638-639 

current  to  use  for 639 

different    appearances    with 
positive  and  with  negative 

electrodes 640 

electrodes  for 640 

yellow-flame  carbons  for.  .  .  .   638 

Enclosed  knife  switch 517 

Energy  losses  in  projection .  .  603-609 
comparison  of  in  water,  etc .  .  607 

example  of 609 

in  the  condenser 604 

in  the  projection  objective  .  .   609 

in  sheet  mica 608 

in  the  specimen 607 

table  of 607 

Energy,  required  for  projecting 

moving  pictures 57°-57J 

with  inductor  and  rheostat .  .   533 

Energy    transmission 606 

table  of  absorption  and  trans- 
mission       607 

English  Mechanic 221 

English  Photographic  Club 29 

Enlargements     with     projection 

apparatus 378 

Enterprise  Mfg.  Co 101 

Episcope  for  drawing 332 

Equivalent  focus 580 

Erect  images,  in  drawing.  .  .  .359-373 
with  two  projection  lenses  368 

Ether  saturator 113 

Ewon's  automatic  lamp 548 

Exhibition,  with,  alcohol  light.  .  132 
alternating  current  lantern  .  .  73 
direct  current  magic  lantern  19 

high  powers 271 

lime  light no 

mazda  lamp 92 

moving  pictures 433~435 

Nernst  lamp 95 

opaque  lantern 179 

projection  microscope 270 

room  lighting,  19,  52,  66,  96-99, 
112,  120,  143,  175,  235,  303,  314, 
.320,  395,  441-443,  626,  633 
with  small  arc  lamp 90 


INDEX 


713 


Exhibition  with  sunlight 161 

Experiments,  on  flicker 423-427 

with  projection  in  physics  621-651 

with  polarized  light 622-626 

with  vision,  normal  and  defec- 
tive     651-672 

Explosion  with  lime  light 107 

Exposing  dry  plates  directly  to 

the  projected  image 381 

for  printing  by  various  lights  214 
with  projection  apparatus .  .  .   380 
Eye,  accommodation  for  distance 

657-658 

as  a  part  of  projection  appara- 
tus          4 

demonstration  of  normal  vi- 
sion       654 

inverted  image  on  the  retina 

655-656 

prevalence  of  defects 659 

refractive  defects 659-672 

two  eyes  unlike 670 

Fan,  electric  in  drawing  room .  .   320 
Faraday  and  moving  pictures ...   680 
Field,  need  of  large,  in  micro- 
projection  254 

and  objectives 254,  255 

Filament,    position    of    (mazda 

lamp) 91 

Filar  micrometer  ocular 234 

Film,  burnt  in  concentrated  light  432 

celluloid    431-432 

direction  of  motion 419-420 

effect  of  opacity  on   energy  571 

inspection  of 427-428 

invented  by  Goodwin 682 

lantern  slides 215 

magazine 407 

security  from  fire  by.  .431-433 

mender    429 

position  in  machine 415 

splicing 428-429 

threading    417-418 

Finimeter 103 

Fire,  danger  from 431 

escapes    443 

-proofiing  curtains 32 1 

Fire-shutter   420,  43i~433 

automatic 420 

Firetrap,  security  o1" 431-433 

Fire  Underwriters 498-505 

regulations 399 


Fish,  Dr.  P.  A 289 

Flexible  metallic  tubing 106 

Flicker 423-427 

curve 424 

experiments  on 426 

formula  of 426 

position  of  shutter  to  prevent 

422-423 

table  on  speed 427 

theory  of 426-427 

Fluorite  in  apochromatic  objec- 
tives     583 

Flux  of  light  in  projection ...  172,  614 

Focus,  or  burning  point 578 

condenser  and  objective .  .  587-592 

conjugate    579 

principal  focus 578-582 

principal,  how  to  obtain 579 

Focusing 28,   30 

device  on  the  microscope ....   241 
device  with  the  magic  lantern 

objective 28 

image  on  screen 28 

for  photographing 379 

of   screen   image   for   micro- 
projection  256 

Folmer  &  Schwing 212 

"Fool-proof"    7 

Formula,  for  absolute  and  centi- 
grade temperature .......    547 

amount  of  gas  in  a  cylinder 
of  acetylene,  hydrogen  or 

oxygen 104,  129 

black  table  stain 289 

determining  visibility 227 

dioptry  and  focus  of  lenses  229-230 
electric  quantities,  Ohm's  law 

483-485,  501-502,  522 
flicker  with  moving  pictures  426 
intrinsic  brilliancy  of  the  sun 

i 38-1 39 
light  flux  passing  an  objective 

172,613 

magnification 260-262 

making  lantern  slides  direct 

20 i,  205-208 
size  of  screen  and  focus  of 

projection  objective.  .  .467-470 
sizing  and  painting  screens .  .   456 

starch  paste 375 

table  black 289 

Foucault,  method  of  detecting 
inhomogeneities  by  dark 
ground  illumination.  .  .650-651 


OPTIC  PROJECTION 


Foucault,  Recueil  des   Travaux 

Scientifiques 138 

Fourtier    9,  202 

et  Moltini 62 1 

Frame   for   darkening   window 

margins 445~447 

for  retouching  slides 29,  203 

French  Congress  of  Photography  200 

French  lantern  slides 200 

Frequency,    of  alternating   cur- 
rents       486 

apparatus  adapted  to 495 

Fuess,  R 56,  138,  150,  159 

Fuse  blocks,  location  of 520 

Fuses 399.5I9 

"blowing"  of 519 

burned  out 47,  519-520 

capacity    520 

and  circuit  breakers 518-519 

location  and  installation  ....   519 

on  each  line 519-520 

on  house  circuit 83,  520 

replacement   of 520-521 

wattmeter 519-520 

Gas,     amount     in     cylinders 

103-104,  129 

lamps 125 

lighter 106 

management 127 

reflector 126 

summary 136 

Gases,  proportion  of  in  lime  light  108 

Gauges,  pressure 102,  103,  129 

Gelatin  for  lantern  slides 205 

General  Electric  Co 553 

cuts  loaned  by 489-495 

Generator 474 

shunt 487 

Gentner  (J.  H.  Gentner  Co.) ....   459 

German  lantern  slides 200 

"Ghost"  from  reflections 245 

Glass  plates  for  polarizing  light .  .   624 
tinted  in  combined  projection  177 

unvarnished  207 

Glassine  ink  for  writing  on  glass  207 
Gloves,  asbestos-patch.  .  .  .21,  22,  109 

Glower  of  Nernst  lamp .  . 92-93 

Golgi  method 240 

Goodwin,    inventor    of    photo- 
graphic film 682-683 

Goring  and  Pritchard 283,  454 

Gothic  type  for  drawings 377 

Gratings  for  spectra.  .  .  .627,  632-635 


s'Gravesande 140,  146 

Ground  of  electric  current 

48,  497-498 

Ground-glass  to  diffuse  light 380 

screen,  transmission. 46 1-462,  465 
Guide  for  making  lantern  slides .  207 
Guides  for  apparatus  blocks ....  292 

Guil  pastils 101 

Gundlach-Manhattan  Optical  Co.  412 

Hand-feed  arc  lamp 12, 15,  62 

lamp  for  alternating  current     68 

lamp  for  small  currents 286 

Hartford  Screw  Co 296 

Hassock  and  Rosenberg 9,  621 

Heat,  getting  rid  of,  in  rheostat  523 

with  small  currents 350 

Heating  unequal  on  condenser ..     58 

Heliostat 139 

clock-driven 145 

for  east  window 144 

for  south  window 143 

for  west  window 145 

hand-regulated 140 

how  to  set  clock- shaft 147 

kinds  of 140 

lens  and  prism 157,  159 

mirror  parallel  to  clock- shaft 

154,  155 

setting  up 154 

one-mirror 145,  146,  148,  150 

setting  up 149 

positions  of  mirror 144,  156 

principle   of 151 

southern  hemisphere 156 

setting  up 158 

two-mirror 145,  152,  153 

arranging  fixed  mirror.  .  .    154 
arranging  movable  mirror  152 

Hepworth,  C.  M 391 

Heyl,  H.  R 680 

Hepworth,  T.  C 9 

Historical  Summary 673-687 

Hitchcock,  Prof.  Romyn 265 

Holland,  translucent  screens  in .  .   462 

Home  kinetoscope 435 

Home-made  optical  bench .  .  .  288-296 

projection  apparatus 4,  287 

rheostats 525~53O 

Home  projectors    435 

Hood,  on  objective 245 

showing   light   centered   and 

not 246 

Hooke's  joint  and  rod 82,  329 


INDEX 


715 


Hopwood,  H.  V 391 

Horizontal  objects 32,  268 

Horner's  Zoetrope 680,  682 

House     system    for    projection 

microscope 285 

fusing    83 

lamp  for 78 

Hubbell  (Hubbell  Inc.) 

86-87,  503-505 

Huygenian  ocular 230,  233,  280 

Hydrogen,  cylinders 103 

substitute  for 113 

Hyperopia  or  long  sight 66 1 

Iban  Al  Haitem  and  the  camera 

obscura 674 

Illumination,  dark  ground .  .  .  647-650 

Foucault's  method  of 650-651 

Toepler's  method  of 650-651 

Illumination,  flashes  per  second 

at  which  flicker  disappears  424 

with  high  powers 599-601 

with  lower  powers 598 

with  magic  lanterns  16-18,584-587 
with  moving  pictures 

411-415,  593-598 

with    projection    microscope 

247-256, 271,  278,  284,  286,  287, 

328,  335,  346,  598-601,  617-620 

Illuminating  gas  for  lime  light ...    112 

for  magic  lantern 125-127 

Image,  brightness  of.  .18-19,  612-617 

carbons 251 

condenser  with  spherical  aber- 
ration       585 

condenser  without   spherical 

aberration 584 

connection  with  aperture.  ...   615 

correct 26-27,  371,  387 

dim  and  brilliant  in  combined 

projection 177 

erect  26-27,    190,  359,  363-365- 
367,  369,371,373,387 
with  opaque  lantern .  .  363-365 
with  translucent  screen 

26,  362 

with  two  lenses 368-369 

formation    with,    the    magic 

lantern    584 

effect  of  an  extended  source 

of  light 586 

moving  pictures 59i~598 

point  not  on  the  axis 617 

projection  microscope.  .  .598-601 


Image  projection  microscope  with 

amplifier  or  ocular.  .  .  .600-601 

a  microscope  objective 616 

hazy 308 

inverted 26-28,  359-371,  584 

for  micro-projection 470 

for  different  microscope  tubes  243 

for  moving  pictures 468 

for  opaque  projection 181 

for  screen  distances 466 

lantern  slides  for 464 

sharpness  of 28-30,  256 

size,  found  by  calculation.  .  .   261 
found  by  measurement 

257-260 
stereoscopic  with  the  magic 

lantern    37~39 

troubles  with  screen  images 

52-57,301,308 

Incandescent  lamp 14 

after   rheostat 17 

before  rheostat 13 

testing  for  voltage 46 

tracing  pictures 374 

Inclined  carbons 61,  70,  543~55O 

Indirect  room  lighting 441-443 

Inductor 352~532 

with  arc  lamp 352,  532 

varying  amperage  with 532 

wiring 352,  532 

Infra-red  radiation 566 

Ink  for  unvarnished  slides 207 

Inks  and  pens  for  varnished  slides 

206-207 
Insulation  with  large  currents 

513-514 

for  long  distance  wiring 513 

of  wires 497-498 

regulating  amperage 5GI 

Intensity   of  light,   in   different 

directions   563 

Intrinsic  brilliancy,  of  crater  of 

arc  564 

of  the  light  source 

138,  564,  612-615 

of  the  sun I38~i39 

Iodized  starch  polarity  indicator  510 
Iris  diaphragm,  effect  on  cone  of 

light 279,  620 

Italian  lantern  slides 201 

Ives,  Fred  E. — Ether  oxygen  lime 
light,  history  and  new  ap- 
paratus    686 


7i6 


OPTIC  PROJECTION 


Janssen  astronomer,  pictures  of 
the  transit  of  Venus  with  a 
photographic  revolver  in- 
vented by  him  in  1873  (see 
Marey,  Hist.  Chronopho- 

tography)   683 

Jablochoff,  arc  lamp  with  paral- 
lel carbons  (see  Silvanus  P. 

Thompson) 686 

Jenkins,  C.  F 391,686 

Journal  of  Applied  Microscopy.   289 
Journal  Royal  Microscopical  So- 
ciety       221 

Journal  Royal  Society  of  Arts  200,  686 

Keller  (P.  Keller  &  Co.) 61 

Kepler.  .  .321,  361,  368,  656-657,  678 
accommodation  for  distance  657 
hypothesis  of  accommodation  657 
inverted  retinal  image.  .  .656-657 

Kilowatt 477 

Kilowatt-hour    477 

Kilowatts  for  candle-power 558 

Kinemacolor  moving  pictures .  .  .   392 

Kinetophone    393 

Kinetoscope  400 

mechanism  418 

Kingsbury,  Dr.  F.  B 324,  327 

Kircher,  A.  magic  lantern  .  .  .676-677 

Knife  switch 515,  517 

Knife  switch  enclosed 517 

Knife  switch  vertical 516 

Kohl,  Max 126,  153 

K6hler  method,  substage  con- 
denser  278,  305,  619-620 

Koristka's  drawing  apparatus.  .  .   323 

Labelling  lantern  slides 218 

Lambert,  F.  C 200 

Lamp  arc,  12,  74,  78,  80,  182,  236,  286 

328,  339,  343,  401,  506,  536,  549, 

628,  639,  652,  670 

Lamp,  for  general  lighting 442 

for  small  currents 78,  286 

£as  •• 125 

incandescent w. .      14 

lime  light 105 

on  house  system 78 

with    special    condenser    for 
house  system 78,  343 

Lamp-house 14,    72 

window  in 14,  72 

Landscape  drawing,  camera  for  166 


Langenheim 673 

Lantern,  college  bench 301 

combined 176,  180,  182 

multiple    34 

testing 20 

Lanternists,  work  of  early.  .  .119-120 
Lantern    slides,    actual    size    of 

opening 202 

American    201 

arranging    30 

black  ground 213 

British,  spotted 203 

coated  with  gelatin  for  writ- 
ing    205 

collecting  at  close  of  exhibi- 
tion         30 

coloring 217 

condenser  to  illuminate 204 

confusion  of  size 202 

correct  position  in  carrier ....     26 

developing    209 

for  showing  eye  defects 

656,665,666,671 
directions  for  making  direct  201 
directions  for  "spotting".  .26,  216 

duplication 19 

film    215 

for  experiments  in  vision. 65 1-672 
for  showing  eye  defects .  .556,  665, 
666,  671 
frame    for    retouching,    203 

with  guide  lines 206,  207 

hand-made 205 

in  individual  carrier 31,  32 

labeling 26,218 

making   200 

mounting    216 

negatives  as 211 

on  mica  or  gelatin 208 

opaque    56 

order  of 19 

photographic   208 

possible  images  of 27 

possible  positions  of  American    28 
possible  positions  of  British .  .      28 

printing  with  camera 213 

printing  from  negative 208 

rapid  preparation 215 

sizes 200 

size  of  condenser  required  for  202 

size  of  image 464 

size  of  print  on 208,  464 

size  of  screen  for 464 

smoked  glass 208 


INDEX 


717 


Lantern,  square 29 

"spotting"  or  marking 

19,  201,  216 

standard 202 

standard  American 25,  200 

storing 218 

summary 220 

troubles  in  making 219 

typewritten    215 

unvarnished,  ink  and  pen  for  207 

varnishing 205 

Lantern  slide  carrier  for  delinea- 

scope 28,    63,    196 

carrier    for    experiments    in 

vision 651-654 

permanent    3I-32 

push-through  form 23 

ways  of  making 203 

writing  them  with  a  pen .  .  204-208 
Lathe  bed  type  of  projection  ap- 
paratus  262, 289 

Latitude  and  heliostat 160 

Lavasseur,  Professor  E 673,  685 

Law  of  magnification  and  reduc- 
tion  357,369 

Law  of  reflection 140,  572-574 

Law  of  refraction 576 

Lecturer,  suggestions  to  on  magic 

lantern  exhibition 19 

Lecture  rooms 439-454 

Lecture  room  with  gallery 448 

with  raised  seats 449-451 

screen  at  side 450-45 1 

transluctent  screen 453 

Lehmann,  fluid  crystals,  etc.  ...   621 

Leiss,   C 138 

Leitz,  Ernst,  79,  178,  225,  299,  360, 

362,  363,  364, 
Lens,  aberration,  chromatic.  .  .  .   583 

spherical 580-586 

how  corrected 581 

achromatic 583 

aplanatic 581 

axis  of,  principal  and  second- 
ary   576-579 

center  of  curvature 577 

center,   optic 576-577 

concave,  convex  and  menis- 

cus 577 

concave  to  make  light  cone 

Parallel    273,276 

convex   to   make   light   cone 
parallel    274-276 


Lens,  crystalline  of  the  eye 657 

change  of  shape  in  accom- 
modation       658 

definition  of 576 

dioptry  of 229-230 

equivalent  focus  of 580 

focus  of,  principal  and  con- 
jugate  578-582 

focus,  equivalent 580 

holder  for  trial  lenses.  .651-654 

meniscus 577 

optic  center  of 576-577 

principal  focus  of 578 

principal  focus,  how  found 

579-580 
principal  and  secondary  axes 

576-579 

radius  of  curvature  of 577 

secondary  and  principal  axes 

of 576-580 

support  for  trial  lenses.  .  .651-654 
unit  of  strength  (diopter) ....   229 
Lenses,  cylindrical  for  eye  defects  664 
detecting  strain  in  by  polar- 
zed  light 626 

for  experiments  in  vision. 65 1-672 

forms  of 577 

position  in  the  triple  and  in 
the  double  lens  condenser 

10,  13,  591-592 

Oculists'  trial 651 

Leonardo  da  Vinci 674 

Lettering,  drawings 374 

fastening  the  letters 375 

size  for  drawings 376 

Letters,  a-k  on  specimen.  .  .  .361,  387 

on  tissue  paper 375 

white  on  black  ground.  ......   375 

Levi  Ben  Gersen 674 

Light  from  alternating  current 

arc  . 565 

absorption  by  lenses  and  ob- 
ject     601-602,  620 

amount  from  sun 138 

amount  with  petroleum  lamp  125 
for  demonstrations  in  vision 

652,  670 

from  arc,  cause  of 546 

from    arc    direct    and    alter- 
nating current 554 

of  arc  dependent  on  tempera- 
ture    547 

avoiding  stray 380 

in  center  of  condenser  face ...     44 


7i8 


OPTIC  PROJECTION 


Light,     centered    on     objective 

face 44 

centering  by  objective  hood  246 
closing    angle    for    screen 

image 611 

direct  on  screen 52 

distribution  of  intensity .  .  562-563 
distribution    in    semi-diffuse 

reflection 459 

distribution  from  white  screen 

462-463 

early  sources 1 19,  686 

energy,    proportion    of   total 

radiation 566,  569 

flux    172,  613,  614 

flux  in  opaque  projection.  ...    172 
from  converging  carbons .  564-565 

from  right-angled  arc 564 

in  exhibition  room.  .112,  161,  441 

increasing  with  sunlight 161 

insufficient    51 

on  screen in 

on  screen,  insufficient 301 

parallelizing. 273,  276,  335 

projection  with  feeble 120 

red  near  exits 443 

reflection  with  various  screens 

249, 250 

relative  amount  with  trans- 
parent and  opaque  projec- 
tion   171 

shield  beyond  objective  ....   246 
shield  on  the  window  frame 

445-447 

sun  most  brilliant.  ...  138,  619 
sources,    arc   lamp,    oil,  gas, 
acetylens,  lime  and  mazda 
lamps ;    sunlight 
u,  79,  100,  119,  125,  127,  138 

sources  for  spectra 627-628 

source,  parts  of 537,  547 

size     and     brilliancy     of 

screen  image 610 

stray,  how  to  cut  off 266 

stray,  vertical  microscope.  .  .   268 
stray,  effect  on  screen  image 

443-444 
turning  on  and  off .  20,  88,  1 10, 162 

unequal  on  screen 308 

weak  for  micro-projection ...   287 

Light  losses  by, 601 

absorption     by     lenses    and 

object 601-602,  619 

polarization    603 


reflection  from  lens  surfaces 

601-602 
shutter  in  moving  pictures .  .  .   603 

sheet  mica 608-609 

small  condenser  or  objective 

601-602 

substage  condenser 618-620 

Lighting,  blackboard 448-449 

entire  opaque  object 187 

indirect   method 442 

Lilliput  lamp 81 

Limes,  for  lime  light 104 

Lime,   arranging 109 

cracks   in 115 

pitting  of 109 

putting  in  place 108 

rotating 109 

warming 108 

Lime  light 100 

burner 102 

connectors    108 

lamp  for 107 

lighting 106 

management 106 

micro-projection  by 287 

oxygen  generator  and  ether 

saturator 113 

oxygen  and  illuminating  gas  112 
regulating  flame 108 


shield 

snaps  out .  . 
starting  .  .  . 
summary  .  . 
troubles  .  .  . 
turning  out. 


in 


106,  no 

117 

114 

no 

Litmus  polarity  indicator 510 

Long  sight  or  hyperopia 66 1 

Loos,  A.  J 673 

Loss  of  light 601-609 

Loss  of  energy 603-609 

Lumen 613 

McAllister    167 

Mclntosh  Bat.  and  Opt.  Co.  ...  123 
Mclntosh  Stereopticon  Co.  .  .62,  301 

Maddox 682 

Magic  disc 680 

Magic  lantern,  9,  68,78, 100,  119,  138, 
414,  584-587 

acetylene 127 

addition  of  micro-projection  309 

advertizing  form 435 

American   forms 59 


INDEX 


719 


Magic  Lantern,  centering 39 

Condenser  with  and  without 
spherical  aberration   .  .584-587 

for  small  drawings 330 

history 676-678 

image  formation  with 584 

inversion  of  the  image 584 

light  source  extended  and  a 

point 586-589 

simplest  with  arc  and  2 -lens 

condenser ;•••.••      IO 

standard    for    projection 

apparatus   9 

wiring  for 15,  71,  82,  504,  5J2 

attachment  for  drawing 327 

automatic  lamp,  inclined  car- 
bons       61 

optical    bench    and  ordinary 

microscope 262 

drawing  accessories 329,  330 

drawing  surface  horizontal.  .   331 

hinged  baseboard 450 

inclined  carbons 70,  543-548 

light  too  far  off 54 

light  too  near 54 

radiant  below  the  axis 53 

lamp  with  microscope.  .   328,  336 

large  source 120 

lathe-bed    form    3-lens   con- 
denser, water-cell 60 

lime  light 100,  101 

mantle,  gas  lamp 125 

mazda  lamp 90,  91 

microscope 265,  365 

moving  pictures 587,  591 

Nernst  lamp 92,  95 

opaque  objects 173 

ordinary  miscrocope 263 

petroleum  lamp 121,  122 

small  arc 85,  86,  90 

small  current 80 

sunlight ; 138,  139 

three-lens  condenser 13 

two-lens  condenser 10 

two-lens  condenser,  hand-fed 

lamp    62 

with  direct  current 9 

with  rods  and  microscope.  .  .   264 
Magnification,  with, 

micro-projection    225,  257 

opaque  projection 181 

opaque  lantern 356 

wall  diagrams 355 


Magnification,  various  objective 

and  screen  distances  .  .  .  .257-261 
Magnesium    oxide    as    standard 

screen    462 

Magic  lantern  with  weak  lights  119 
amplifiers  and  oculars 

258,  262,  598-601 
Magnification 

181,  228,  257-262,  357,  369 

actual    257-261 

calculated 261 

due  to  the  amplifier 262,  600 

due  to  the  ocular 232,  262 

how  calculated 261 

how  found 260,  353 

law  for 357-369 

varying 351 

Management    of   apparatus   for 

projection,  24,  73,  92,  106,  123, 
127,  414,  433 

Mantle,  gas  lamp 125 

inverted 126 

position 125 

upright    126 

Masks,  how  to  employ 253 

kind  and  color 253 

for  microscopic  slides 252 

Masked  sections  and  slides.  .253,  270 
Marcy's  petroleum  lamp  ....  121-122 

Marey 685-686 

Mayer,  Alfred  M 138,  157,  159 

Mazda  lamp 90 

summary  for 98 

wiring  of 90 

Measurement  of  electric  quanti- 
ties     478 

Mechanism  of  cameragraph 410 

for  moving  the  film 406 

moving  picture  machine 

400,  402,  416 

Mercury  arc  rectifier 490-491 

Metallic  screens 458-459 

Meter,  unit  of  length 318 

candles    612-615 

number  for  reading 612 

Method  of  striae 647-650 

Metric  rule 27,  318,  371 

system 318 

Micrometer  ocular 232,  233 

Micrometer  ocular,  filar 234 

Micrometers  by  photography .  .  .   353 

Micro-planar 224,  225 

Micro-projection,  221,  223,  236,  262, 
267,  284,  286-287,  296,  313,  344, 
385,  598,615-620 


720 


OPTIC  PROJECTION 


Micro-projection,  addition  to  the 

magic  lantern 307-309 

advantages 223 

alternating  current 284 

apparatus 221,   319 

centering 246 

college,  bench  lantern 301 

condenser  for 237 

conditions  for  good 309 

direct  current  arc  lamp 236 

illumination  for  236,  248,  271,  284, 

286-287,  305,  328,  335,  339,  598, 

616-620 

house-circuit 285 

lime  light 287 

objectives  224-226 

ordinary  microscope 262 

outfit  for  photography  .  .382,  386 

room    235 

screen    235 

size  of  image 470 

size  of  screen 470 

size  of  specimens 224,  255 

summary 313 

sunlight 286 

troubles  with 301 

weak  lights  for 287 

Microscope  with  amplifier.  .  .238,  600 
for    drawing    on    horizontal 

surface 344 

drawing  with  inclined 345 

focusing  device 241 

house  circuit  drawing  outfit .  .   343 
getting    light     through    with 

concave  mirror 347 

magic  lantern  lamp  and  con- 
denser    336 

objectives,  objects  to  project 

with 269 

photo-micrography 385 

plane  mirror 346 

position  for  drawing 345 

projection  and  drawing 275 

projection  with  ordinary.262,  263 

projection  with  vertical 267 

slide,  masks  for 252 

specimens,  projection  of 269 

tube 241 

size  and  image 243 

vertical  with  high  powers ....   283 

vertical,  stray  light 268 

without   ocular   or   substage 

condenser 334 

Microsummar 225 


Microtessar    225-579 

Milliet  de  Chales 676,  679 

Mirror 574 

at  end  of  clock-shaft  (helios- 

tat)    152,153 

concave 573 

convex 575 

reflection 572-573 

attached  to  drawing  table ...   324 

concave,  as  reflector 127 

concave,     without     substage 

condenser 349 

45  degree  with  vertical  objec- 
tive     46,  267 

to  illuminate  opaque  object .  .    1 75 
light  through  microscope  with 

concave 347 

with  plane 346 

on  drawing  table 324 

parallel  to  clock- shaft  (helios- 

tat)    154 

position  at  equator,  for  helio- 

stat    158 

position  at  poles  for  a  helicstat  1 58 
position  and  time  of  day  with 

the  heliostat 144 

screen    459 

silvered  on  face  with  dim  light  33 1 

to  get  erect  image 190,  361 

to  reflect  image  to  drawing 

surface 322,  337 

Mirroroide   459 

Misframe    434 

Mixed  jet 105,  107 

Models,  drawing  for 357 

Moler,  Geo.  S 218,  219 

Molyneux <  .  .   677 

Motion  of  picture  film 419 

Motion  Picture  Screen  Co 459 

Motor-generator  set 489 

Mounting,  condenser 17,  62 

lantern  slides 216 

low  power  objectives 241 

Movement  of  lamp  at  its  limit .  .     47 
intermittent     with     camera- 
graph  417 

Moving  pictures, 

390,  409,  435,  468,  570,  591,  ooo 

apparatus  for 390 

apparatus,  getting  in  correct 

position 412 

development  of  the  art .  .  .  391 ,  680 
and  education 394 


INDEX 


721 


Moving  Pictures,  energy  required 

for  projection    57° 

at  home 435 

history ....680-686 

illumination  and  optics 

409,  591,  593-598 

image  formation 594~598 

in  science 394~395 

size  of  image  and  screen 468 

summary  and  troubles .  .  .  436,  438 
Moving  picture  and  magic  lan- 
tern projection 587,  591 

machine.  .  .400-408,  410-418,  435 

inside  shutter  for 406-407 

installing 4°7 

mechanism  of 415-417 

objective  for 406,  412 

operating  room  for 396-397 

operator  for 39° 

optics  of 409,  591 

phonograph   with 393 

position     in     the     operating 

room    396 

principle   of 4l6 

Moving  Picture  World 

101,  207,  390-391,  397,  406 
Multiple  lanterns,  composition  of     34 

dissolving  views 35 

effects  with 35 

use  of 34 

wiring  for 34 

Municipal  regulations  for  wiring  499 

Muybridge 393,  681-684 

Myopia  or  short  sight 660 

Nash,  A.  E.,  painting  screens.  .  .   456 
National  Board  of  Fire  Under- 
writers     499 

National  Electric  Code 

499-500,  502,  515,  528 

National  X-Ray  Reflector  Co ...   442 

Negative    carbon,    12-13,    222,    250, 

507-508, 537-538, 547-549, 55i- 

552, 640 

Negative  photographic  for  lan- 
tern slide 208,  211 

making  with  projection  ap- 
paratus     384 

Nelson 274 

Nernst  lamp 92,  93,  94 

automatic 92 

for  drawing 323 


for  spectra 628 

summary 98 

Nernst  Lamp,  wiring  alternating 

current 93 

wiring  direct  current 93 

Neuhauss 9,  202,  677 

Newton  and   chromatic  aberra- 
tion    583 

composition  of  white  light ...   391 
experiments      with      the 

spectrum 62*7 

&  Co.,  lanterns  and  lantern 

slides 124 

Nicholas  Power  Co., 

391,  402,  404,  410,  417 

Nichols,  physics 521 

Nicol  prism 623 

for  analyzer 623-624 

polarizer 623-624 

Nicol  prism  spectacles 37 

Normal  vision,  projection  experi- 
ments     , 652-659 

apparatus  for  demonstrating 

651-655 

trial  lenses  for 651-655 

lantern  slides  for 656 

source  of  light  for 652 

Norris,  electrical  engineering   .  .   522 

Norton    9 

Nose-piece  for  objectives .  .  .  .249,  250 

Obliquity,  avoidance  of 41 

Observation    window    in  lamp- 
house  72 

Oculars,  230-234,  280,  337,  366,  369- 
372,600 

compensation 232,  600 

designation  of 231 

drawing,  for 337 

eye-lens  of 366,  600-601 

field  lens  of 366,  600 

filar  micrometer 234 

image  formation  when  used 

366,  601 

Huygenian 230 

magnification  by.  .  .232,   258-262 

designation 231 

for  drawing 337 

filar  mictometer 234 

micrometer 232,  233 

position  of  image  and  course 

of  light 366,601 

projection  .230,  232,  280,  366,  601 


722 


OPTIC  PROJECTION 


Oculist's  trial  lenses,  651,  655 

where  to  be  obtained 651 

Ohm 477,  521 

Ohmage 477 

Ohmage  by  Ohm's  law 521-522 

Ohm's  law 521 

Oil  lamp 121 

Object,  position  on  stage 257 

for  projection,  objectives ....  268 
projection  when  horizontal.  .  268 
size  for  opaque  projection ...  179 

visibility  of 227 

to     draw     with     projection 

microscope 333 

erect  image  when  vertical.  .  .    190 

horizontal,  image  erect 190 

how  to  light 335 

illumination  with  main  con- 
denser    234 

large,     drawing     with     low 

powers .  . . 333 

for  opaque  projections 179 

position    with    different    ob- 
jectives     256 

Objective 18,  224,  226,  281,  609 

aperture     for     opaque     and 

transparent   objects 172 

apparatus  at  rear  of  room .  .  .  464 
apparatus  not  at  rear  of  room 

464, 468 
brilliancy  of  the  screen  image  613 

centering 247 

demonstrations     with     high 

powers 271 

diaphragm  of 380 

distance  from  condenser  43,  247 

drawing 335 

focus    and    brightness    of 

screen    613 

focus  for  room  and  screen .  .   467 

for  different  sized  fields 254 

formula   for    getting    proper 

focus 467 

with    hood    for 245 

immersion 225,  349 

low  power,  mounting 241 

magic  lantern 

1 8,  464,  597,  60 1,  609 
microscope  construction  ....   616 
microscope  and  image  forma- 
tion with 616 

moving  picture 406 

painting  black 246 


Objective,     photographic,     con- 
struction     225 

for  photography 384 

for  photographic  printing .  .  .   379 
proportioned  to  condenser  43,  277 

revolving  nose-piece  for 249 

screen  limited  in  size  by 469 

shield  for 30-31,  246,  293 

to  use  with  substage  condens- 
er     281 

various  fields 255 

various  objects 268-269 

Old  age  sight  or  presbyopia 66 1 

Opaque  lantern  for  drawing.  .  .  .   332 
drawing  on  horizontal  surface  332 

one  source 173 

two  sources 173 

with  sunlight 174 

Opaque  objects,  bright  images  of  174 

lighting  entire 187 

magnification    and     size    of 

image 181 

projection,  166,  168,  183,  184,  187, 
188,  191,  192,  196 

screen  and  distance 181 

size  and  kind  of  objects 179 

summary 198 

troubles 195 

versus  transparency.  1 68,  169,  170 
Operating  room,  for  moving  pic- 
tures.  396-398,  453 

construction  (M.  P.  World) .  .   397 

permanent    396 

temporary 396 

Operation    of    moving    picture 

machine    420 

Operator,  competent 390 

Optical  bench 288-289,  294 

deceptions 36 

system  for  spectra 628,  641 

Optics  of  moving  pictures.  .  .409,  591 

of  projection,  general 572 

Orange  G  for  staining  masks ....   253 
Oscillogram,  anode  currents.  .  .  .   495 
current     supplied     and     de- 
livered     493 

potential   and   electromotive 

force    494 

rectifier 493~495 

Oxodium 113 

Oxone    113 

Oxygen,  cylinder 101 


INDEX 


723 


Oxygen-ether 114 

generator 113 

use  of 113 

tank 102 

Oxy-hydrogen  flame 100 

Oxylithe    113 

Paint,  amount  for  cloth  screen .  .   456 

for  blackening  apparatus 245 

for  projection  room 440 

Painted  cloth  screen 457,  460 

Painted  wall  screen 454 

Paper,  holding  while  printing.  .  .   379 

Paper  for  masks 253 

Paper  for  printing  with  projec- 
tion     379 

Paper  screen 458 

Parallel  light,  how  to  obtain.  ...   273 

Parallel,  rheostats  in 531 

Parallelizing  light 276,  335 

Patching  a  film 428 

Pen  for  slides 206,  207 

Pennock,  Edward 673 

Pennsylvania    Metallic    Tubing 

Co 108 

Petroleum,  amount  to  use 125 

lamp 121,  123,  124 

lamp,  amount  of  oil 125 

lamp,   candle-power 125 

chimney    122 

condenser  position 125 

management 123 

lamp  for  photo-micrography  386 

position  of  flame 125 

reflector 122 

smoking    133 

summary 136 

Phenakistoscope  or  magic  disc  of 

Plateau  680-681 

Photographic  booth 32 1 

camera  for  drawing 332 

enlargements     with     projec- 
tion   . , 378 

lantern  slides 200,  208 

objectives  for  prints 379 

for  projection 224 

objects  to  project  with ....   268 

paper  with  projection 379 

room    320 

Society,  British 217 

Photography  apparatus 319 

with     projection     apparatus 

3i9,38i 
with  Quartz  spectrograph .  .  .   642 


Photography,  with  spectra  .....   643 

summary 388 

troubles 384 

Photo-micrographic  camera.  ...   382 

microscope 385 

vertical  camera 384 

Photo-Mineature  Series 200 

Physical  Review 567-569 

Physics,    special   projection    ex- 
periments in 622 

Plaster  Paris  screen 454 

Plate,  exposing  directly 381 

Plateau  and  moving  pictures .  .  .   680 

Pointer  for  lecturer 271 

Polarity  determination 506-511 

by  arc  lamp 506 

with  direct  current.  .496,  506-511 

direct  current  ammeter 506 

direct  current  voltmeter ....   508 

incorrect 51 ,  507 

indicating  on  wires 51,  508 

indicators,  chemical 510 

Nernst  lamp 95 

right  and  wrong  in  arc  lamp 

51,507 

small   arc 84 

test 51,  506-511 

testing  by  ammeter 506 

testing  by  voltmeter 508 

Polarized  attachment  plugs.  .87,  504 

light 622-626 

brightness  of  screen  image  626 
condensers  and  objectives 

for   625 

dark  room  needed  for  experi- 
ments       626 

detecting  strain  in  lenses  by .   626 
projection  experiments  with 

622-626 

setting  up  the  experiments.  .   626 
showing    rings    and    brushes 

with 625 

small  Nicol  prism  for 624 

specimens  for 623 

use  of  piles  of  glass  plates .  623-624 

use  of  small  prisms 624 

wall  socket 86,  503 

Popular  Science  Monthly.  .  .321,  521 
Porta,  Baptista  and  the  camera 

obscura  ....". 674 

Porte  lumiere 140,  144 

installation 140 

with  microscope 142 

operation    143 


724 


OPTIC  PROJECTION 


Porte  lumere,  setting  up 142 

for  south  window 143 

for  west  window. 145 

Porter,  T.  C 424-426 

Position  of  Ballast 512-513 

condenser  and  objective.  .  .43,  251 
of  filament  (mazda  lamp) ...  91 
of  moving  picture  machine.  .  396 

of  object  on  stage 257 

of   object   with    various   ob- 
jectives    256 

of   switch 515-518 

Post-card   projector 183 

Potato  polarity  indicator 511 

Potential  drop  and  arc  length  536-539 

and  current 536 

and  electrodes 536 

Potential,    oscillogram,    electro- 
motive force  supplied 494 

Potter,  A.  O 177 

Power  at  arc,  wattmeter  connec- 
tions       482 

Power,  Nicholas  Company 

391,402,404,410,417 
Power      consumption,      candle- 
power 559,  561 

factor 485 

Precautions,  alcohol  lamp 133 

ammeter,  wiring 481-482 

heavy  currents 186,  513 

lime  light 116 

in  polarity  tests 506 

for  wiring. 498,  513 

Precision  Machine  Co 406 

Presbyopia,  old  age  sight.  .  .  .661-662 

Pressure  gauges 102,  103 

gauge,  acetylene  tank 128 

of  one  atmosphere 103 

reducing  valve 102 

Prestolite  tanks 127,  128,  129 

Principal   axis 577~58 1 

focus  and  how  obtained.  .578-579 

Print,  size  for  lantern  slides 208 

Printing,  artificial  light 214 

on    paper    or    a    dry    plate, 

directly   381 

exposure  for 380 

how  to  hold  paper 379 

lantern  slides 209 

lantern  slides  with  camera.  .  .   213 

paper  to  use 379 

Prints,  enlarged  from  negatives.   378 

white  on  black 213 

Prism,  37,  46,  190,  196,  322,  337,  344, 
368,  629-632,  640-643 


erecting 368 

direct  vision 627,  631 

drawing 322, 337,  344 

glass 627,  629-630 

hollow  for  liquids.  .  .627,  631-632 

polarizing 622-626 

quartz  for  ultra-violet 641 

right-angled      with     vertical 

objective 46 

Rutherford's 630-631 

spectacles   of 37 

Prism,    Nicol   for   polarizing   or 

analyzing  light.  .  .622,  624,  626 
Proceedings    of     the    American 
Association    for    the    Ad- 
vancement of  Science  ..211,321 
Proceedings  of  the  International 

Electrical  Congress 566 

of  the  American    Microscop- 
ical Society 320 

of  the  Royal  Society 426 

Projection  apparatus,  with  mir- 
ror on  lecture  table.  .  .  .451-452 

outside  the  room 453 

for     photographic     enlarge- 
ments       378 

rules  for  construction 6 

aid  to  drawing 319 

apparatus,    best   position   in 

room    446 

early  use  with  drawing.  .  .  .   372 

form  for  drawing 321 

home-made 287 

similarity  of  all 221 

from  three  standpoints 3 

universal 178,  185,  191 

of  human  face 170 

horizontal  objects 

33,  185,  190,  268 
illumination  in  high  power ...   271 

in  physics 621-651 

material   and   apparatus   for 

622,  626-627,  643,  644,  651 

of  spectra 626-642 

special  experiments  in 622 

with  polarized  light 622-626 

with  ultra-violet  light 

627, 641-642 

microscope,    178,    221,    222,    271, 
282,  299,  300,  303,  309,  322,  598 
601 
microscope  with  amplifier 

238,  366,  599-600 

drawing  with 333 

erect  images  with 3^>7~373 


INDEX 


725 


Projection  apparatus,  exhibition  270 

on  house  circuit 234,  285 

illumination   for,   237,    247,   251, 
271-282,  328,  335,  347,  598,  601 

limitations 223-224 

on  market 296 

solar 142 

microscope 221,  234,  249,  271 

use 249 

defective   vision    demonstra- 
tions   659-672 

moving  pictures 

409,  420,  433,  593 

microscopic  specimens 269 

multiple  lanterns 34 

normal  vision 652-659 

objective 18,  224,  464 

shield  for 292 

objectives,     micro-projection 

224-226,  244,  263 
objective  for  moving  pictures 

406,412 

ocular 230-234,  280,  366,  600 

optics  of 572 

position  of  carbons  for 
49-50,  249,  342,  415,  507,  537-550 
room,  form  and  darkness .  440-444 

specimens  for 252,  268 

table    287 

with  drawing  shelf 325 

with  vertical  microscope.  .  .  .   267 

with  weak  lights 120 

Projectors  for  home  use 435 

Projectoscope  combination.  .185-186 

universal 302 

Proportion  of  gases  for  lime  light  108 

Pseudoscopic   effect 39 

"Push-through"  slide  carrier.  ...     23 
Pupil  for  eye  experiments.  .  .651,  669 

effect  of  size  in  vision 669 

Ptolemy,  phenomena  of  refrac- 
tion       576 

Quartz  optical  system  for  ultra 

violet  .  .  .  . " 641 

spectrograph  for  ultra  violet  642 
system   for   spectra   of   ultra 

violet 641 

Queen  &  Co 166 

Radial    lines    for    showing    eye 

defects 662-667 

Radiant  or  light  source,  1 1,  68,  78,  90, 
92,  100,  113,  119,  121,  125,  127, 
130,  138,  553 


Radiant,  arrangement  and  cen- 
tering  39-46,  56 

distance  from  condenser.  ...     41 

for  drawing 328,  329 

Radiant  efficiency 567 

and  mirror 189 

energy   for  moving   pictures 
with  alternating  current.  .   570 

direct  current 571 

for  opaque  projection 174 

position  and  illumination  of 

condenser 42 

position  of  in  opaque  projec- 
tion        174 

tilted 188-196 

Radiation,  light  energy  of.  .  .566,  569 

visible  and  invisible 566 

Ramsden's  Circle  or  disc 230 

type   ocular 234 

Reactor 532 

Read,  E.  A 673 

Rectifier  for  alternating  current  489 

Red  lights  near  exits 443 

Reducing  valve  for  gas  cylinders  102 

Reflection 572 

irregular   572~573 

law  of 140,  572 

losses  by 175,  602 

metallic  surfaced  screens .  .   458 

mirror 573 

regular 572 

semi-regular 573~574 

of  various  screens.  .460,  463,  465 
visibility  of  reflected  beam.  .    573 

Reflector,  acetylene  lamp 127 

gas  lamp 126 

in  opaque  projection 175 

petroleum  lamp 122 

Reflectoscope 180 

Refraction    575~576 

at  plane  surfaces 575 

at  curved  surfaces 575 

air  to  glass 575 

air  to  water 575 

density  and  refraction 576 

index  of 576 

law  of  sines 576 

Ptolemy's  laws  of  refraction  576 
Snell  and   Descartes  law  of 

sines 576 

Refractive  eye  defects 659-672 

Regulations,    fire    underwriters, 

for  wiring 499~5°5 

Reichert 82,  83,  93 


726 


OPTIC  PROJECTION 


Reinhold,  use  of  camera  obscura 

in  eclipses 674 

Requirements  for  good   projec- 
tion 

7,  1 8,  222-223,  313-317,  433 
for    projection     with     weak 

lights 120 

Resistance,  to  control  current .  .  .   539 
how  to  get  amount  needed ...    52 1 

unit   of 477 

Retouching  frame 29,  203 

Rewinding  moving  picture  film .  .   430 

Reynolds' 374 

Rheostat 521-531 

adjustable 263,  326,  523-524 

alternating  current 69,  532 

barrel,  saltwater 525-526 

calibration  of 525 

direct  current 1 1 ,  52 1 

energy  used 533 

fixed 523 

heat  developed  in 523 

home-made 526-530 

installing 84,  339,  504,  524 

in  parallel 531 

on  1 10  volt  line 542 

range  of 263,    326,    523-524 

salt  water 525-526 

series 531 

tin  strips 529~53O 

water  cooled 527-528 

wiring    504,  524 

Richardson's    Motion    Picture 
Handbook 

390-391,397,420,439 

Riley,  Dr 327,  331 

Ripolin  white  paint 264 

Roaring  of  alcohol  lamp 134 

Roaring  of  lime  light 115 

Rods  for  optical  bench 290 

Roller  screens 456 

Rooms,  for  projection 439 

Room,  apparatus  on  lecture  table 

451-452 

darkening  443 

darkness  for  opaque  projec- 
tion     175 

for  drawing  and  photography  32 1 

for  micro-projection 235 

for  moving  pictures 395 

form  for  projection 440 

light  in  projection 441,  443 

limiting  size  of  screen 469 


Room,  size  with  sunlight 162 

tint  and  decoration 440 

troubles  with 471 

Salt  and  water  polarity  indicator  511 
water  rheostat 525-526 

Scale,  diagrams  with  opaque  lan- 
tern    356 

wall  diagrams 355 

Scheiner    361,  679 

demonstration      of      retinal 

image 656 

theory  of  accommodation  658 

Oculus 368 

Science    2 1 1 ,  327 

Scientific  American 221 

Schmidt  &  Haensch 102,  126 

Screens   454-466 

anthracene 627,  635-642 

Screen,  in  alcove 443 

cloth    457 

dark  spots  on 309 

distance 464-471 

distance,  for  high  powers.  .  .  .   282 
distance,  micro-projection 

225,282 
for  opaque  projection.  .  .  181,  454 

for  high  powers 272,  282 

hinged  to  meet  axial  ray 449 

image,  actual  size 257 

with  amplifiers  and  oculars  258 
perfection    and    brilliancy 

18,612-617 
image,  not  photographically 

sharp 256 

with  polarized  light 626 

size  of 464 

spectra 627,    635,    642 

stereoscopic    37 

with  vision  experiments 654 

not  light  enough 301 

with  metallic  surface ....  458-463 

micro-projection    235,  454 

moving  pictures 395,  454 

painted  cloth 455 

painted  on  wall 454 

paper 458 

plaster  of  Paris 454 

qualities  of  goods 454 

reflection 460-465 

reflection,  semi-diffuse 459 

reflection,    white.  .246,    460,    462 


INDEX 


727 


Screen,  roller 456 

size 464 

size  dependent  on  distance.  .   466 

size  for  lantern  slides 464 

size  limited  by  room 469 

size  for  micro-projection.  .  .  .   470 
size  for  moving  pictures.  .395,  468 

size  for  petroleum  lamp 125 

size  for  sunlight 162 

tipped  to  meet  axial  ray.  .449,  452 

translucent 461 

travelling    458 

troubles 471 

white  washed  wall 455 

Screws  and  nuts,  thumb 296 

Search-light,  use  in  opaque  pro- 
jection      187 

Sections,  masked 253 

Separable,  attachments .  .  87,  503-504 

extension  connector 87 

wall  receptacles 86,  503 

Serial  sections,  masked 253 

Series,  rheostats  in 531 

Shadow  projection  with  the  lan- 
tern     62 1 

Shadows,    alternating    current 

lantern    75 

avoidance  of 190 

on  screen 53 

Shedd,  Dr.  on  history  of  Ohm's  law  52 1 

Shellac  for  sizing 456 

Shield,  asbestos  paper  for 266 

beyond   objective 246 

for  drawing  in  dark  room ....   351 
for  drawing  in  light  room ....   351 

objective 31,  in,  292 

on  condenser  tube 349 

to  hold  objective 293 

Shock,  electric,  if  on  damp  floor     59 

Short  sight  or  myopia 660 

Short  circuit 47,  496 

Shunt  with  ammeter 479 

generator    487-488 

adaptibility  for  arc  lamp. 487-488 

connected  to  arc  lamp 488 

Shutter,  automatic  fire 420 

Shutters  for  darkening  room ....   446 
on  moving  picture  machine 

406-408,  410,  680 
early  use,  by  Heyl  and  Muy- 

bridge 680-682 

best  position 422 

on  moving  picture  machine 
inside 407 


Shutters,  number  of  wings 425 

outside    408,  410 

setting  or  "timing" 422 

speed  to  prevent  flicker .  .423-427 
Size  of  carbons  for  house  system 

87-88,  285,  341 

of  condenser  for  drawings.  .  .  331 
of  drawing,  how  to  obtain .  .  .  329 
of  screen 

125,  162,  395,  464,  468-470 
Sizing,    and    amount    for    cloth 

screen    456 

Slide  box 271 

cabinet    219 

carrier,  individual 31,  32 

lantern 23 

on  moving  picture  machine .  .   404 

push- through  form 23 

masks  and  masking. 2 52-2 53,  387 
thickness,  and  position  of  sub- 
stage  condenser 281,  337 

tray 270 

Slip-slides 36 

Slit  for  spectra 626,  629-630 

for  Abbe  diffraction  experi- 
ments       644 

home-made  slit 630 

illumination  of  slit 636 

Smoke  to  show  the  path  of  light 

rays  247,582 

Snap  switch 515,  518 

Snell,   mathematical  law  of  re- 
fraction     576 

Sockets  for  apparatus 292 

Socket,  railing  flange  and 293 

separable  attachments.  .  .  .87,  504 

wall 86,503 

Sodium  peroxide 1 13 

Sodium  tungstate  for  fireproof- 
ing  cloth 321 

Source    of    light    for   projection 

n,  68,  78,  90,  92,  100,  113,  119, 
121,  125,  127,  130,  138,  553 
of  light  in  demonstrations  in 
normal  and  defective  vision 

652,  670 
Specimen,  effect  of  heat  on.  .252,  607 

for  high  powers 283 

for  high  power  drawing 338 

for  projection 224,  252 

Spectacle  makers  for  supplying 

trial  lenses 651 

Spectra 626-643 


728 


OFTIC  PROJECTION 


Sp  ictra,  absorption  and  substan- 
ces for 637-638 

apparatus  and  material  for,  626,643 

arc  lamp  for 639 

chemicals  to  use  with  627,  637,  640 

comparison  spectra 638 

current  to  use  for  emission 

spectra    639 

electrodes  and  stuffed  elecc- 

rodes 627,638 

emission  spectra 638-640 

gratings  for 627,  632-635 

illumination  of  the  slit  for ...   636 

optical  system  for 628 

optical  quartz  system  for .  627-641 

photography  of 643 

prisms  for 627,  631-632,  642 

quartz  optical  system.  ..  .627,  641 
screens,  white  and  anthracene 

for 627,635,  642 

slit  for 626,  629-630 

substance  to  use  for  absorp- 
tion spectra 637-638 

for  emission  spectra 640 

ultra-violet 641-642 

Spectrograph 642 

Spectrographic   camera 643 

Specimen,  stage  for 239-240 

Spectacles  for  correcting  eye  de- 
fects   659-672 

prism  and  colored  for  stereo- 
scopic screen  images 37-3*3 

Spectrum,  showing  visible  radia- 
tion       566 

Speed  of  shutter  to  prevent  flick- 
er   422-427 

of  moving  picture  machine .  .   420 

Spencer  Lens  Co.,  28, 63,  81,  190,  193- 

196,  233,  241,  271,309-312,354, 

355 

Spherical  and  chromatic  aberra- 
tion     580-583 

Splicing  moving  picture  film.  ...   428 

Spots  on  screen 309 

Spotting  lantern  slides 

19,  26,  29,  201,  203,  216 

Stage  cooling  device 239,  240 

Stage,  mechanical 239,  241,  242 

in  micro-projection 239-240 

water-cell    239,  240,  608 

Stain  for  tables 289 

Stampfer's  magic  disc 680 

Standard,  aperture 571 


Standard,  material  for  installing 

arc  lamp 502 

Stanley,  Geo.  C 375 

Stan  ton,  Theodore 673 

Starch  polarity  indicator 510 

Starrett  (L.  S.  Starrett  Co.) 295 

Stenopaeic  slit 651-670 

Stereoscopic  screen  images.  .  .  .37-38 

Stimson  Hall 155 

C.  H.  vStoelting  Co.  .33,  141,  185,302 

vStoring  lantern  slides 218 

Straite,  inventor  of  automatic  arc 

lamp    ooo 

Stray  light,  avoiding 380 

Striae,  method  of  demonstrating 

647-650 

Style  Brief,  Wistar  Institute 374 

Substage  condenser 

232,271,347,619,626 

achromatic  282,  626 

aperture  of 278 

centering 280 

and  concave  mirror 348 

Kohler  method  with 278,  619 

objectives  with 281 

plane  mirror  with 348 

condenser,  position 337 

position  depending  on  slide.  .   281 
position  with  different  ob- 
jectives     282 

position  with  parallelizing  lens  277 
reduction  of  brillaincy  by 

235, 618-620 

Summaries  of  the  different  chap- 
ters: I.  64;  II,  76;  III,  97; 
IV,  117;  V,  135;  VI,  164; 
VII,  198;  VIII,  220;  IX,  313; 
X,  388;  XI,  438;  XII,  472 
Summary,  alternating  current.  .  76 

direct  current  lanterns 64 

drawing  and  photography.  .  .   388 

heliostats  and  sunlight 164 

house  circuit 97 

lantern-slide   making 220 

lime  light 117 

micro-projection    313 

moving  pictures 438 

oil,  acetylene,  alcohol 135 

opaque  projection 198 

rooms  and  screens 472 

sunlight 164 

weak  lights 135 

Sunlight  for  opaque  projection.  .    174 


INDEX 


729 


Sunlight,  for  spectra 628 

troubles  with 162 

Support  for  magic  lantern 80 

Sun,  apparent  positions 163 

intrinsic  brilliancy 138,  613 

with  projection  microscope .  .   286 
temperature 547 

Sunlight,  apparatus  for 138 

condenser  for 161 

for  micro-projection 286 

for  spectra 628 

Switch  for  opening  and  closing 

the  electric  circuit 514-518 

closed  and  open. 515 

double  pole 12,  70 

double    pole    double    throw 

knife  switch 297 

enclosed    517 

end  of  knife  switch  next  sup- 
ply   516-517 

for  all  the  wires  of  a  circuit ..   514 

installation  of 515 

knife  switch 515 

lamp  or  table  switch 

12,  15,71,238,263.288 

open  and  closed 515 

position  of 516-517 

snap  switch 515 

table  or  lamp  switch 

12,  15,71,238 

Table  for  drawing  with  attached 

mirror 323~324 

for  projection 287 

for  projection  with  drawing 

shelf 325 

stain  for 289 

stand     for    moving    picture 

machine 401 

switch  for  opening  and  clos- 
ing electric  circuit 

12,15,71,238 

Talbot,  F.  A 391,  439 

Temperature,  absolute 547 

of  crater,  of  arc  lamp 546 

of  sun 547 

Tent  of  cloth  for  drawing 350 

for  drawing .- 166 

Terminals,  carbon 12,  539 

Tests  of  polarity 506-51 1 

Theory  of  flicke'r 426 

Thompson,  A.  T.  &  Co 

80,  180,  191,  286,  303,  334,  687 
Silvanus  P.  History  of  arc  lamp  686 


Thorium  discs 101 

Three-lens  condenser.  . .  .61,  587-592 
Three- wire  automatic  lamp .  .  238,  263 

Thumb  screws  and  nuts 296 

Tin  for  rheostat 530 

Tinted  glass  in  combined  projec- 
tion    177 

Tint  for  projection  room 440 

Toepler's  method  of  striae .  .  .  647-650 

"Schlieren-Methode" 647 

Tracing  pictures 374 

Track 290 

fixing  to  baseboard 290 

for  optical  bench 290 

Transformer 533~535 

with  arc  lamp 534 

wiring    532 

Translucent   screen. .  .  .453,  461-462 
Transparency  and  opaque  pro- 
jection   168-170 

Troubles  in  the  various  chapters: 

I,  46-59;  II,  74-75;  III,  96; 
IV,  114-116;  V,  133-134; 
VI,  162-163;  VII,  195-197; 
VIII,  219;  IX,  301-309;  X, 
384-388;  XI,  43M37;  XII, 
47i 

alternating  current. 74 

direct  current  magic  lantern     46 
drawing  and  photography.  .  .   384 

house  circuit 96 

lime  light 114 

making  lantern  slides 219 

micro-projection    301 

moving  pictures 436 

opaque  projections 195 

rooms  and  screens 471 

sunlight 162 

weak  lights,  oil,  gas,  acety- 
lene        133 

Trutat 202,  621 

Tube  of  miscroscope 241 

Tubing,  metallic 108 

for  optical  bench 290 

Tungsten  lamp  for  spectra 628 

Twilight  vision 121,    175,   444 

use  with  weak  lights 121 

Two-lens  condenser 62,  587-591 

Tyndall 621 

Uchatius,  moving  picture  projec- 
tion (1853) 680,  683 

Ultra-violet,  anthracene  screen 

for  627,  635,  642 


730 


OPTIC  PROJECTION 


Ultra-violet,  quartz  optical  sys- 
tem for 641-642 

radiation 567,  641-642 

projection  of 640-  642 

United  States  Geological  Survey  147 
Units  of  alternating  current .  484-486 

Units  of  direct  current 475-483 

Units  of  electricity 475,  484 

University  of  Pennsylvania  and 

animal  movement    682 

Unpacking  moving  picture  ma- 
chine    401 

Uranium  arc  for  spectra 628 

when  in  the  positive,  and 
when  in  the  negative  elec- 
trode    640 

Valspar  varnish  for  water  cells 

205,  264 

Varnishing  lantern  slides 205 

Vertical     microscope    for     high 

powers 283 

Vertical  knife  switch 516 

Vertical  projection 33 

Violet  lines  in  arc  stream 546 

Visibility  of  objects 227 

Vision,    daylight    and    twilight 

121,    175,    444 
Vision,  experiments  with  normal 

651-659 

defective 659-672 

persistence  of  in  moving  pic- 
tures   

Visual  angle 228 

Voigtlander  und  Sohn 226,  250 

Volt  and  voltage 476,  484 

Voltage,  alternating  current ....   484 

by  Ohm's  law 521 

intermediate 544 

of  line  and  arc 544 

of  line,  voltmeter  connections  475 

not  present  in  the  line 46 

Voltmeter,    connection    for    arc 

voltage    .  .„ 476 

connection  to  circuit 478 

direct  current 478 

for  line  voltage 475 

for  polarity  testing 508 

soft  cored 510 

Walgensten 3,  675-677 

Wall  diagrams 329,  355 

Wall  receptacle 86,  503 


Water-cell,  13,  15,  18,  57,  63,  71,  161, 
194,  222-223,  237,  239-240, 
264-266,  276-277,  281-282, 
287,  299,  301,  309,  355,  365, 
421,  432,  571,  592,  604-609 
absorption  of  radiant  heat  not 
affected  by  temperature .  .  .  606 

effect  of  alum,  etc.  in 607 

efficiency 568-569 

light  and  energy  absorbed  by 

604-607 

in  micro-projection 237 

the  microscope  stage  in 608 

how  to  make 264 

need  in  moving  pictures .  .  42 1 .  432 
with  ordinary  microscope.  .  .   266 
position       in     micro-projec- 
tion  239,  240 

shadow  from 57 

stage    239-240 

sunlight 161 

sun  and  micro-projection.  .  .  .   287 
with  two-lens  condenser .  .  365,  42 1 

Water  cooled  rheostat 527 

Waterhouse,  Gen.  J 673 

Watt,   and  wattmeter. 477,   482-483 

Watts,  alternating  current 485 

equal  volts  times  amperes 

477,521 

Wattmeter 482-483 

amperage  by 483 

connecting    482 

power  at  arc  by 482 

power  from  dynamo 483 

White,  Dr.  A.  C 673 

White-washed  wall  screen 455 

Wiedemann's  Annalen 647 

Williams,  Brown  &  Earle 

59,  91,  107,  130,  131,  182,  183,  300 

Wimmer 9,  202 

Winding  and  rewinding    moving 

picture  film 430 

Widow  in  lamp -house 

72,  267,  402-403 
Window    shades    for    darkening 

the   room 445-447 

Wires  for  the  electric  circuit .  496-505 

how  to  connect 503 

insulation   of 497-498 

wrong  polarity 51,  506-5 1 1 

Wiring  the  arc  lamp,  10,  13,  15,  71, 
84,  263,  324,  339-340.  399, 496- 
535,  639 


INDEX 


731 


depending  on  amperage 496 

for  large  currents 513 

for  automatic  lamp 512 

for  moving  pictures 399 

for  multiple  lanterns 34,  297 

for  a  distant  supply 513 

inductor  and  transformer.  .  .   532 

mazda  lamp 90 

municipal  regulations  for.  ...   499 
Nernst  lamp  alternating  cur- 
rent       93 

Nernst  lamp,  direct  current .  .     94 

on  three- wire  supply 514 

regulations  for 498-501 

rheostat 504 


small  arc  lamp 82,  84 

Wistar  Institute 320,  374 

Woodward,  use  of  lime  light  for 

projection   (1824) 686 

Wright,  Lewis 

9,  105,  207,  243,  245,  256,  282 

Zahn,  early  projection  apparatus  679 
Zeiss,  176,  181,  187,  221,  225,  231, 

232,  250,  272,  281,  382,  385,  386 

Zeitschrift  fur  Instr 221 

Zeitschrift  fur  wissenschaftliche 

Mikroskopie 221 

Zirconium  discs 100 

Zoetrope  and  magic  disc  ....  680-682 


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